From: Oleg Nesterov <oleg@redhat.com>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
John Stultz <jstultz@google.com>, Marco Elver <elver@google.com>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@kernel.org>,
"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: Thu, 4 Apr 2024 15:43:57 +0200 [thread overview]
Message-ID: <20240404134357.GA7153@redhat.com> (raw)
In-Reply-To: <CACT4Y+a-kdkAjmACJuDzrhmUPmv9uMpYOg6LLVviMQn=+9JRgA@mail.gmail.com>
Perhaps I am totally confused, but.
On 04/04, Dmitry Vyukov wrote:
>
> On Wed, 3 Apr 2024 at 17:43, Thomas Gleixner <tglx@linutronix.de> wrote:
> >
> > > Why distribution_thread() can't simply exit if got_signal != 0 ?
> > >
> > > See https://lore.kernel.org/all/20230128195641.GA14906@redhat.com/
> >
> > Indeed. It's too obvious :)
>
> This test models the intended use-case that was the motivation for the change:
> We want to sample execution of a running multi-threaded program, it
> has multiple active threads (that don't exit), since all threads are
> running and consuming CPU,
Yes,
> they all should get a signal eventually.
Well, yes and no.
No, in a sense that the motivation was not to ensure that all threads
get a signal, the motivation was to ensure that cpu_timer_fire() paths
will use the current task as the default target for signal_wake_up/etc.
This is just optimization.
But yes, all should get a signal eventually. And this will happen with
or without the commit bcb7ee79029dca ("posix-timers: Prefer delivery of
signals to the current thread"). Any thread can dequeue a shared signal,
say, on return from interrupt.
Just without that commit this "eventually" means A_LOT_OF_TIME statistically.
> If threads will exit once they get a signal,
just in case, the main thread should not exit ...
> then the test will pass
> even if signal delivery is biased towards a single running thread all
> the time (the previous kernel impl).
See above.
But yes, I agree, if thread exits once it get a signal, then A_LOT_OF_TIME
will be significantly decreased. But again, this is just statistical issue,
I do not see how can we test the commit bcb7ee79029dca reliably.
OTOH. If the threads do not exit after they get signal, then _in theory_
nothing can guarantee that this test-case will ever complete even with
that commit. It is possible that one of the threads will "never" have a
chance to run cpu_timer_fire().
In short, I leave this to you and Thomas. I have no idea how to write a
"good" test for that commit.
Well... perhaps the main thread should just sleep in pause(), and
distribution_handler() should check that gettid() != getpid() ?
Something like this maybe... We need to ensure that the main thread
enters pause before timer_settime().
Oleg.
next prev parent reply other threads:[~2024-04-04 13:45 UTC|newest]
Thread overview: 51+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-16 12:30 [PATCH v6 1/2] posix-timers: Prefer delivery of signals to the current thread Marco Elver
2023-03-16 12:30 ` [PATCH v6 2/2] selftests/timers/posix_timers: Test delivery of signals across threads Marco Elver
2024-04-06 20:53 ` Muhammad Usama Anjum
2024-04-06 21:13 ` Oleg Nesterov
2024-04-06 21:32 ` Muhammad Usama Anjum
2023-03-30 10:19 ` [PATCH v6 1/2] posix-timers: Prefer delivery of signals to the current thread Marco Elver
2023-04-06 14:12 ` Marco Elver
2023-04-06 15:13 ` Frederic Weisbecker
2023-04-06 20:22 ` Peter Zijlstra
2024-04-01 20:17 ` John Stultz
2024-04-02 9:07 ` Dmitry Vyukov
2024-04-02 14:57 ` Thomas Gleixner
2024-04-02 17:23 ` John Stultz
2024-04-03 12:41 ` Thomas Gleixner
2024-04-03 15:03 ` Oleg Nesterov
2024-04-03 15:43 ` Thomas Gleixner
2024-04-03 16:32 ` Thomas Gleixner
2024-04-03 18:16 ` John Stultz
2024-04-03 19:09 ` Thomas Gleixner
2024-04-03 19:35 ` John Stultz
2024-04-03 22:24 ` Thomas Gleixner
2024-04-04 14:54 ` Oleg Nesterov
2024-04-04 18:08 ` Thomas Gleixner
2024-04-06 15:09 ` [PATCH] selftests/timers/posix_timers: reimplement check_timer_distribution() Oleg Nesterov
2024-04-06 15:10 ` Oleg Nesterov
2024-04-06 22:00 ` Thomas Gleixner
2024-04-08 8:30 ` Dmitry Vyukov
2024-04-08 10:01 ` Thomas Gleixner
2024-04-08 10:26 ` Oleg Nesterov
2024-04-08 18:49 ` Oleg Nesterov
2024-04-08 22:17 ` Thomas Gleixner
2024-04-09 11:10 ` Oleg Nesterov
2024-04-09 11:45 ` Dmitry Vyukov
2024-04-09 12:02 ` Thomas Gleixner
2024-04-09 13:38 ` [PATCH v2] " Oleg Nesterov
2024-04-10 22:21 ` John Stultz
2024-04-10 22:31 ` Thomas Gleixner
2024-04-10 22:33 ` John Stultz
2024-04-11 12:41 ` [PATCH] " Mark Brown
2024-04-11 15:33 ` John Stultz
2024-04-11 12:44 ` Mark Brown
2024-04-11 14:17 ` Thomas Gleixner
2024-04-11 15:50 ` Oleg Nesterov
2024-04-11 16:03 ` Mark Brown
2024-04-12 12:35 ` [PATCH] selftests: fix build failure with NOLIBC Oleg Nesterov
2024-04-14 7:42 ` Mark Brown
2024-04-04 8:55 ` [PATCH v6 1/2] posix-timers: Prefer delivery of signals to the current thread Dmitry Vyukov
2024-04-04 13:43 ` Oleg Nesterov [this message]
2024-04-04 15:10 ` Thomas Gleixner
2024-04-04 15:23 ` Oleg Nesterov
2024-04-05 4:28 ` Dmitry Vyukov
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