Re: vmstat: On demand vmstat workers V8

From: Andrew Morton
Date: Mon Jul 28 2014 - 17:54:48 EST

On Mon, 28 Jul 2014 13:55:17 -0500 (CDT) Christoph Lameter <cl@xxxxxxxxxx> wrote:

> On Fri, 25 Jul 2014, Sasha Levin wrote:
> > This patch doesn't interact well with my fuzzing setup. I'm seeing
> > the following:
> >
> > [ 490.446927] BUG: using __this_cpu_read() in preemptible [00000000] code: kworker/16:1/7368
> > [ 490.447909] caller is __this_cpu_preempt_check+0x13/0x20
> __this_cpu_read() from vmstat_update is only called from a kworker that
> is bound to a single cpu. A false positive?

schedule_delayed_work() uses system_wq. The comment in workqueue.h says

* system_wq is the one used by schedule[_delayed]_work[_on]().
* Multi-CPU multi-threaded. There are users which expect relatively
* short queue flush time. Don't queue works which can run for too
* long.

but the code itself does

system_wq = alloc_workqueue("events", 0, 0);

ie: it didn't pass WQ_UNBOUND in the flags.

Tejun, wazzup?

Also, Sasha's report showed this:

[ 490.464613] kernel BUG at mm/vmstat.c:1278!

That's your VM_BUG_ON() in vmstat_update(). That ain't no false

Is this code expecting that schedule_delayed_work() will schedule the
work on the current CPU? I don't think it will do that. Maybe you
should be looking at schedule_delayed_work_on().

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