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SUNRPC in case of backlog, hand free slots directly to waiting task

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If sunrpc.tcp_max_slot_table_entries is small and there are tasks
on the backlog queue, then when a request completes it is freed and the
first task on the queue is woken.  The expectation is that it will wake
and claim that request.  However if it was a sync task and the waiting
process was killed at just that moment, it will wake and NOT claim the
request.

As long as TASK_CONGESTED remains set, requests can only be claimed by
tasks woken from the backlog, and they are woken only as requests are
freed, so when a task doesn't claim a request, no other task can ever
get that request until TASK_CONGESTED is cleared.  Each time this
happens the number of available requests is decreased by one.

With a sufficiently high workload and sufficiently low setting of
max_slot (16 in the case where this was seen), TASK_CONGESTED can remain
set for an extended period, and the above scenario (of a process being
killed just as its task was woken) can repeat until no requests can be
allocated.  Then traffic stops.

This patch addresses the problem by introducing a positive handover of a
request from a completing task to a backlog task - the request is never
freed when there is a backlog.

When a task is woken it might not already have a request attached in
which case it is *not* freed (as with current code) but is initialised
(if needed) and used.  If it isn't used it will eventually be freed by
rpc_exit_task().  xprt_release() is enhanced to be able to correctly
release an uninitialised request.

Fixes: ba60eb25ff ("SUNRPC: Fix a livelock problem in the xprt->backlog queue")
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
This commit is contained in:
NeilBrown 2021-05-17 09:59:10 +10:00 committed by Trond Myklebust
parent d1d973950a
commit e877a88d1f
2 changed files with 47 additions and 28 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
net/sunrpc/clnt.c
View File

@ -1677,13 +1677,6 @@ call_reserveresult(struct rpc_task *task)
return;
}
/*
* Even though there was an error, we may have acquired
* a request slot somehow. Make sure not to leak it.
*/
if (task->tk_rqstp)
xprt_release(task);
switch (status) {
case -ENOMEM:
rpc_delay(task, HZ >> 2);

68
net/sunrpc/xprt.c
View File

@ -70,6 +70,7 @@
static void xprt_init(struct rpc_xprt *xprt, struct net *net);
static __be32 xprt_alloc_xid(struct rpc_xprt *xprt);
static void xprt_destroy(struct rpc_xprt *xprt);
static void xprt_request_init(struct rpc_task *task);
static DEFINE_SPINLOCK(xprt_list_lock);
static LIST_HEAD(xprt_list);
@ -1612,10 +1613,26 @@ static void xprt_add_backlog(struct rpc_xprt *xprt, struct rpc_task *task)
rpc_sleep_on(&xprt->backlog, task, NULL);
}
static void xprt_wake_up_backlog(struct rpc_xprt *xprt)
static bool __xprt_set_rq(struct rpc_task *task, void *data)
{
if (rpc_wake_up_next(&xprt->backlog) == NULL)
struct rpc_rqst *req = data;
if (task->tk_rqstp == NULL) {
memset(req, 0, sizeof(*req)); /* mark unused */
task->tk_status = -EAGAIN;
task->tk_rqstp = req;
return true;
}
return false;
}
static bool xprt_wake_up_backlog(struct rpc_xprt *xprt, struct rpc_rqst *req)
{
if (rpc_wake_up_first(&xprt->backlog, __xprt_set_rq, req) == NULL) {
clear_bit(XPRT_CONGESTED, &xprt->state);
return false;
}
return true;
}
static bool xprt_throttle_congested(struct rpc_xprt *xprt, struct rpc_task *task)
@ -1703,11 +1720,11 @@ EXPORT_SYMBOL_GPL(xprt_alloc_slot);
void xprt_free_slot(struct rpc_xprt *xprt, struct rpc_rqst *req)
{
spin_lock(&xprt->reserve_lock);
if (!xprt_dynamic_free_slot(xprt, req)) {
if (!xprt_wake_up_backlog(xprt, req) &&
!xprt_dynamic_free_slot(xprt, req)) {
memset(req, 0, sizeof(*req)); /* mark unused */
list_add(&req->rq_list, &xprt->free);
}
xprt_wake_up_backlog(xprt);
spin_unlock(&xprt->reserve_lock);
}
EXPORT_SYMBOL_GPL(xprt_free_slot);
@ -1795,6 +1812,10 @@ xprt_request_init(struct rpc_task *task)
struct rpc_xprt *xprt = task->tk_xprt;
struct rpc_rqst *req = task->tk_rqstp;
if (req->rq_task)
/* Already initialized */
return;
req->rq_task = task;
req->rq_xprt = xprt;
req->rq_buffer = NULL;
@ -1855,8 +1876,10 @@ void xprt_retry_reserve(struct rpc_task *task)
struct rpc_xprt *xprt = task->tk_xprt;
task->tk_status = 0;
if (task->tk_rqstp != NULL)
if (task->tk_rqstp != NULL) {
xprt_request_init(task);
return;
}
task->tk_status = -EAGAIN;
xprt_do_reserve(xprt, task);
@ -1881,23 +1904,26 @@ void xprt_release(struct rpc_task *task)
}
xprt = req->rq_xprt;
xprt_request_dequeue_xprt(task);
spin_lock(&xprt->transport_lock);
xprt->ops->release_xprt(xprt, task);
if (xprt->ops->release_request)
xprt->ops->release_request(task);
xprt_schedule_autodisconnect(xprt);
spin_unlock(&xprt->transport_lock);
if (req->rq_buffer)
xprt->ops->buf_free(task);
xdr_free_bvec(&req->rq_rcv_buf);
xdr_free_bvec(&req->rq_snd_buf);
if (req->rq_cred != NULL)
put_rpccred(req->rq_cred);
task->tk_rqstp = NULL;
if (req->rq_release_snd_buf)
req->rq_release_snd_buf(req);
if (xprt) {
xprt_request_dequeue_xprt(task);
spin_lock(&xprt->transport_lock);
xprt->ops->release_xprt(xprt, task);
if (xprt->ops->release_request)
xprt->ops->release_request(task);
xprt_schedule_autodisconnect(xprt);
spin_unlock(&xprt->transport_lock);
if (req->rq_buffer)
xprt->ops->buf_free(task);
xdr_free_bvec(&req->rq_rcv_buf);
xdr_free_bvec(&req->rq_snd_buf);
if (req->rq_cred != NULL)
put_rpccred(req->rq_cred);
if (req->rq_release_snd_buf)
req->rq_release_snd_buf(req);
} else
xprt = task->tk_xprt;
task->tk_rqstp = NULL;
if (likely(!bc_prealloc(req)))
xprt->ops->free_slot(xprt, req);
else