Re: [PATCH 3/4] mm, page_alloc: Drain per-cpu pages from workqueue context

[Mel Gorman <mgorman@techsingularity.net>]

On Fri, Jan 20, 2017 at 03:26:05PM +0100, Vlastimil Babka wrote:
> > @@ -2392,8 +2404,24 @@ void drain_all_pages(struct zone *zone)
> >  		else
> >  			cpumask_clear_cpu(cpu, &cpus_with_pcps);
> >  	}
> > -	on_each_cpu_mask(&cpus_with_pcps, (smp_call_func_t) drain_local_pages,
> > -								zone, 1);
> > +
> > +	if (works) {
> > +		for_each_cpu(cpu, &cpus_with_pcps) {
> > +			struct work_struct *work = per_cpu_ptr(works, cpu);
> > +			INIT_WORK(work, drain_local_pages_wq);
> > +			schedule_work_on(cpu, work);
> 
> This translates to queue_work_on(), which has the comment of "We queue
> the work to a specific CPU, the caller must ensure it can't go away.",
> so is this safe? lru_add_drain_all() uses get_online_cpus() around this.
> 

get_online_cpus() would be required.

> schedule_work_on() also uses the generic system_wq, while lru drain has
> its own workqueue with WQ_MEM_RECLAIM so it seems that would be useful
> here as well?
> 

I would be reluctant to introduce a dedicated queue unless there was a
definite case where an OOM occurred because pages were pinned on per-cpu
lists and couldn't be drained because the buddy allocator was depleted.
As it was, I thought the fallback case was excessively paranoid.

> > +		}
> > +		for_each_cpu(cpu, &cpus_with_pcps)
> > +			flush_work(per_cpu_ptr(works, cpu));
> > +	} else {
> > +		for_each_cpu(cpu, &cpus_with_pcps) {
> > +			struct work_struct work;
> > +
> > +			INIT_WORK(&work, drain_local_pages_wq);
> > +			schedule_work_on(cpu, &work);
> > +			flush_work(&work);
> 
> Totally out of scope, but I wonder if schedule_on_each_cpu() could use
> the same fallback that's here?
> 

I'm not aware of a case where it really has been a problem. I only considered
it here as the likely caller is in a context that is failing allocations.

-- 
Mel Gorman
SUSE Labs

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