Date: Tue, 29 Jul 2014 14:23:03 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Tejun Heo <tj@kernel.org>
Cc: Christoph Lameter <cl@gentwo.org>, Sasha Levin <sasha.levin@oracle.com>,
        akpm@linux-foundation.org, Gilad Ben-Yossef <gilad@benyossef.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        John Stultz <johnstul@us.ibm.com>, Mike Frysinger <vapier@gentoo.org>,
        Minchan Kim <minchan.kim@gmail.com>,
        Hakan Akkan <hakanakkan@gmail.com>,
        Max Krasnyansky <maxk@qualcomm.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org, hughd@google.com,
        viresh.kumar@linaro.org, hpa@zytor.com, mingo@kernel.org
Subject: Re: vmstat: On demand vmstat workers V8
Message-ID: <20140729122303.GA3935@laptop>
References: <alpine.DEB.2.11.1407100903130.12483@gentwo.org>
 <53D31101.8000107@oracle.com>
 <alpine.DEB.2.11.1407281353450.15405@gentwo.org>
 <20140729075637.GA19379@twins.programming.kicks-ass.net>
 <20140729120525.GA28366@mtj.dyndns.org>
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On Tue, Jul 29, 2014 at 08:05:25AM -0400, Tejun Heo wrote:
> 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.

Just force flush on unplug and fix those that complain. No auditing
needed for that.

> 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.

I think before cmwq we'd run into the broken affinity warning in the
scheduler.
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