From: Peter Zijlstra <peterz@infradead.org>
To: "Jörn Engel" <joern@purestorage.com>
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>
Subject: Re: sched: wakeup setting TIF_NEED_RESCHED too frequently
Date: Wed, 28 Apr 2021 10:30:05 +0200 [thread overview]
Message-ID: <YIkdDeb9c+XmDRvM@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <YIh/QubidJcE5IIv@cork>
On Tue, Apr 27, 2021 at 02:16:50PM -0700, Jörn Engel wrote:
> Anyway, trying to find a cause, I noticed the following call chain:
> set_nr_if_polling()
> ttwu_queue_remote()
> ttwu_queue()
> try_to_wake_up()
> default_wake_function()
> curr->func()
> __wake_up_common()
> __wake_up_common_lock()
> __wake_up()
> wake_up()
>
> Call chain above is manually created from source code. Closest sample I
> caught with instrumentation is missing the leaf calls after
> try_to_wake_up():
> _raw_spin_unlock_irqrestore+0x1f/0x40
> try_to_wake_up+0x425/0x5e0
> wake_up_q+0x3f/0x80
> futex_wake+0x159/0x180
> do_futex+0xcd/0xba0
>
> Afaics, the result is us setting TIF_NEED_RESCHED on any wakeup, unless
> wake_list is already populated. Is that actually intentional? And is
> that useful for performance of latency? I think it isn't, but I am
> probably missing something here.
set_nr_if_polling() only sets TIF_NEED_RESCHED IFF TIF_POLLING_NRFLAG is
set. TIF_POLLING_NRFLAG is only set when idle and is then sufficient to
wake that cpu and reschedule.
If not TIF_POLLING_NRFLAG, then we send an IPI to deal with the
non-empty wake_list.
next prev parent reply other threads:[~2021-04-28 8:30 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-27 21:16 sched: wakeup setting TIF_NEED_RESCHED too frequently Jörn Engel
2021-04-28 8:30 ` Peter Zijlstra [this message]
2021-04-28 18:51 ` Jörn Engel
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