Re: frequent lockups in 3.18rc4

From: Linus Torvalds
Date: Thu Nov 27 2014 - 14:17:25 EST

On Wed, Nov 26, 2014 at 2:57 PM, Dave Jones <davej@xxxxxxxxxx> wrote:
> So 3.17 also has this problem.
> Good news I guess in that it's not a regression, but damn I really didn't
> want to have to go digging through the mists of time to find the last 'good' point.

So I'm looking at the watchdog code, and it seems racy wrt parking and startup.

In particular, it sets the high priority *after* starting the hrtimer,
and it goes back to SCHED_NORMAL *before* canceling the timer.

Which seems completely ass-backwards. And the smp_hotplug_thread stuff
explicitly enables preemption around the setup/cleanup/part/unpark

However, that would be an issue only if trinity might be doing things
that enable and disable the watchdog. And doing so under insane loads.
Even then it seems unlikely.

The insane loads you have. But even then, could a load average of 169
possibly delay running a non-RT process for 22 seconds? Doubtful.

But just in case: do you do cpu hotplug events (that will disable and
re-enable the watchdog process?). Anything else that will part/unpark
the hotplug thread?

Quite frankly, I'm just grasping for straws here, but a lot of the
watchdog traces really have seemed spurious...

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