Re: [PATCH v6 1/2] posix-timers: Prefer delivery of signals to the current thread

From: Dmitry Vyukov <dvyukov@google.com>
To: John Stultz <jstultz@google.com>
Cc: Marco Elver <elver@google.com>,
	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,
	kasan-dev@googlegroups.com,  Edward Liaw <edliaw@google.com>,
	Carlos Llamas <cmllamas@google.com>,
	 Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [PATCH v6 1/2] posix-timers: Prefer delivery of signals to the current thread
Date: Tue, 2 Apr 2024 11:07:32 +0200	[thread overview]
Message-ID: <CACT4Y+a6E8wg3PZhG_AoZtZwozhqUC+LPgMV3G_gQZXkr1rGzw@mail.gmail.com> (raw)
In-Reply-To: <CANDhNCqBGnAr_MSBhQxWo+-8YnPPggxoVL32zVrDB+NcoKXVPQ@mail.gmail.com>

On Mon, 1 Apr 2024 at 22:17, John Stultz <jstultz@google.com> wrote:
>
> On Thu, Mar 16, 2023 at 5:30 AM Marco Elver <elver@google.com> wrote:
> >
> > 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 trees?
>
> 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.

I don't have objections per se. But I wonder how other tests deal with
such situations. It should happen for any test for new functionality.
Can we do the same other tests are doing?