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From: Dmitry Vyukov <dvyukov@google.com>
Date: Thu, 4 Apr 2024 10:55:13 +0200
Message-ID: <CACT4Y+a-kdkAjmACJuDzrhmUPmv9uMpYOg6LLVviMQn=+9JRgA@mail.gmail.com>
Subject: Re: [PATCH v6 1/2] posix-timers: Prefer delivery of signals to the
 current thread
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Oleg Nesterov <oleg@redhat.com>, 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>
Content-Type: text/plain; charset="UTF-8"

On Wed, 3 Apr 2024 at 17:43, Thomas Gleixner <tglx@linutronix.de> wrote:
>
> On Wed, Apr 03 2024 at 17:03, Oleg Nesterov wrote:
> > On 04/03, Thomas Gleixner wrote:
> >> The test if fragile as hell as there is absolutely no guarantee that the
> >> signal target distribution is as expected. The expectation is based on a
> >> statistical assumption which does not really hold.
> >
> > Agreed. I too never liked this test-case.
> >
> > I forgot everything about this patch and test-case, I can't really read
> > your patch right now (sorry), so I am sure I missed something, but
> >
> >>  static void *distribution_thread(void *arg)
> >>  {
> >> -    while (__atomic_load_n(&remain, __ATOMIC_RELAXED));
> >> -    return NULL;
> >> +    while (__atomic_load_n(&remain, __ATOMIC_RELAXED) && !done) {
> >> +            if (got_signal)
> >> +                    usleep(10);
> >> +    }
> >> +
> >> +    return (void *)got_signal;
> >>  }
> >
> > 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, they all should get a signal eventually.

If threads will exit once they get a signal, then the test will pass
even if signal delivery is biased towards a single running thread all
the time (the previous kernel impl).