Re: vmstat: On demand vmstat workers V8

[Tejun Heo <tj@kernel.org>]

On Tue, Jul 29, 2014 at 09:56:37AM +0200, Peter Zijlstra wrote:
> On Mon, Jul 28, 2014 at 01:55:17PM -0500, Christoph Lameter 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?
> 
> kworkers are never guaranteed to be so, its a 'feature' :/

It's because we don't distinguish work items which are per-cpu for
optimization and per-cpu for correctness and can't automatically flush
/ cancel / block per-cpu work items when a cpu goes down.  I like the
idea of distingushing them but it's gonna take a lot of auditing.

Any work item usage which requires per-cpu for correctness should
implement cpu down hook to flush in-flight work items and block
further issuance.  This hasn't changed from the beginning and was
necessary even before cmwq.

Thanks.

-- 
tejun

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