linux-kernel - Re: vmstat: On demand vmstat workers V8

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Date:	Tue, 29 Jul 2014 08:05:25 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Christoph Lameter <cl@...two.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	akpm@...ux-foundation.org, Gilad Ben-Yossef <gilad@...yossef.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <johnstul@...ibm.com>,
	Mike Frysinger <vapier@...too.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Max Krasnyansky <maxk@...lcomm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, hughd@...gle.com,
	viresh.kumar@...aro.org, hpa@...or.com, mingo@...nel.org
Subject: Re: vmstat: On demand vmstat workers V8

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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