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From:   Peter Oskolkov <posk@google.com>
Date:   Fri, 21 May 2021 15:01:24 -0700
Message-ID: <CAPNVh5foSzWj3XHoONjvzmLLXOi9u3ojNmdfegohpufx1YYXgg@mail.gmail.com>
Subject: Re: [RFC PATCH v0.1 4/9] sched/umcg: implement core UMCG API
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Paul Turner <pjt@google.com>, Ben Segall <bsegall@google.com>,
        Peter Oskolkov <posk@posk.io>,
        Joel Fernandes <joel@joelfernandes.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrei Vagin <avagin@google.com>,
        Jim Newsome <jnewsome@torproject.org>
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On Fri, May 21, 2021 at 12:32 PM Andy Lutomirski <luto@kernel.org> wrote:
>
> On Thu, May 20, 2021 at 11:36 AM Peter Oskolkov <posk@google.com> wrote:
> >
> > Implement version 1 of core UMCG API (wait/wake/swap).
> >
> > As has been outlined in
> > https://lore.kernel.org/lkml/20200722234538.166697-1-posk@posk.io/,
> > efficient and synchronous on-CPU context switching is key
> > to enabling two broad use cases: in-process M:N userspace scheduling
> > and fast X-process RPCs for security wrappers.
> >
> > High-level design considerations/approaches used:
> > - wait & wake can race with each other;
> > - offload as much work as possible to libumcg in tools/lib/umcg,
> >   specifically:
> >   - most state changes, e.g. RUNNABLE <=> RUNNING, are done in
> >     the userspace (libumcg);
> >   - retries are offloaded to the userspace.
>
> Do you have some perf numbers as to how long a UMCG context switch
> takes compared to a normal one?

I'm not sure what is a "normal context switch" in this context. Futex
wakeup on a remote idle CPU takes 5-10usec; an on-CPU UMCG context
switch takes less than 1usec; futex wake + futex wait on the same CPU
(taskset ***) takes about 1-1.5usec in my benchmarks.

>
> --Andy