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rtmutex: update rt-mutex-design

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The rt-mutex-design documents didn't gotten meaningful update from its
first version. Even after owner's pending bit was removed in commit 8161239a8b
("rtmutex: Simplify PI algorithm and make highest prio task get lock")
and priority list 'plist' changed to rbtree. And Peter Zijlstra did some
clean up and fix for deadline task changes on tip tree.

So update it to latest code and make it meaningful.
Steven Rostedt and Sebastian Siewior gave much of comments and input
in this doc. Thanks!

Signed-off-by: Alex Shi <alex.shi@linaro.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Juri Lelli <juri.lelli@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
To: linux-doc@vger.kernel.org
To: linux-kernel@vger.kernel.org
To: Jonathan Corbet <corbet@lwn.net>
To: Ingo Molnar <mingo@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Alex Shi 2017-07-31 09:50:53 +08:00 committed by Jonathan Corbet
parent 0e4c2b7589
commit f1824df12e
1 changed files with 102 additions and 324 deletions
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426
Documentation/locking/rt-mutex-design.txt
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@ -97,9 +97,9 @@ waiter - A waiter is a struct that is stored on the stack of a blocked
a process being blocked on the mutex, it is fine to allocate
the waiter on the process's stack (local variable). This
structure holds a pointer to the task, as well as the mutex that
the task is blocked on. It also has the plist node structures to
place the task in the waiter_list of a mutex as well as the
pi_list of a mutex owner task (described below).
the task is blocked on. It also has rbtree node structures to
place the task in the waiters rbtree of a mutex as well as the
pi_waiters rbtree of a mutex owner task (described below).
waiter is sometimes used in reference to the task that is waiting
on a mutex. This is the same as waiter->task.
@ -179,53 +179,34 @@ again.
|
F->L5-+
If process G has the highest priority in the chain, then all the tasks up
the chain (A and B in this example), must have their priorities increased
to that of G.
Plist
-----
Before I go further and talk about how the PI chain is stored through lists
on both mutexes and processes, I'll explain the plist. This is similar to
the struct list_head functionality that is already in the kernel.
The implementation of plist is out of scope for this document, but it is
very important to understand what it does.
There are a few differences between plist and list, the most important one
being that plist is a priority sorted linked list. This means that the
priorities of the plist are sorted, such that it takes O(1) to retrieve the
highest priority item in the list. Obviously this is useful to store processes
based on their priorities.
Another difference, which is important for implementation, is that, unlike
list, the head of the list is a different element than the nodes of a list.
So the head of the list is declared as struct plist_head and nodes that will
be added to the list are declared as struct plist_node.
Mutex Waiter List
Mutex Waiters Tree
-----------------
Every mutex keeps track of all the waiters that are blocked on itself. The mutex
has a plist to store these waiters by priority. This list is protected by
a spin lock that is located in the struct of the mutex. This lock is called
wait_lock. Since the modification of the waiter list is never done in
interrupt context, the wait_lock can be taken without disabling interrupts.
Every mutex keeps track of all the waiters that are blocked on itself. The
mutex has a rbtree to store these waiters by priority. This tree is protected
by a spin lock that is located in the struct of the mutex. This lock is called
wait_lock.
Task PI List
Task PI Tree
------------
To keep track of the PI chains, each process has its own PI list. This is
a list of all top waiters of the mutexes that are owned by the process.
Note that this list only holds the top waiters and not all waiters that are
To keep track of the PI chains, each process has its own PI rbtree. This is
a tree of all top waiters of the mutexes that are owned by the process.
Note that this tree only holds the top waiters and not all waiters that are
blocked on mutexes owned by the process.
The top of the task's PI list is always the highest priority task that
The top of the task's PI tree is always the highest priority task that
is waiting on a mutex that is owned by the task. So if the task has
inherited a priority, it will always be the priority of the task that is
at the top of this list.
at the top of this tree.
This list is stored in the task structure of a process as a plist called
pi_list. This list is protected by a spin lock also in the task structure,
This tree is stored in the task structure of a process as a rbtree called
pi_waiters. It is protected by a spin lock also in the task structure,
called pi_lock. This lock may also be taken in interrupt context, so when
locking the pi_lock, interrupts must be disabled.
@ -312,15 +293,12 @@ Mutex owner and flags
The mutex structure contains a pointer to the owner of the mutex. If the
mutex is not owned, this owner is set to NULL. Since all architectures
have the task structure on at least a four byte alignment (and if this is
not true, the rtmutex.c code will be broken!), this allows for the two
least significant bits to be used as flags. This part is also described
in Documentation/rt-mutex.txt, but will also be briefly described here.
Bit 0 is used as the "Pending Owner" flag. This is described later.
Bit 1 is used as the "Has Waiters" flags. This is also described later
in more detail, but is set whenever there are waiters on a mutex.
have the task structure on at least a two byte alignment (and if this is
not true, the rtmutex.c code will be broken!), this allows for the least
significant bit to be used as a flag. Bit 0 is used as the "Has Waiters"
flag. It's set whenever there are waiters on a mutex.
See Documentation/locking/rt-mutex.txt for further details.
cmpxchg Tricks
--------------
@ -359,40 +337,31 @@ Priority adjustments
--------------------
The implementation of the PI code in rtmutex.c has several places that a
process must adjust its priority. With the help of the pi_list of a
process must adjust its priority. With the help of the pi_waiters of a
process this is rather easy to know what needs to be adjusted.
The functions implementing the task adjustments are rt_mutex_adjust_prio,
__rt_mutex_adjust_prio (same as the former, but expects the task pi_lock
to already be taken), rt_mutex_getprio, and rt_mutex_setprio.
The functions implementing the task adjustments are rt_mutex_adjust_prio
and rt_mutex_setprio. rt_mutex_setprio is only used in rt_mutex_adjust_prio.
rt_mutex_getprio and rt_mutex_setprio are only used in __rt_mutex_adjust_prio.
rt_mutex_adjust_prio examines the priority of the task, and the highest
priority process that is waiting any of mutexes owned by the task. Since
the pi_waiters of a task holds an order by priority of all the top waiters
of all the mutexes that the task owns, we simply need to compare the top
pi waiter to its own normal/deadline priority and take the higher one.
Then rt_mutex_setprio is called to adjust the priority of the task to the
new priority. Note that rt_mutex_setprio is defined in kernel/sched/core.c
to implement the actual change in priority.
rt_mutex_getprio returns the priority that the task should have. Either the
task's own normal priority, or if a process of a higher priority is waiting on
a mutex owned by the task, then that higher priority should be returned.
Since the pi_list of a task holds an order by priority list of all the top
waiters of all the mutexes that the task owns, rt_mutex_getprio simply needs
to compare the top pi waiter to its own normal priority, and return the higher
priority back.
(Note: For the "prio" field in task_struct, the lower the number, the
higher the priority. A "prio" of 5 is of higher priority than a
"prio" of 10.)
(Note: if looking at the code, you will notice that the lower number of
prio is returned. This is because the prio field in the task structure
is an inverse order of the actual priority. So a "prio" of 5 is
of higher priority than a "prio" of 10.)
__rt_mutex_adjust_prio examines the result of rt_mutex_getprio, and if the
result does not equal the task's current priority, then rt_mutex_setprio
is called to adjust the priority of the task to the new priority.
Note that rt_mutex_setprio is defined in kernel/sched/core.c to implement the
actual change in priority.
It is interesting to note that __rt_mutex_adjust_prio can either increase
It is interesting to note that rt_mutex_adjust_prio can either increase
or decrease the priority of the task. In the case that a higher priority
process has just blocked on a mutex owned by the task, __rt_mutex_adjust_prio
process has just blocked on a mutex owned by the task, rt_mutex_adjust_prio
would increase/boost the task's priority. But if a higher priority task
were for some reason to leave the mutex (timeout or signal), this same function
would decrease/unboost the priority of the task. That is because the pi_list
would decrease/unboost the priority of the task. That is because the pi_waiters
always contains the highest priority task that is waiting on a mutex owned
by the task, so we only need to compare the priority of that top pi waiter
to the normal priority of the given task.
@ -412,9 +381,10 @@ priorities.
rt_mutex_adjust_prio_chain is called with a task to be checked for PI
(de)boosting (the owner of a mutex that a process is blocking on), a flag to
check for deadlocking, the mutex that the task owns, and a pointer to a waiter
check for deadlocking, the mutex that the task owns, a pointer to a waiter
that is the process's waiter struct that is blocked on the mutex (although this
parameter may be NULL for deboosting).
parameter may be NULL for deboosting), a pointer to the mutex on which the task
is blocked, and a top_task as the top waiter of the mutex.
For this explanation, I will not mention deadlock detection. This explanation
will try to stay at a high level.
@ -424,133 +394,14 @@ that the state of the owner and lock can change when entered into this function.
Before this function is called, the task has already had rt_mutex_adjust_prio
performed on it. This means that the task is set to the priority that it
should be at, but the plist nodes of the task's waiter have not been updated
with the new priorities, and that this task may not be in the proper locations
in the pi_lists and wait_lists that the task is blocked on. This function
should be at, but the rbtree nodes of the task's waiter have not been updated
with the new priorities, and this task may not be in the proper locations
in the pi_waiters and waiters trees that the task is blocked on. This function
solves all that.
A loop is entered, where task is the owner to be checked for PI changes that
was passed by parameter (for the first iteration). The pi_lock of this task is
taken to prevent any more changes to the pi_list of the task. This also
prevents new tasks from completing the blocking on a mutex that is owned by this
task.
If the task is not blocked on a mutex then the loop is exited. We are at
the top of the PI chain.
A check is now done to see if the original waiter (the process that is blocked
on the current mutex) is the top pi waiter of the task. That is, is this
waiter on the top of the task's pi_list. If it is not, it either means that
there is another process higher in priority that is blocked on one of the
mutexes that the task owns, or that the waiter has just woken up via a signal
or timeout and has left the PI chain. In either case, the loop is exited, since
we don't need to do any more changes to the priority of the current task, or any
task that owns a mutex that this current task is waiting on. A priority chain
walk is only needed when a new top pi waiter is made to a task.
The next check sees if the task's waiter plist node has the priority equal to
the priority the task is set at. If they are equal, then we are done with
the loop. Remember that the function started with the priority of the
task adjusted, but the plist nodes that hold the task in other processes
pi_lists have not been adjusted.
Next, we look at the mutex that the task is blocked on. The mutex's wait_lock
is taken. This is done by a spin_trylock, because the locking order of the
pi_lock and wait_lock goes in the opposite direction. If we fail to grab the
lock, the pi_lock is released, and we restart the loop.
Now that we have both the pi_lock of the task as well as the wait_lock of
the mutex the task is blocked on, we update the task's waiter's plist node
that is located on the mutex's wait_list.
Now we release the pi_lock of the task.
Next the owner of the mutex has its pi_lock taken, so we can update the
task's entry in the owner's pi_list. If the task is the highest priority
process on the mutex's wait_list, then we remove the previous top waiter
from the owner's pi_list, and replace it with the task.
Note: It is possible that the task was the current top waiter on the mutex,
in which case the task is not yet on the pi_list of the waiter. This
is OK, since plist_del does nothing if the plist node is not on any
list.
If the task was not the top waiter of the mutex, but it was before we
did the priority updates, that means we are deboosting/lowering the
task. In this case, the task is removed from the pi_list of the owner,
and the new top waiter is added.
Lastly, we unlock both the pi_lock of the task, as well as the mutex's
wait_lock, and continue the loop again. On the next iteration of the
loop, the previous owner of the mutex will be the task that will be
processed.
Note: One might think that the owner of this mutex might have changed
since we just grab the mutex's wait_lock. And one could be right.
The important thing to remember is that the owner could not have
become the task that is being processed in the PI chain, since
we have taken that task's pi_lock at the beginning of the loop.
So as long as there is an owner of this mutex that is not the same
process as the tasked being worked on, we are OK.
Looking closely at the code, one might be confused. The check for the
end of the PI chain is when the task isn't blocked on anything or the
task's waiter structure "task" element is NULL. This check is
protected only by the task's pi_lock. But the code to unlock the mutex
sets the task's waiter structure "task" element to NULL with only
the protection of the mutex's wait_lock, which was not taken yet.
Isn't this a race condition if the task becomes the new owner?
The answer is No! The trick is the spin_trylock of the mutex's
wait_lock. If we fail that lock, we release the pi_lock of the
task and continue the loop, doing the end of PI chain check again.
In the code to release the lock, the wait_lock of the mutex is held
the entire time, and it is not let go when we grab the pi_lock of the
new owner of the mutex. So if the switch of a new owner were to happen
after the check for end of the PI chain and the grabbing of the
wait_lock, the unlocking code would spin on the new owner's pi_lock
but never give up the wait_lock. So the PI chain loop is guaranteed to
fail the spin_trylock on the wait_lock, release the pi_lock, and
try again.
If you don't quite understand the above, that's OK. You don't have to,
unless you really want to make a proof out of it ;)
Pending Owners and Lock stealing
--------------------------------
One of the flags in the owner field of the mutex structure is "Pending Owner".
What this means is that an owner was chosen by the process releasing the
mutex, but that owner has yet to wake up and actually take the mutex.
Why is this important? Why can't we just give the mutex to another process
and be done with it?
The PI code is to help with real-time processes, and to let the highest
priority process run as long as possible with little latencies and delays.
If a high priority process owns a mutex that a lower priority process is
blocked on, when the mutex is released it would be given to the lower priority
process. What if the higher priority process wants to take that mutex again.
The high priority process would fail to take that mutex that it just gave up
and it would need to boost the lower priority process to run with full
latency of that critical section (since the low priority process just entered
it).
There's no reason a high priority process that gives up a mutex should be
penalized if it tries to take that mutex again. If the new owner of the
mutex has not woken up yet, there's no reason that the higher priority process
could not take that mutex away.
To solve this, we introduced Pending Ownership and Lock Stealing. When a
new process is given a mutex that it was blocked on, it is only given
pending ownership. This means that it's the new owner, unless a higher
priority process comes in and tries to grab that mutex. If a higher priority
process does come along and wants that mutex, we let the higher priority
process "steal" the mutex from the pending owner (only if it is still pending)
and continue with the mutex.
The main operation of this function is summarized by Thomas Gleixner in
rtmutex.c. See the 'Chain walk basics and protection scope' comment for further
details.
Taking of a mutex (The walk through)
------------------------------------
@ -563,14 +414,14 @@ done when we have CMPXCHG enabled (otherwise the fast taking automatically
fails). Only when the owner field of the mutex is NULL can the lock be
taken with the CMPXCHG and nothing else needs to be done.
If there is contention on the lock, whether it is owned or pending owner
we go about the slow path (rt_mutex_slowlock).
If there is contention on the lock, we go about the slow path
(rt_mutex_slowlock).
The slow path function is where the task's waiter structure is created on
the stack. This is because the waiter structure is only needed for the
scope of this function. The waiter structure holds the nodes to store
the task on the wait_list of the mutex, and if need be, the pi_list of
the owner.
the task on the waiters tree of the mutex, and if need be, the pi_waiters
tree of the owner.
The wait_lock of the mutex is taken since the slow path of unlocking the
mutex also takes this lock.
@ -581,102 +432,45 @@ contention).
try_to_take_rt_mutex is used every time the task tries to grab a mutex in the
slow path. The first thing that is done here is an atomic setting of
the "Has Waiters" flag of the mutex's owner field. Yes, this could really
be false, because if the mutex has no owner, there are no waiters and
the current task also won't have any waiters. But we don't have the lock
yet, so we assume we are going to be a waiter. The reason for this is to
play nice for those architectures that do have CMPXCHG. By setting this flag
now, the owner of the mutex can't release the mutex without going into the
slow unlock path, and it would then need to grab the wait_lock, which this
code currently holds. So setting the "Has Waiters" flag forces the owner
to synchronize with this code.
the "Has Waiters" flag of the mutex's owner field. By setting this flag
now, the current owner of the mutex being contended for can't release the mutex
without going into the slow unlock path, and it would then need to grab the
wait_lock, which this code currently holds. So setting the "Has Waiters" flag
forces the current owner to synchronize with this code.
Now that we know that we can't have any races with the owner releasing the
mutex, we check to see if we can take the ownership. This is done if the
mutex doesn't have a owner, or if we can steal the mutex from a pending
owner. Let's look at the situations we have here.
The lock is taken if the following are true:
1) The lock has no owner
2) The current task is the highest priority against all other
waiters of the lock
1) Has owner that is pending
----------------------------
If the task succeeds to acquire the lock, then the task is set as the
owner of the lock, and if the lock still has waiters, the top_waiter
(highest priority task waiting on the lock) is added to this task's
pi_waiters tree.
The mutex has a owner, but it hasn't woken up and the mutex flag
"Pending Owner" is set. The first check is to see if the owner isn't the
current task. This is because this function is also used for the pending
owner to grab the mutex. When a pending owner wakes up, it checks to see
if it can take the mutex, and this is done if the owner is already set to
itself. If so, we succeed and leave the function, clearing the "Pending
Owner" bit.
If the pending owner is not current, we check to see if the current priority is
higher than the pending owner. If not, we fail the function and return.
There's also something special about a pending owner. That is a pending owner
is never blocked on a mutex. So there is no PI chain to worry about. It also
means that if the mutex doesn't have any waiters, there's no accounting needed
to update the pending owner's pi_list, since we only worry about processes
blocked on the current mutex.
If there are waiters on this mutex, and we just stole the ownership, we need
to take the top waiter, remove it from the pi_list of the pending owner, and
add it to the current pi_list. Note that at this moment, the pending owner
is no longer on the list of waiters. This is fine, since the pending owner
would add itself back when it realizes that it had the ownership stolen
from itself. When the pending owner tries to grab the mutex, it will fail
in try_to_take_rt_mutex if the owner field points to another process.
2) No owner
-----------
If there is no owner (or we successfully stole the lock), we set the owner
of the mutex to current, and set the flag of "Has Waiters" if the current
mutex actually has waiters, or we clear the flag if it doesn't. See, it was
OK that we set that flag early, since now it is cleared.
3) Failed to grab ownership
---------------------------
The most interesting case is when we fail to take ownership. This means that
there exists an owner, or there's a pending owner with equal or higher
priority than the current task.
We'll continue on the failed case.
If the mutex has a timeout, we set up a timer to go off to break us out
of this mutex if we failed to get it after a specified amount of time.
Now we enter a loop that will continue to try to take ownership of the mutex, or
fail from a timeout or signal.
Once again we try to take the mutex. This will usually fail the first time
in the loop, since it had just failed to get the mutex. But the second time
in the loop, this would likely succeed, since the task would likely be
the pending owner.
If the mutex is TASK_INTERRUPTIBLE a check for signals and timeout is done
here.
The waiter structure has a "task" field that points to the task that is blocked
on the mutex. This field can be NULL the first time it goes through the loop
or if the task is a pending owner and had its mutex stolen. If the "task"
field is NULL then we need to set up the accounting for it.
If the lock is not taken by try_to_take_rt_mutex(), then the
task_blocks_on_rt_mutex() function is called. This will add the task to
the lock's waiter tree and propagate the pi chain of the lock as well
as the lock's owner's pi_waiters tree. This is described in the next
section.
Task blocks on mutex
--------------------
The accounting of a mutex and process is done with the waiter structure of
the process. The "task" field is set to the process, and the "lock" field
to the mutex. The plist nodes are initialized to the processes current
priority.
to the mutex. The rbtree node of waiter are initialized to the processes
current priority.
Since the wait_lock was taken at the entry of the slow lock, we can safely
add the waiter to the wait_list. If the current process is the highest
priority process currently waiting on this mutex, then we remove the
previous top waiter process (if it exists) from the pi_list of the owner,
and add the current process to that list. Since the pi_list of the owner
add the waiter to the task waiter tree. If the current process is the
highest priority process currently waiting on this mutex, then we remove the
previous top waiter process (if it exists) from the pi_waiters of the owner,
and add the current process to that tree. Since the pi_waiter of the owner
has changed, we call rt_mutex_adjust_prio on the owner to see if the owner
should adjust its priority accordingly.
If the owner is also blocked on a lock, and had its pi_list changed
If the owner is also blocked on a lock, and had its pi_waiters changed
(or deadlock checking is on), we unlock the wait_lock of the mutex and go ahead
and run rt_mutex_adjust_prio_chain on the owner, as described earlier.
@ -686,30 +480,23 @@ mutex (waiter "task" field is not NULL), then we go to sleep (call schedule).
Waking up in the loop
---------------------
The schedule can then wake up for a few reasons.
1) we were given pending ownership of the mutex.
2) we received a signal and was TASK_INTERRUPTIBLE
3) we had a timeout and was TASK_INTERRUPTIBLE
The task can then wake up for a couple of reasons:
1) The previous lock owner released the lock, and the task now is top_waiter
2) we received a signal or timeout
In any of these cases, we continue the loop and once again try to grab the
ownership of the mutex. If we succeed, we exit the loop, otherwise we continue
and on signal and timeout, will exit the loop, or if we had the mutex stolen
we just simply add ourselves back on the lists and go back to sleep.
In both cases, the task will try again to acquire the lock. If it
does, then it will take itself off the waiters tree and set itself back
to the TASK_RUNNING state.
Note: For various reasons, because of timeout and signals, the steal mutex
algorithm needs to be careful. This is because the current process is
still on the wait_list. And because of dynamic changing of priorities,
especially on SCHED_OTHER tasks, the current process can be the
highest priority task on the wait_list.
In first case, if the lock was acquired by another task before this task
could get the lock, then it will go back to sleep and wait to be woken again.
Failed to get mutex on Timeout or Signal
----------------------------------------
If a timeout or signal occurred, the waiter's "task" field would not be
NULL and the task needs to be taken off the wait_list of the mutex and perhaps
pi_list of the owner. If this process was a high priority process, then
the rt_mutex_adjust_prio_chain needs to be executed again on the owner,
but this time it will be lowering the priorities.
The second case is only applicable for tasks that are grabbing a mutex
that can wake up before getting the lock, either due to a signal or
a timeout (i.e. rt_mutex_timed_futex_lock()). When woken, it will try to
take the lock again, if it succeeds, then the task will return with the
lock held, otherwise it will return with -EINTR if the task was woken
by a signal, or -ETIMEDOUT if it timed out.
Unlocking the Mutex
@ -739,25 +526,12 @@ owner still needs to make this check. If there are no waiters then the mutex
owner field is set to NULL, the wait_lock is released and nothing more is
needed.
If there are waiters, then we need to wake one up and give that waiter
pending ownership.
If there are waiters, then we need to wake one up.
On the wake up code, the pi_lock of the current owner is taken. The top
waiter of the lock is found and removed from the wait_list of the mutex
as well as the pi_list of the current owner. The task field of the new
pending owner's waiter structure is set to NULL, and the owner field of the
mutex is set to the new owner with the "Pending Owner" bit set, as well
as the "Has Waiters" bit if there still are other processes blocked on the
mutex.
The pi_lock of the previous owner is released, and the new pending owner's
pi_lock is taken. Remember that this is the trick to prevent the race
condition in rt_mutex_adjust_prio_chain from adding itself as a waiter
on the mutex.
We now clear the "pi_blocked_on" field of the new pending owner, and if
the mutex still has waiters pending, we add the new top waiter to the pi_list
of the pending owner.
waiter of the lock is found and removed from the waiters tree of the mutex
as well as the pi_waiters tree of the current owner. The "Has Waiters" bit is
marked to prevent lower priority tasks from stealing the lock.
Finally we unlock the pi_lock of the pending owner and wake it up.
@ -772,10 +546,14 @@ Credits
-------
Author: Steven Rostedt <rostedt@goodmis.org>
Updated: Alex Shi <alex.shi@linaro.org> - 7/6/2017
Reviewers: Ingo Molnar, Thomas Gleixner, Thomas Duetsch, and Randy Dunlap
Original Reviewers: Ingo Molnar, Thomas Gleixner, Thomas Duetsch, and
Randy Dunlap
Update (7/6/2017) Reviewers: Steven Rostedt and Sebastian Siewior
Updates
-------
This document was originally written for 2.6.17-rc3-mm1
was updated on 4.12