From: Peter Zijlstra <peterz@infradead.org>
To: Peter Oskolkov <posk@google.com>
Cc: Peter Oskolkov <posk@posk.io>, Ingo Molnar <mingo@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
Andrew Morton <akpm@linux-foundation.org>,
Dave Hansen <dave.hansen@linux.intel.com>,
Andy Lutomirski <luto@kernel.org>,
Linux Memory Management List <linux-mm@kvack.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
linux-api@vger.kernel.org, Paul Turner <pjt@google.com>,
Ben Segall <bsegall@google.com>, Andrei Vagin <avagin@google.com>,
Jann Horn <jannh@google.com>,
Thierry Delisle <tdelisle@uwaterloo.ca>
Subject: Re: [PATCH v0.9.1 3/6] sched/umcg: implement UMCG syscalls
Date: Thu, 20 Jan 2022 12:07:44 +0100 [thread overview]
Message-ID: <YelCgKeprVDNoal7@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CAPNVh5eYinGEK2Ece45fLYzU8hMWiqAzVdVbdFxd-P5fPXuFSA@mail.gmail.com>
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@infradead.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@infradead.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.
next prev parent reply other threads:[~2022-01-20 11:08 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-22 21:13 [PATCH v0.9.1 0/6] sched,mm,x86/uaccess: implement User Managed Concurrency Groups Peter Oskolkov
2021-11-22 21:13 ` [PATCH v0.9.1 1/6] sched/umcg: add WF_CURRENT_CPU and externise ttwu Peter Oskolkov
2021-11-22 21:13 ` [PATCH v0.9.1 2/6] mm, x86/uaccess: add userspace atomic helpers Peter Oskolkov
2021-11-24 14:31 ` Peter Zijlstra
2021-11-22 21:13 ` [PATCH v0.9.1 3/6] sched/umcg: implement UMCG syscalls Peter Oskolkov
2021-11-24 18:36 ` kernel test robot
2021-11-24 20:08 ` Peter Zijlstra
2021-11-24 21:32 ` Peter Zijlstra
2021-11-25 17:28 ` Peter Oskolkov
2021-11-26 17:09 ` Peter Zijlstra
2021-11-26 21:08 ` Thomas Gleixner
2021-11-26 21:59 ` Peter Zijlstra
2021-11-26 22:07 ` Peter Zijlstra
2021-11-27 0:45 ` Thomas Gleixner
2021-11-29 15:05 ` Peter Zijlstra
2021-11-26 22:16 ` Peter Zijlstra
2021-11-27 1:16 ` Thomas Gleixner
2021-11-29 15:07 ` Peter Zijlstra
2021-11-29 0:29 ` Peter Oskolkov
2021-11-29 16:41 ` Peter Zijlstra
2021-11-29 17:34 ` Peter Oskolkov
2021-11-29 21:08 ` Peter Zijlstra
2021-11-29 21:29 ` Peter Zijlstra
2021-11-29 23:38 ` Peter Oskolkov
2021-12-06 11:32 ` Peter Zijlstra
2021-12-06 12:04 ` Peter Zijlstra
2021-12-13 13:55 ` Peter Zijlstra
2021-12-06 11:47 ` Peter Zijlstra
2022-01-19 17:26 ` Peter Oskolkov
2022-01-20 11:07 ` Peter Zijlstra [this message]
2021-11-24 21:19 ` Peter Zijlstra
2021-11-26 21:11 ` Thomas Gleixner
2021-11-26 21:52 ` Peter Zijlstra
2021-11-29 22:07 ` Thomas Gleixner
2021-11-29 22:22 ` Peter Zijlstra
2021-11-24 21:41 ` Peter Zijlstra
2021-11-24 21:58 ` Peter Zijlstra
2021-11-24 22:18 ` Peter Zijlstra
2021-11-22 21:13 ` [PATCH v0.9.1 4/6] sched/umcg, lib/umcg: implement libumcg Peter Oskolkov
2021-11-22 21:13 ` [PATCH v0.9.1 5/6] sched/umcg: add Documentation/userspace-api/umcg.txt Peter Oskolkov
2021-11-22 21:13 ` [PATCH v0.9.1 6/6] sched/umcg, lib/umcg: add tools/lib/umcg/libumcg.txt Peter Oskolkov
2021-11-24 14:06 ` [PATCH v0.9.1 0/6] sched,mm,x86/uaccess: implement User Managed Concurrency Groups Peter Zijlstra
2021-11-24 16:28 ` Peter Oskolkov
2021-11-24 17:20 ` Peter Zijlstra
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