linux-kernel - Re: [PATCH v0.9.1 3/6] sched/umcg: implement UMCG syscalls

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Date:   Thu, 20 Jan 2022 12:07:44 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Peter Oskolkov <posk@...gle.com>
Cc:     Peter Oskolkov <posk@...k.io>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-api@...r.kernel.org, Paul Turner <pjt@...gle.com>,
        Ben Segall <bsegall@...gle.com>,
        Andrei Vagin <avagin@...gle.com>, Jann Horn <jannh@...gle.com>,
        Thierry Delisle <tdelisle@...terloo.ca>
Subject: Re: [PATCH v0.9.1 3/6] sched/umcg: implement UMCG syscalls

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@...radead.org> 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@...radead.org> 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.