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task_work: remove fifo ordering guarantee 

In commit f341861fb0 ("task_work: add a scheduling point in
task_work_run()") I fixed a latency problem adding a cond_resched()
call.

Later, commit ac3d0da8f3 added yet another loop to reverse a list,
bringing back the latency spike :

I've seen in some cases this loop taking 275 ms, if for example a
process with 2,000,000 files is killed.

We could add yet another cond_resched() in the reverse loop, or we
can simply remove the reversal, as I do not think anything
would depend on order of task_work_add() submitted works.

Fixes: ac3d0da8f3 ("task_work: Make task_work_add() lockless")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Maciej Żenczykowski <maze@google.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Eric Dumazet 2015-08-28 19:42:30 -07:00 committed by Linus Torvalds
parent f377ea88b8
commit c821990610
1 changed files with 2 additions and 10 deletions
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12
kernel/task_work.c
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@ -18,6 +18,8 @@ static struct callback_head work_exited; /* all we need is ->next == NULL */
* This is like the signal handler which runs in kernel mode, but it doesn't
* try to wake up the @task.
*
* Note: there is no ordering guarantee on works queued here.
*
* RETURNS:
* 0 if succeeds or -ESRCH.
*/
@ -108,16 +110,6 @@ void task_work_run(void)
raw_spin_unlock_wait(&task->pi_lock);
smp_mb();
/* Reverse the list to run the works in fifo order */
head = NULL;
do {
next = work->next;
work->next = head;
head = work;
work = next;
} while (work);
work = head;
do {
next = work->next;
work->func(work);