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SubjectRe: [PATCH] schedule: use unlikely()

* Mikulas Patocka <mpatocka@redhat.com> wrote:

>
>
> 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().

That needs to be in the changelog and the patch needs to be Cc:-ed to the
affected maintainer as well.

> > > 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.

That too should be in the changelog.

Thanks,

Ingo

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