linux-kernel - Re: [PATCH 2/4 v0.5] sched/umcg: RFC: add userspace atomic helpers

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Message-ID: <CAPNVh5f+=CmP_hhtVdMTh3kjsFDFNEWdeCBZ8TOhvAXWKe-N+A@mail.gmail.com>
Date:   Tue, 14 Sep 2021 11:48:27 -0700
From:   Peter Oskolkov <posk@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jann Horn <jannh@...gle.com>, Peter Oskolkov <posk@...k.io>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        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>,
        Thierry Delisle <tdelisle@...terloo.ca>
Subject: Re: [PATCH 2/4 v0.5] sched/umcg: RFC: add userspace atomic helpers

On Tue, Sep 14, 2021 at 11:29 AM Peter Oskolkov <posk@...gle.com> wrote:
>
> On Tue, Sep 14, 2021 at 11:05 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Tue, Sep 14, 2021 at 09:29:00AM -0700, Peter Oskolkov wrote:
> > > In the version of the patchset that I'm preparing to send I've decided
> > > to punt on the issue and just ask the userspace to deal with locking
> > > the memory as it sees fit: mlock() is available and as far as I can
> >
> > Sadly mlock() does not imply no faults. Someone had a too literal
> > reading of the POSIX-RT spec (of which mlock is part) and figured that
> > all that was required was to keep the page in memory, not avoid faults.
> >
> > Linux has had this bahviour for ages, PREEMPT_RT has tried to change
> > this, but so far to no avail. At some point sys_mpin() was proposed to
> > meet the original POSIX-RT intent, but afaict that never actually
> > happened.
> >
> > In short, mlock() does not avoid minor faults, or even migration faults,
> > which can take a fair while to resolve.
>
> Ok, I'll go with transiently pinning pages in
> __syscall_enter_from_user_work(), as you suggested. Seems easy enough
> to do.

Actually, I think pinning these pages when the worker exits to the
userspace (i.e. is scheduled on a CPU) and releasing them when the
worker is descheduled (blocks) would be better - this way we will be
able to wake the server not only on blocking syscalls but also on
pagefaults (on other pages) as well.

Do you think this approach is acceptable?

>
> Thanks for the suggestion!