On Thu, 4 Apr 2024 at 15:45, Oleg Nesterov oleg@redhat.com wrote:
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.
I agree that any thread can pick the signal, but this A_LOT_OF_TIME makes it impossible for the test to reliably repeatedly pass w/o the change in any reasonable testing system. With the change the test was finishing/passing for me immediately all the time.
Again, if the test causes practical problems (flaky), then I don't mind relaxing it (flaky tests suck). I was just against giving up on testing proactively just in case.
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.