On Mon, Nov 29, 2021 at 09:34:49AM -0800, Peter Oskolkov wrote: > On Mon, Nov 29, 2021 at 8:41 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > Also, timeout on sys_umcg_wait() gets you the exact same situation (or > > worse, multiple running workers). > > It should not. Timed out workers should be added to the runnable list > and not become running unless a server chooses so. So sys_umcg_wait() > with a timeout should behave similarly to a normal sleep, in that the > server is woken upon the worker blocking, and upon the worker wakeup > the worker is added to the woken workers list and waits for a server > to run it. The only difference is that in a sleep the worker becomes > BLOCKED, while in sys_umcg_wait() the worker is RUNNABLE the whole > time. > > Why then have sys_umcg_wait() with a timeout at all, instead of > calling nanosleep()? Because the worker in sys_umcg_wait() can be > context-switched into by another worker, or made running by a server; > if the worker is in nanosleep(), it just sleeps. I've been trying to figure out the semantics of that timeout thing, and I can't seem to make sense of it. Consider two workers: S0 running A S1 running B therefore: S0::state == RUNNABLE S1::state == RUNNABLE A::server_tid == S0.tid B::server_tid = S1.tid A::state == RUNNING B::state == RUNNING Doing: self->state = RUNNABLE; self->state = RUNNABLE; sys_umcg_wait(0); sys_umcg_wait(10); umcg_enqueue_runnable() umcg_enqueue_runnable() umcg_wake() umcg_wake() umcg_wait() umcg_wait() hrtimer_start() In both cases we get the exact same outcome: A::state == RUNNABLE B::state == RUNNABLE S0::state == RUNNING S1::state == RUNNING S0::runnable_ptr == &A S1::runnable_ptr = &B Which is, AFAICT, the exact state you wanted to achieve, except B now has an active timer, but what do you want it to do when that goes? I'm tempted to say workers cannot have timeout, and servers can use it to wake themselves.
