Re: [PATCH 1/4] workqueue:Fix unbound workqueue's node affinity detection

[Kamezawa Hiroyuki]

(2014/12/16 14:30), Lai Jiangshan wrote:
> On 12/15/2014 07:14 PM, Kamezawa Hiroyuki wrote:
> > Unbound wq pool's node attribute is calculated at its allocation.
> > But it's now calculated based on possible cpu<->node information
> > which can be wrong after cpu hotplug/unplug.
> >
> > If wrong pool->node is set, following allocation error will happen.
> > ==
> >   SLUB: Unable to allocate memory on node 2 (gfp=0x80d0)
> >    cache: kmalloc-192, object size: 192, buffer size: 192, default order:
> > 1, min order: 0
> >    node 0: slabs: 6172, objs: 259224, free: 245741
> >    node 1: slabs: 3261, objs: 136962, free: 127656
> > ==
> >
> > This patch fixes the node detection by making use of online cpu info.
> > Unlike cpumask, the best node can be calculated by degree of overlap
> > between attr->cpumask and numanode->online_cpumask.
> > This change doesn't corrupt original purpose of the old calculation.
> >
> > Note: it's expected that this function is called as
> >        pool_detect_best_node
> >        get_unbound_pool
> >        alloc_unbound_pwq
> >        wq_update_unbound_numa
> >        called at CPU_ONLINE/CPU_DOWN_PREPARE
> > and the latest online cpu info can be applied to a new wq pool,
> > which replaces old one.
> >
> > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> > ---
> >   kernel/workqueue.c | 38 ++++++++++++++++++++++++++------------
> >   1 file changed, 26 insertions(+), 12 deletions(-)
> >
> > diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> > index 09b685d..7809154 100644
> > --- a/kernel/workqueue.c
> > +++ b/kernel/workqueue.c
> > @@ -3440,6 +3440,31 @@ static void put_unbound_pool(struct worker_pool *pool)
> >   }
> >
> >   /**
> > + * pool_detect_best_node - detect a node which contains specified cpumask.
> > + * Should be called with wq_pool_mutex held.
> > + * Returns a online node where the most of given cpus are tied to.
> > + */
> > +static int pool_detect_best_node(const struct cpumask *cpumask)
> > +{
> > +	int node, best, match, selected = NUMA_NO_NODE;
> > +	static struct cpumask andmask; /* under wq_pool_mutex */
> > +
> > +	if (!wq_numa_enabled ||
> > +		cpumask_subset(cpu_online_mask, cpumask))
> > +		goto out;
> > +	best = 0;
> > +	/* select a node which contains the most number of cpu */
> > +	for_each_node_state(node, N_ONLINE) {
> > +		cpumask_and(&andmask, cpumask, cpumask_of_node(node));
> > +		match = cpumask_weight(&andmask);
> > +		if (match > best)
> > +			selected = best;
> > +	}
> > +out:
> > +	return selected;
> > +}
>
>
> This is a mixture of fix and development.  Why not just keep the original calculation?
Just because wq_numa_possible_mask is broken, it shouldn't be used if possible.
In this patch series, the mask is updated only when a node is coming up.
Is it better to clear it at node offline ?

> if the mask cover multiple nodes, NUMA_NO_NODE is the best for pool->node
> after the pool was created. The memory allocation will select the best node
> for manage_workers(), from which CPU that the worker actually is running on.

I'll drop this and try to keep original code as much as possible.

Thanks,
-Kame


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