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From: Peter Oskolkov <posk@google.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Jann Horn <jannh@google.com>, Peter Oskolkov <posk@posk.io>,
	Ingo Molnar <mingo@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	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>,
	Thierry Delisle <tdelisle@uwaterloo.ca>
Subject: Re: [PATCH 2/4 v0.5] sched/umcg: RFC: add userspace atomic helpers
Date: Tue, 14 Sep 2021 11:48:27 -0700	[thread overview]
Message-ID: <CAPNVh5f+=CmP_hhtVdMTh3kjsFDFNEWdeCBZ8TOhvAXWKe-N+A@mail.gmail.com> (raw)
In-Reply-To: <CAPNVh5eodc7_oSd9bb83eBucT5Vxy4cExkVSxw3pZ4McFAtjmw@mail.gmail.com>

On Tue, Sep 14, 2021 at 11:29 AM Peter Oskolkov <posk@google.com> wrote:
>
> On Tue, Sep 14, 2021 at 11:05 AM Peter Zijlstra <peterz@infradead.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!

  reply	other threads:[~2021-09-14 18:48 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-08 18:49 [PATCH 0/4 v0.5] sched/umcg: RFC UMCG patchset Peter Oskolkov
2021-09-08 18:49 ` [PATCH 1/4 v0.5] sched/umcg: add WF_CURRENT_CPU and externise ttwu Peter Oskolkov
2021-09-08 18:49 ` [PATCH 2/4 v0.5] sched/umcg: RFC: add userspace atomic helpers Peter Oskolkov
2021-09-08 23:38   ` Jann Horn
2021-09-09  1:16     ` Jann Horn
2021-09-09 19:06     ` Peter Oskolkov
2021-09-09 21:20       ` Jann Horn
2021-09-09 22:09         ` Peter Oskolkov
2021-09-09 23:13           ` Jann Horn
2021-09-14 16:52         ` Andy Lutomirski
2021-09-14 18:11           ` Peter Zijlstra
2021-09-14 18:40             ` Andy Lutomirski
2021-09-15 15:42               ` Peter Zijlstra
2021-09-15 16:50                 ` Andy Lutomirski
2021-09-15 19:10                   ` Peter Zijlstra
2021-09-14  8:07       ` Peter Zijlstra
2021-09-14 16:29         ` Peter Oskolkov
2021-09-14 18:04           ` Peter Zijlstra
2021-09-14 18:15             ` Peter Zijlstra
2021-09-14 18:29             ` Peter Oskolkov
2021-09-14 18:48               ` Peter Oskolkov [this message]
2021-09-08 18:49 ` [PATCH 3/4 v0.5] sched/umcg: RFC: implement UMCG syscalls Peter Oskolkov
2021-09-09  1:39   ` Jann Horn
2021-09-14 16:51     ` Peter Oskolkov
2021-09-08 18:49 ` [PATCH 4/4 v0.5] sched/umcg: add Documentation/userspace-api/umcg.rst Peter Oskolkov
2021-09-14 16:35   ` Tao Zhou
2021-09-14 16:57     ` Peter Oskolkov

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