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From:   Andy Lutomirski <luto@kernel.org>
Date:   Fri, 21 May 2021 12:32:07 -0700
Message-ID: <CALCETrXo=5r+i6f3qvSp4mEHcR93U3F0S0kFr8d5JGU6WetSqw@mail.gmail.com>
Subject: Re: [RFC PATCH v0.1 4/9] sched/umcg: implement core UMCG API
To:     Peter Oskolkov <posk@google.com>
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 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?

--Andy