Perhaps I am totally confused, but.
On 04/04, Dmitry Vyukov wrote:
>
> On Wed, 3 Apr 2024 at 17:43, Thomas Gleixner <t...@linutronix.de> wrote:
> >
> > > Why distribution_thread() can't simply exit if got_signal != 0 ?
> > >
> > > See https://lore.kernel.org/all/20230128195641.ga14...@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.
