linux-kernel - Re: [RFC workqueue/driver-core PATCH 1/5] workqueue: Provide queue_work_near to queue work near a given NUMA node

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Message-ID: <4eebc017-23a2-a26e-095c-66433061a141@linux.intel.com>
Date:   Mon, 1 Oct 2018 14:54:39 -0700
From:   Alexander Duyck <alexander.h.duyck@...ux.intel.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     linux-nvdimm@...ts.01.org, gregkh@...uxfoundation.org,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, len.brown@...el.com,
        dave.jiang@...el.com, rafael@...nel.org, vishal.l.verma@...el.com,
        jiangshanlai@...il.com, pavel@....cz, zwisler@...nel.org,
        dan.j.williams@...el.com
Subject: Re: [RFC workqueue/driver-core PATCH 1/5] workqueue: Provide
 queue_work_near to queue work near a given NUMA node

On 10/1/2018 9:01 AM, Tejun Heo wrote:
> Hello,
> 
> On Wed, Sep 26, 2018 at 03:19:21PM -0700, Alexander Duyck wrote:
>> On 9/26/2018 3:09 PM, Tejun Heo wrote:
>> I could just use queue_work_on probably, but is there any issue if I
>> am passing CPU values that are not in the wq_unbound_cpumask? That
> 
> That should be fine.  If it can't find any available cpu, it'll fall
> back to round-robin.  We probably can improve it so that it can
> consider the numa distance when falling back.
> 
>> was mostly my concern. Also for an unbound queue do I need to worry
>> about the hotplug lock? I wasn't sure if that was the case or not as
> 
> Issuers don't need to worry about them.
> 
>> I know it is called out as something to be concerned with using
>> queue_work_on, but in __queue_work the value is just used to
>> determine which node to grab a work queue from.
> 
> It might be better to leave queue_work_on() to be used for per-cpu
> workqueues and introduce queue_work_near() as you suggseted.  I just
> don't want it to duplicate the node selection code in it.  Would that
> work?

So if I understand what you are saying correctly we default to 
round-robin on a given node has no CPUs attached to it. I could probably 
work with that if that is the default behavior instead of adding much of 
the complexity I already have.

The question I have then is what should I do about workqueues that 
aren't WQ_UNBOUND if they attempt to use queue_work_near? In that case I 
should be looking for some way to go from a node to a CPU shouldn't I? 
If so should I look at doing something like wq_select_unbound_cpu that 
uses the node cpumask instead of the wq_unbound_cpumask?

- Alex