linux-kernel - Re: [PATCH 2/2] workqueue:Fix affinity of an unbound worker of a node with 1 online CPU

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Date:	Wed, 15 Jun 2016 18:20:33 +0530
From:	Gautham R Shenoy <ego@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tejun Heo <htejun@...il.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Abdul Haleem <abdhalee@...ux.vnet.ibm.com>,
	Aneesh Kumar <aneesh.kumar@...ux.vnet.ibm.com>,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] workqueue:Fix affinity of an unbound worker of a
 node with 1 online CPU

On Wed, Jun 15, 2016 at 01:32:49PM +0200, Peter Zijlstra wrote:
> On Wed, Jun 15, 2016 at 03:49:36PM +0530, Gautham R Shenoy wrote:
> 
> > Also, with the first patch in the series (which ensures that
> > restore_unbound_workers are called *after* the new workers for the
> > newly onlined CPUs are created) and without this one, you can
> > reproduce this WARN_ON on both x86 and PPC by offlining all the CPUs
> > of a node and bringing just one of them online.
> 
> Ah good.
> 
> > I am not sure about that. The workqueue creates unbound workers for a
> > node via wq_update_unbound_numa() whenever the first CPU of every node
> > comes online. So that seems legitimate. It then tries to affine these
> > workers to the cpumask of that node. Again this seems right. As an
> > optimization, it does this only when the first CPU of the node comes
> > online. Since this online CPU is not yet active, and since
> > nr_cpus_allowed > 1, we will hit the WARN_ON().
> 
> So I had another look and isn't the below a much simpler solution?
> 
> It seems to work on my x86 with:
> 
>   for i in /sys/devices/system/cpu/cpu*/online ; do echo 0 > $i ; done
>   for i in /sys/devices/system/cpu/cpu*/online ; do echo 1 > $i ; done
> 
> without complaint.

Yup. This will work on PPC as well. 

We will no longer have the optimization in
restore_unbound_workers_cpumask() but I suppose we don't lose much by
resetting the affinity every time a CPU in the pool->attr->cpumask
comes online.

--
Thanks and Regards
gautham.