linux-kernel - Re: sched: wakeup setting TIF_NEED

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Message-ID: <YImurOENWBEg8UP+@cork>
Date:   Wed, 28 Apr 2021 11:51:24 -0700
From:   Jörn Engel <joern@...estorage.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>
Subject: Re: sched: wakeup setting TIF_NEED_RESCHED too frequently

On Wed, Apr 28, 2021 at 10:30:05AM +0200, Peter Zijlstra wrote:
> 
> 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.

Yeah, saw that myself.  My explanation of the behaviour is wrong.  The
observed behaviour remains, though.

Guess I have to add more instrumentation if I cannot find the cause by
reading the code.

Thank you!

Jörn

--
Optimizations always bust things, because all optimizations are, in
the long haul, a form of cheating, and cheaters eventually get caught.
-- Larry Wall