From: Peter Zijlstra <peterz@infradead.org>
To: Qais Yousef <qais.yousef@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>,
Valentin Schneider <valentin.schneider@arm.com>,
Juri Lelli <juri.lelli@redhat.com>,
Vincent Guittot <vincent.guittot@linaro.org>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Steven Rostedt <rostedt@goodmis.org>,
Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
Patrick Bellasi <patrick.bellasi@matbug.net>,
Chris Redpath <chris.redpath@arm.com>,
Lukasz Luba <lukasz.luba@arm.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v6 2/2] sched/uclamp: Protect uclamp fast path code with static key
Date: Tue, 30 Jun 2020 21:06:43 +0200 [thread overview]
Message-ID: <20200630190643.GC4817@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200630175502.otw4seymlynghje7@e107158-lin.cambridge.arm.com>
On Tue, Jun 30, 2020 at 06:55:02PM +0100, Qais Yousef wrote:
> On 06/30/20 19:07, Peter Zijlstra wrote:
> > There's a fun race described in 9107c89e269d ("perf: Fix race between
> > event install and jump_labels"), are we sure this isn't also susceptible
> > to something similar?
> >
> > I suspect not, but I just wanted to make sure.
>
> IIUC, the worry is that not all CPUs might have observed the change in the
> static key state; hence could not be running the patched
> enqueue/dequeue_task(), so we could end up with some CPUs accounting for
> uclamp in the enqueue/dequeue path but not others?
>
> I was hoping this synchronization is guaranteed by the static_branch_*() call.
It is, that isn't quite the the problem. Looking at it more I think
commit 1dbb6704de91 ("jump_label: Fix concurrent
static_key_enable/disable()") fixed some of it.
From what I can remember there were two parts to this problem, one being
fixed by the above commit, the other being that if we enable while a
task is running we miss the switch-in event (exactly how in this patch
we miss the enqueue).
Due to the missing switch-in, the state is 'weird' and the subsequent
IPI to install a remote event didn't quite work.
So I put that sync_sched() call in to guarantee all CPUs have done a
schedule() cycle after having the key switched. This makes sure that
every running task has seen the switch-in and thus the state is as
expected.
But like I said, I think we're good, that one extra branch deals with
the half-state.
next prev parent reply other threads:[~2020-06-30 19:07 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-30 11:21 [PATCH v6 0/2] sched: Optionally skip uclamp logic in fast path Qais Yousef
2020-06-30 11:21 ` [PATCH v6 1/2] sched/uclamp: Fix initialization of struct uclamp_rq Qais Yousef
2020-07-09 8:45 ` [tip: sched/core] " tip-bot2 for Qais Yousef
2020-06-30 11:21 ` [PATCH v6 2/2] sched/uclamp: Protect uclamp fast path code with static key Qais Yousef
2020-06-30 17:07 ` Peter Zijlstra
2020-06-30 17:55 ` Qais Yousef
2020-06-30 19:06 ` Peter Zijlstra [this message]
2020-06-30 19:28 ` Qais Yousef
2020-07-09 8:45 ` [tip: sched/core] " tip-bot2 for Qais Yousef
2020-07-01 16:32 ` [PATCH v6 0/2] sched: Optionally skip uclamp logic in fast path Lukasz Luba
2020-07-03 12:09 ` Vincent Guittot
2020-07-06 10:41 ` Qais Yousef
2020-07-07 12:29 ` Vincent Guittot
2020-07-07 13:11 ` Qais Yousef
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20200630190643.GC4817@hirez.programming.kicks-ass.net \
--to=peterz@infradead.org \
--cc=bsegall@google.com \
--cc=chris.redpath@arm.com \
--cc=dietmar.eggemann@arm.com \
--cc=juri.lelli@redhat.com \
--cc=linux-kernel@vger.kernel.org \
--cc=lukasz.luba@arm.com \
--cc=mgorman@suse.de \
--cc=mingo@redhat.com \
--cc=patrick.bellasi@matbug.net \
--cc=qais.yousef@arm.com \
--cc=rostedt@goodmis.org \
--cc=valentin.schneider@arm.com \
--cc=vincent.guittot@linaro.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox