On Wed, Jan 19, 2022 at 09:26:41AM -0800, Peter Oskolkov wrote: > On Mon, Dec 6, 2021 at 3:47 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > > > 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() > > sys_umcg_wait() should not enqueue the worker as runnable; workers are > enqueued to indicate wakeup events. Oooh... I see. > So worker timeouts in sys_umcg_wait are treated as wakeup events, with > the difference that when the worker is eventually scheduled by a > server, sys_umcg_wait returns with ETIMEDOUT. Right.. OK, let me go fold and polish what I have now before I go change things again though.
