linux-kernel - Re: [PATCH V2] workqueue: Ensure that cpumask set for pools created after boot

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Message-ID: <20170525145907.GD23493@htj.duckdns.org>
Date:   Thu, 25 May 2017 10:59:07 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Michael Bringmann <mwb@...ux.vnet.ibm.com>
Cc:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org,
        Nathan Fontenot <nfont@...ux.vnet.ibm.com>
Subject: Re: [PATCH V2] workqueue: Ensure that cpumask set for pools created
 after boot

Hello, Michael.

On Thu, May 25, 2017 at 09:55:10AM -0500, Michael Bringmann wrote:
> On NUMA systems with dynamic processors, the content of the cpumask
> may change over time.  As new processors are added via DLPAR operations,
> workqueues are created for them.  This patch ensures that the pools
> created for new workqueues will be initialized with a cpumask before
> the first worker is created, attached, and woken up.  If the mask is
> not set up, then the kernel will crash when 'wakeup_process' is unable
> to find a valid CPU to which to assign the new worker.
> 
> Signed-off-by: Michael Bringmann <mwb@...ux.vnet.ibm.com>
> ---
> Changes in V2:
>   -- Only fill in empty cpumasks.
> ---
>  kernel/workqueue.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index c74bf39..1b16ac4 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -3366,6 +3366,9 @@ static struct worker_pool *get_unbound_pool(const struct workqueue_attrs *attrs)
>  	copy_workqueue_attrs(pool->attrs, attrs);
>  	pool->node = target_node;
>  
> +	if (!cpumask_weight(pool->attrs->cpumask))
> +		cpumask_copy(pool->attrs->cpumask, cpumask_of(smp_processor_id()));
> +

So, the problem is that control is reaching here with empty mask.
Filling it with an arbitrary cpu id doesn't really fix the bug.  I
think I just messed up wq_calc_node_cpumask().  Will send a fix soon.

Thanks.

-- 
tejun