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From: John Stultz <jstultz@google.com>
Date: Mon, 1 Apr 2024 13:17:09 -0700
Message-ID: <CANDhNCqBGnAr_MSBhQxWo+-8YnPPggxoVL32zVrDB+NcoKXVPQ@mail.gmail.com>
Subject: Re: [PATCH v6 1/2] posix-timers: Prefer delivery of signals to the
 current thread
To: Marco Elver <elver@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>, Thomas Gleixner <tglx@linutronix.de>, 
	Ingo Molnar <mingo@kernel.org>, Oleg Nesterov <oleg@redhat.com>, 
	"Eric W. Biederman" <ebiederm@xmission.com>, linux-kernel@vger.kernel.org, 
	linux-kselftest@vger.kernel.org, Dmitry Vyukov <dvyukov@google.com>, 
	kasan-dev@googlegroups.com, Edward Liaw <edliaw@google.com>, 
	Carlos Llamas <cmllamas@google.com>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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On Thu, Mar 16, 2023 at 5:30=E2=80=AFAM Marco Elver <elver@google.com> wrot=
e:
>
> From: Dmitry Vyukov <dvyukov@google.com>
>
> POSIX timers using the CLOCK_PROCESS_CPUTIME_ID clock prefer the main
> thread of a thread group for signal delivery.     However, this has a
> significant downside: it requires waking up a potentially idle thread.
>
> Instead, prefer to deliver signals to the current thread (in the same
> thread group) if SIGEV_THREAD_ID is not set by the user. This does not
> change guaranteed semantics, since POSIX process CPU time timers have
> never guaranteed that signal delivery is to a specific thread (without
> SIGEV_THREAD_ID set).
>
> The effect is that we no longer wake up potentially idle threads, and
> the kernel is no longer biased towards delivering the timer signal to
> any particular thread (which better distributes the timer signals esp.
> when multiple timers fire concurrently).
>
> Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
> Suggested-by: Oleg Nesterov <oleg@redhat.com>
> Reviewed-by: Oleg Nesterov <oleg@redhat.com>
> Signed-off-by: Marco Elver <elver@google.com>

Apologies for drudging up this old thread.

I wanted to ask if anyone had objections to including this in the -stable t=
rees?

After this and the follow-on patch e797203fb3ba
("selftests/timers/posix_timers: Test delivery of signals across
threads") landed, folks testing older kernels with the latest
selftests started to see the new test checking for this behavior to
stall.  Thomas did submit an adjustment to the test here to avoid the
stall: https://lore.kernel.org/lkml/20230606142031.071059989@linutronix.de/=
,
but it didn't seem to land, however that would just result in the test
failing instead of hanging.

This change does seem to cherry-pick cleanly back to at least
stable/linux-5.10.y cleanly, so it looks simple to pull this change
back. But I wanted to make sure there wasn't anything subtle I was
missing before sending patches.

thanks
-john