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SubjectRe: [PATCH] schedule: use unlikely()


On Fri, 24 Nov 2017, Ingo Molnar wrote:

> * Mikulas Patocka <mpatocka@redhat.com> wrote:
>
> > A small patch for schedule(), so that the code goes straght in the common
> > case.
> >
> > Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> >
> > ---
> > include/linux/blkdev.h | 2 +-
> > kernel/sched/core.c | 2 +-
> > 2 files changed, 2 insertions(+), 2 deletions(-)
> >
> > Index: linux-2.6/include/linux/blkdev.h
> > ===================================================================
> > --- linux-2.6.orig/include/linux/blkdev.h
> > +++ linux-2.6/include/linux/blkdev.h
> > @@ -1308,7 +1308,7 @@ static inline bool blk_needs_flush_plug(
> > {
> > struct blk_plug *plug = tsk->plug;
> >
> > - return plug &&
> > + return unlikely(plug != NULL) &&
> > (!list_empty(&plug->list) ||
> > !list_empty(&plug->mq_list) ||
> > !list_empty(&plug->cb_list));
>
> That's an unrelated change.

It is related, because this function gets inlined into schedule().

> > Index: linux-2.6/kernel/sched/core.c
> > ===================================================================
> > --- linux-2.6.orig/kernel/sched/core.c
> > +++ linux-2.6/kernel/sched/core.c
> > @@ -3405,7 +3405,7 @@ void __noreturn do_task_dead(void)
> >
> > static inline void sched_submit_work(struct task_struct *tsk)
> > {
> > - if (!tsk->state || tsk_is_pi_blocked(tsk))
> > + if (!tsk->state || unlikely(tsk_is_pi_blocked(tsk)))
> > return;
> > /*
> > * If we are going to sleep and we have plugged IO queued,
>
> Do these changes actually change the generated assembly code?
>
> Thanks,
>
> Ingo

Yes. It saves two jumps because the compiler falsely assumes that
tsk_is_pi_blocked would return true. Likewise, the compiler falsely
assumes that tsk->plug would be true.

The static branch prediction in gcc assumes that pointers are usually not
NULL, but in this case tsk->pi_blocked_on and tsk->plug are usually NULL.

Mikulas

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