Re: [PATCH 3/6] sched: add a few helpers to wake up tasks on the current cpu

[Andrei Vagin <avagin@google.com>]

On Wed, Apr 26, 2023 at 7:43 AM Bernd Schubert <bschubert@ddn.com> wrote:
>
> > Add complete_on_current_cpu, wake_up_poll_on_current_cpu helpers to wake
> > up tasks on the current CPU.
>
> > These two helpers are useful when the task needs to make a synchronous context
> > switch to another task. In this context, synchronous means it wakes up the
> > target task and falls asleep right after that.
>
> > One example of such workloads is seccomp user notifies. This mechanism allows
> > the  supervisor process handles system calls on behalf of a target process.
> > While the supervisor is handling an intercepted system call, the target process
> > will be blocked in the kernel, waiting for a response to come back.
>
> > On-CPU context switches are much faster than regular ones.
>
> > Signed-off-by: Andrei Vagin <avagin@google.com>
>
> Avoiding cpu switches is very desirable for fuse, I'm working on fuse over uring
> with per core queues. In my current branch and running a single threaded bonnie++
> I get about 9000 creates/s when I bind the process to a core, about 7000 creates/s
> when I set SCHED_IDLE for the ring threads and back to 9000 with SCHED_IDLE and
> disabling cpu migration in fs/fuse/dev.c request_wait_answer() before going into
> the waitq and enabling it back after waking up.
>
> I had reported this a few weeks back
> https://lore.kernel.org/lkml/d0ed1dbd-1b7e-bf98-65c0-7f61dd1a3228@ddn.com/
> and had been pointed to your and Prateeks patch series. I'm now going
> through these series. Interesting part is that a few weeks I didn't need
> SCHED_IDLE, just disabling/enabling migration before/after waking up was
> enough.
>
> [...]
>
> > EXPORT_SYMBOL(swake_up_one);
> > diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
> > index 133b74730738..47803a0b8d5d 100644
> > --- a/kernel/sched/wait.c
> > +++ b/kernel/sched/wait.c
> > @@ -161,6 +161,11 @@ int __wake_up(struct wait_queue_head *wq_head, unsigned int mode,
> >  }
> >  EXPORT_SYMBOL(__wake_up);
>
> > +void __wake_up_on_current_cpu(struct wait_queue_head *wq_head, unsigned int mode, void *key)
> > +{
> > +     __wake_up_common_lock(wq_head, mode, 1, WF_CURRENT_CPU, key);
> > +}
>
> I'm about to test this instead of migrate_disable/migrate_enable, but the symbol needs
> to be exported - any objection to do that right from the beginning in your patch?

I think EXPORT_SYMBOL_GPL should not trigger any objections and it
covers your case, doesn't it?

Thanks,
Andrei