From: Peter Zijlstra <peterz@infradead.org>
To: Will Deacon <will.deacon@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
Arnaldo Carvalho de Melo <acme@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
jeremy.linton@arm.com, Boqun Feng <boqun.feng@gmail.com>,
Paul McKenney <paulmck@linux.vnet.ibm.com>
Subject: Re: Perf hotplug lockup in v4.9-rc8
Date: Thu, 22 Dec 2016 15:00:10 +0100 [thread overview]
Message-ID: <20161222140010.GY3174@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20161222084509.GX3174@twins.programming.kicks-ass.net>
On Thu, Dec 22, 2016 at 09:45:09AM +0100, Peter Zijlstra wrote:
> On Mon, Dec 12, 2016 at 01:42:28PM +0100, Peter Zijlstra wrote:
> > > What are you trying to order here?
> >
> > I suppose something like this:
> >
> >
> > CPU0 CPU1 CPU2
> >
> > (current == t)
> >
> > t->perf_event_ctxp[] = ctx;
> > smp_mb();
> > cpu = task_cpu(t);
> >
> > switch(t, n);
> > migrate(t, 2);
> > switch(p, t);
> >
> > ctx = t->perf_event_ctxp[]; // must not be NULL
> >
>
> So I think I can cast the above into a test like:
>
> W[x] = 1 W[y] = 1 R[z] = 1
> mb mb mb
> R[y] = 0 W[z] = 1 R[x] = 0
>
> Where x is the perf_event_ctxp[], y is our task's cpu and z is our task
> being placed on the rq of cpu2.
>
> See also commit: 8643cda549ca ("sched/core, locking: Document
> Program-Order guarantees"), Independent of which cpu initiates the
> migration between CPU1 and CPU2 there is ordering between the CPUs.
I think that when we assume RCpc locks, the above CPU1 mb ends up being
something like an smp_wmb() (ie. non transitive). CPU2 needs to do a
context switch between observing the task on its runqueue and getting to
switching in perf-events for the task, which keeps that a full mb.
Now, if only this model would have locks in ;-)
> This would then translate into something like:
>
> C C-peterz
>
> {
> }
>
> P0(int *x, int *y)
> {
> int r1;
>
> WRITE_ONCE(*x, 1);
> smp_mb();
> r1 = READ_ONCE(*y);
> }
>
> P1(int *y, int *z)
> {
> WRITE_ONCE(*y, 1);
> smp_mb();
And this modified to: smp_wmb()
> WRITE_ONCE(*z, 1);
> }
>
> P2(int *x, int *z)
> {
> int r1;
> int r2;
>
> r1 = READ_ONCE(*z);
> smp_mb();
> r2 = READ_ONCE(*x);
> }
>
> exists
> (0:r1=0 /\ 2:r1=1 /\ 2:r2=0)
Still results in the same outcome.
If however we change P2's barrier into a smp_rmb() it does become
possible, but as said above, there's a context switch in between which
implies a full barrier so no worries.
Similar if I replace everything z with smp_store_release() and
smp_load_acquire().
Of course, its entirely possible the litmus test doesn't reflect
reality, I still find it somewhat hard to write these things.
next prev parent reply other threads:[~2016-12-22 14:00 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-12-07 13:53 Perf hotplug lockup in v4.9-rc8 Mark Rutland
2016-12-07 14:30 ` Mark Rutland
2016-12-07 16:39 ` Mark Rutland
2016-12-07 17:53 ` Mark Rutland
2016-12-07 18:34 ` Peter Zijlstra
2016-12-07 19:56 ` Mark Rutland
2016-12-09 13:59 ` Peter Zijlstra
2016-12-12 11:46 ` Will Deacon
2016-12-12 12:42 ` Peter Zijlstra
2016-12-22 8:45 ` Peter Zijlstra
2016-12-22 14:00 ` Peter Zijlstra [this message]
2016-12-22 16:33 ` Paul E. McKenney
2017-01-11 14:59 ` Mark Rutland
2017-01-11 16:03 ` Peter Zijlstra
2017-01-11 16:26 ` Mark Rutland
2017-01-11 19:51 ` Peter Zijlstra
2017-01-14 12:28 ` [tip:perf/urgent] perf/core: Fix sys_perf_event_open() vs. hotplug tip-bot for Peter Zijlstra
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=20161222140010.GY3174@twins.programming.kicks-ass.net \
--to=peterz@infradead.org \
--cc=acme@kernel.org \
--cc=bigeasy@linutronix.de \
--cc=boqun.feng@gmail.com \
--cc=jeremy.linton@arm.com \
--cc=linux-kernel@vger.kernel.org \
--cc=mark.rutland@arm.com \
--cc=mingo@redhat.com \
--cc=paulmck@linux.vnet.ibm.com \
--cc=tglx@linutronix.de \
--cc=will.deacon@arm.com \
/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;
as well as URLs for NNTP newsgroup(s).