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:

quoted

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.

quoted

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.