Re: [PATCH v4 1/3] mm/free_pcppages_bulk: update pcp->count inside

[Aaron Lu <aaron.lu@intel.com>]

On Mon, Mar 12, 2018 at 02:22:28PM +0100, Vlastimil Babka wrote:
> On 03/01/2018 07:28 AM, Aaron Lu wrote:
> > Matthew Wilcox found that all callers of free_pcppages_bulk() currently
> > update pcp->count immediately after so it's natural to do it inside
> > free_pcppages_bulk().
> > 
> > No functionality or performance change is expected from this patch.
> 
> Well, it's N decrements instead of one decrement by N / assignment of
> zero. But I assume the difference is negligible anyway, right?

Yes.

> 
> > Suggested-by: Matthew Wilcox <willy@infradead.org>
> > Signed-off-by: Aaron Lu <aaron.lu@intel.com>
> 
> Acked-by: Vlastimil Babka <vbabka@suse.cz>

Thanks!

> > ---
> >  mm/page_alloc.c | 10 +++-------
> >  1 file changed, 3 insertions(+), 7 deletions(-)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index cb416723538f..faa33eac1635 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1148,6 +1148,7 @@ static void free_pcppages_bulk(struct zone *zone, int count,
> >  			page = list_last_entry(list, struct page, lru);
> >  			/* must delete as __free_one_page list manipulates */
> >  			list_del(&page->lru);
> > +			pcp->count--;
> >  
> >  			mt = get_pcppage_migratetype(page);
> >  			/* MIGRATE_ISOLATE page should not go to pcplists */
> > @@ -2416,10 +2417,8 @@ void drain_zone_pages(struct zone *zone, struct per_cpu_pages *pcp)
> >  	local_irq_save(flags);
> >  	batch = READ_ONCE(pcp->batch);
> >  	to_drain = min(pcp->count, batch);
> > -	if (to_drain > 0) {
> > +	if (to_drain > 0)
> >  		free_pcppages_bulk(zone, to_drain, pcp);
> > -		pcp->count -= to_drain;
> > -	}
> >  	local_irq_restore(flags);
> >  }
> >  #endif
> > @@ -2441,10 +2440,8 @@ static void drain_pages_zone(unsigned int cpu, struct zone *zone)
> >  	pset = per_cpu_ptr(zone->pageset, cpu);
> >  
> >  	pcp = &pset->pcp;
> > -	if (pcp->count) {
> > +	if (pcp->count)
> >  		free_pcppages_bulk(zone, pcp->count, pcp);
> > -		pcp->count = 0;
> > -	}
> >  	local_irq_restore(flags);
> >  }
> >  
> > @@ -2668,7 +2665,6 @@ static void free_unref_page_commit(struct page *page, unsigned long pfn)
> >  	if (pcp->count >= pcp->high) {
> >  		unsigned long batch = READ_ONCE(pcp->batch);
> >  		free_pcppages_bulk(zone, batch, pcp);
> > -		pcp->count -= batch;
> >  	}
> >  }
> >  
> > 
>