From: Scott Wood <swood@redhat.com>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Steven Rostedt <rostedt@goodmis.org>,
Peter Zijlstra <peterz@infradead.org>,
Juri Lelli <juri.lelli@redhat.com>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
Clark Williams <williams@redhat.com>,
linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org
Subject: Re: [PATCH RT 7/8] sched: migrate_enable: Use select_fallback_rq()
Date: Tue, 24 Sep 2019 13:05:47 -0500 [thread overview]
Message-ID: <b07c0ad99393cfa0968a926bd7302adef4c6a7e4.camel@redhat.com> (raw)
In-Reply-To: <20190917160030.i24gvyye2bpdykfy@linutronix.de>
On Tue, 2019-09-17 at 18:00 +0200, Sebastian Andrzej Siewior wrote:
> On 2019-07-27 00:56:37 [-0500], Scott Wood wrote:
> > migrate_enable() currently open-codes a variant of select_fallback_rq().
> > However, it does not have the "No more Mr. Nice Guy" fallback and thus
> > it will pass an invalid CPU to the migration thread if cpus_mask only
> > contains a CPU that is !active.
> >
> > Signed-off-by: Scott Wood <swood@redhat.com>
> > ---
> > This scenario will be more likely after the next patch, since
> > the migrate_disable_update check goes away. However, it could happen
> > anyway if cpus_mask was updated to a CPU other than the one we were
> > pinned to, and that CPU subsequently became inactive.
>
> I'm unclear about the problem / side effect this has (before and after
> the change). It is possible (before and after that change) that a CPU is
> selected which is invalid / goes offline after the "preempt_enable()"
> statement and before stop_one_cpu() does its job, correct?
By "pass an invalid CPU" I don't mean offline; I mean >= nr_cpu_ids which is
what cpumask_any_and() returns if the sets don't intersect (a CPU going
offline is merely a way to end up in that situation). At one point I
observed that causing a crash. I guess is_cpu_allowed() returned true by
chance based on the contents of data beyond the end of the cpumask? That
doesn't seem likely based on what comes after p->cpus_mask (at that point
migrate_disable should be zero), but maybe I had something else at that
point in the struct while developing. In any case, not something to be
relied on. :-)
Going offline after selection shouldn't be a problem. migration_cpu_stop()
won't do anything if is_cpu_allowed() returns false, and we'll get migrated
off the CPU by migrate_tasks(). Even if we get migrated after reading
task_cpu(p) but before queuing the stop machine work, it'll either get
flushed when the stopper thread parks, or rejected due to stopper->enabled
being false.
-Scott
next prev parent reply other threads:[~2019-09-24 18:05 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-27 5:56 [RT PATCH 0/8] migrate disable fixes and performance Scott Wood
2019-07-27 5:56 ` [PATCH RT 1/8] sched: migrate_enable: Use sleeping_lock to indicate involuntary sleep Scott Wood
2019-07-29 13:26 ` Steven Rostedt
2019-07-27 5:56 ` [PATCH RT 2/8] sched: __set_cpus_allowed_ptr: Check cpus_mask, not cpus_ptr Scott Wood
2019-09-17 14:57 ` Sebastian Andrzej Siewior
2019-09-17 15:23 ` Scott Wood
2019-07-27 5:56 ` [PATCH RT 3/8] sched: Remove dead __migrate_disabled() check Scott Wood
2019-07-27 5:56 ` [PATCH RT 4/8] sched: migrate disable: Protect cpus_ptr with lock Scott Wood
2019-09-26 16:39 ` Sebastian Andrzej Siewior
2019-09-26 16:52 ` Scott Wood
2019-09-27 12:19 ` Sebastian Andrzej Siewior
2019-09-27 20:02 ` Scott Wood
2019-07-27 5:56 ` [PATCH RT 5/8] sched/deadline: Reclaim cpuset bandwidth in .migrate_task_rq() Scott Wood
2019-09-17 15:10 ` Sebastian Andrzej Siewior
2019-09-27 8:11 ` Juri Lelli
2019-09-27 16:40 ` Scott Wood
2019-09-30 7:12 ` Juri Lelli
2019-09-30 16:24 ` Scott Wood
2019-10-01 8:52 ` Juri Lelli
2019-10-09 6:25 ` Scott Wood
2019-10-09 7:27 ` Juri Lelli
2019-10-09 19:12 ` Scott Wood
2019-10-10 8:18 ` Juri Lelli
2019-07-27 5:56 ` [PATCH RT 6/8] sched: migrate_enable: Set state to TASK_RUNNING Scott Wood
2019-09-17 15:31 ` Sebastian Andrzej Siewior
2019-09-17 17:54 ` Scott Wood
2019-07-27 5:56 ` [PATCH RT 7/8] sched: migrate_enable: Use select_fallback_rq() Scott Wood
2019-09-17 16:00 ` Sebastian Andrzej Siewior
2019-09-24 18:05 ` Scott Wood [this message]
2019-07-27 5:56 ` [PATCH RT 8/8] sched: Lazy migrate_disable processing Scott Wood
2019-09-17 16:50 ` Sebastian Andrzej Siewior
2019-09-17 17:06 ` Scott Wood
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