Perhaps I am totally confused, but. On 04/04, Dmitry Vyukov wrote: > > On Wed, 3 Apr 2024 at 17:43, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: > > > > > Why distribution_thread() can't simply exit if got_signal != 0 ? > > > > > > See https://lore.kernel.org/all/20230128195641.GA14906@xxxxxxxxxx/ > > > > 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.
