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sched/core: Introduce SM_IDLE and an idle re-entry fast-path in __schedule()

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Since commit b2a02fc43a ("smp: Optimize
send_call_function_single_ipi()") an idle CPU in TIF_POLLING_NRFLAG mode
can be pulled out of idle by setting TIF_NEED_RESCHED flag to service an
IPI without actually sending an interrupt. Even in cases where the IPI
handler does not queue a task on the idle CPU, do_idle() will call
__schedule() since need_resched() returns true in these cases.

Introduce and use SM_IDLE to identify call to __schedule() from
schedule_idle() and shorten the idle re-entry time by skipping
pick_next_task() when nr_running is 0 and the previous task is the idle
task.

With the SM_IDLE fast-path, the time taken to complete a fixed set of
IPIs using ipistorm improves noticeably. Following are the numbers
from a dual socket Intel Ice Lake Xeon server (2 x 32C/64T) and
3rd Generation AMD EPYC system (2 x 64C/128T) (boost on, C2 disabled)
running ipistorm between CPU8 and CPU16:

cmdline: insmod ipistorm.ko numipi=100000 single=1 offset=8 cpulist=8 wait=1

   ==================================================================
   Test          : ipistorm (modified)
   Units         : Normalized runtime
   Interpretation: Lower is better
   Statistic     : AMean
   ======================= Intel Ice Lake Xeon ======================
   kernel:				time [pct imp]
   tip:sched/core			1.00 [baseline]
   tip:sched/core + SM_IDLE		0.80 [20.51%]
   ==================== 3rd Generation AMD EPYC =====================
   kernel:				time [pct imp]
   tip:sched/core			1.00 [baseline]
   tip:sched/core + SM_IDLE		0.90 [10.17%]
   ==================================================================

[ kprateek: Commit message, SM_RTLOCK_WAIT fix ]

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Not-yet-signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20240809092240.6921-1-kprateek.nayak@amd.com
This commit is contained in:
Peter Zijlstra 2024-08-09 09:22:40 +00:00
parent b2d70222db
commit 3dcac251b0
1 changed files with 26 additions and 19 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

45
kernel/sched/core.c
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@ -6410,19 +6410,12 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
* Constants for the sched_mode argument of __schedule().
*
* The mode argument allows RT enabled kernels to differentiate a
* preemption from blocking on an 'sleeping' spin/rwlock. Note that
* SM_MASK_PREEMPT for !RT has all bits set, which allows the compiler to
* optimize the AND operation out and just check for zero.
* preemption from blocking on an 'sleeping' spin/rwlock.
*/
#define SM_NONE 0x0
#define SM_PREEMPT 0x1
#define SM_RTLOCK_WAIT 0x2
#ifndef CONFIG_PREEMPT_RT
# define SM_MASK_PREEMPT (~0U)
#else
# define SM_MASK_PREEMPT SM_PREEMPT
#endif
#define SM_IDLE (-1)
#define SM_NONE 0
#define SM_PREEMPT 1
#define SM_RTLOCK_WAIT 2
/*
* __schedule() is the main scheduler function.
@ -6463,9 +6456,14 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
*
* WARNING: must be called with preemption disabled!
*/
static void __sched notrace __schedule(unsigned int sched_mode)
static void __sched notrace __schedule(int sched_mode)
{
struct task_struct *prev, *next;
/*
* On PREEMPT_RT kernel, SM_RTLOCK_WAIT is noted
* as a preemption by schedule_debug() and RCU.
*/
bool preempt = sched_mode > SM_NONE;
unsigned long *switch_count;
unsigned long prev_state;
struct rq_flags rf;
@ -6476,13 +6474,13 @@ static void __sched notrace __schedule(unsigned int sched_mode)
rq = cpu_rq(cpu);
prev = rq->curr;
schedule_debug(prev, !!sched_mode);
schedule_debug(prev, preempt);
if (sched_feat(HRTICK) || sched_feat(HRTICK_DL))
hrtick_clear(rq);
local_irq_disable();
rcu_note_context_switch(!!sched_mode);
rcu_note_context_switch(preempt);
/*
* Make sure that signal_pending_state()->signal_pending() below
@ -6511,12 +6509,20 @@ static void __sched notrace __schedule(unsigned int sched_mode)
switch_count = &prev->nivcsw;
/* Task state changes only considers SM_PREEMPT as preemption */
preempt = sched_mode == SM_PREEMPT;
/*
* We must load prev->state once (task_struct::state is volatile), such
* that we form a control dependency vs deactivate_task() below.
*/
prev_state = READ_ONCE(prev->__state);
if (!(sched_mode & SM_MASK_PREEMPT) && prev_state) {
if (sched_mode == SM_IDLE) {
if (!rq->nr_running) {
next = prev;
goto picked;
}
} else if (!preempt && prev_state) {
if (signal_pending_state(prev_state, prev)) {
WRITE_ONCE(prev->__state, TASK_RUNNING);
} else {
@ -6547,6 +6553,7 @@ static void __sched notrace __schedule(unsigned int sched_mode)
}
next = pick_next_task(rq, prev, &rf);
picked:
clear_tsk_need_resched(prev);
clear_preempt_need_resched();
#ifdef CONFIG_SCHED_DEBUG
@ -6588,7 +6595,7 @@ static void __sched notrace __schedule(unsigned int sched_mode)
psi_account_irqtime(rq, prev, next);
psi_sched_switch(prev, next, !task_on_rq_queued(prev));
trace_sched_switch(sched_mode & SM_MASK_PREEMPT, prev, next, prev_state);
trace_sched_switch(preempt, prev, next, prev_state);
/* Also unlocks the rq: */
rq = context_switch(rq, prev, next, &rf);
@ -6664,7 +6671,7 @@ static void sched_update_worker(struct task_struct *tsk)
}
}
static __always_inline void __schedule_loop(unsigned int sched_mode)
static __always_inline void __schedule_loop(int sched_mode)
{
do {
preempt_disable();
@ -6709,7 +6716,7 @@ void __sched schedule_idle(void)
*/
WARN_ON_ONCE(current->__state);
do {
__schedule(SM_NONE);
__schedule(SM_IDLE);
} while (need_resched());
}