Re: [PATCH v2 1/8] mm/page_alloc: fix pcp high, batch management

[Joonsoo Kim]

On Tue, Aug 12, 2014 at 01:24:09AM +0000, Minchan Kim wrote:
> Hey Joonsoo,
> 
> On Wed, Aug 06, 2014 at 04:18:28PM +0900, Joonsoo Kim wrote:
> > per cpu pages structure, aka pcp, has high and batch values to control
> > how many pages we perform caching. This values could be updated
> > asynchronously and updater should ensure that this doesn't make any
> > problem. For this purpose, pageset_update() is implemented and do some
> > memory synchronization. But, it turns out to be wrong when I implemented
> > new feature using this. There is no corresponding smp_rmb() in read-side
> > so that it can't guarantee anything. Without correct updating, system
> > could hang in free_pcppages_bulk() due to larger batch value than high.
> > To properly update this values, we need to synchronization primitives on
> > read-side, but, it hurts allocator's fastpath.
> > 
> > There is another choice for synchronization, that is, sending IPI. This
> > is somewhat expensive, but, this is really rare case so I guess it has
> > no problem here. However, reducing IPI is very helpful here. Current
> > logic handles each CPU's pcp update one by one. To reduce sending IPI,
> > we need to re-ogranize the code to handle all CPU's pcp update at one go.
> > This patch implement these requirements.
> 
> Let's add right reviewer for the patch.
> Cced Cody and Thomas.

Okay. I will do it next time.

> 
> > 
> > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> > ---
> >  mm/page_alloc.c |  139 ++++++++++++++++++++++++++++++++-----------------------
> >  1 file changed, 80 insertions(+), 59 deletions(-)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index b99643d4..44672dc 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -3797,7 +3797,7 @@ static void build_zonelist_cache(pg_data_t *pgdat)
> >   * not check if the processor is online before following the pageset pointer.
> >   * Other parts of the kernel may not check if the zone is available.
> >   */
> > -static void setup_pageset(struct per_cpu_pageset *p, unsigned long batch);
> > +static void setup_pageset(struct per_cpu_pageset __percpu *pcp);
> >  static DEFINE_PER_CPU(struct per_cpu_pageset, boot_pageset);
> >  static void setup_zone_pageset(struct zone *zone);
> >  
> > @@ -3843,9 +3843,9 @@ static int __build_all_zonelists(void *data)
> >  	 * needs the percpu allocator in order to allocate its pagesets
> >  	 * (a chicken-egg dilemma).
> >  	 */
> > -	for_each_possible_cpu(cpu) {
> > -		setup_pageset(&per_cpu(boot_pageset, cpu), 0);
> > +	setup_pageset(&boot_pageset);
> >  
> > +	for_each_possible_cpu(cpu) {
> >  #ifdef CONFIG_HAVE_MEMORYLESS_NODES
> >  		/*
> >  		 * We now know the "local memory node" for each node--
> > @@ -4227,24 +4227,59 @@ static int zone_batchsize(struct zone *zone)
> >   * outside of boot time (or some other assurance that no concurrent updaters
> >   * exist).
> >   */
> > -static void pageset_update(struct per_cpu_pages *pcp, unsigned long high,
> > -		unsigned long batch)
> > +static void pageset_update(struct zone *zone, int high, int batch)
> >  {
> > -       /* start with a fail safe value for batch */
> > -	pcp->batch = 1;
> > -	smp_wmb();
> > +	int cpu;
> > +	struct per_cpu_pages *pcp;
> > +
> > +	/* start with a fail safe value for batch */
> > +	for_each_possible_cpu(cpu) {
> > +		pcp = &per_cpu_ptr(zone->pageset, cpu)->pcp;
> > +		pcp->batch = 1;
> > +	}
> > +	kick_all_cpus_sync();
> > +
> > +	/* Update high, then batch, in order */
> > +	for_each_possible_cpu(cpu) {
> > +		pcp = &per_cpu_ptr(zone->pageset, cpu)->pcp;
> > +		pcp->high = high;
> > +	}
> > +	kick_all_cpus_sync();
> >  
> > -       /* Update high, then batch, in order */
> > -	pcp->high = high;
> > -	smp_wmb();
> > +	for_each_possible_cpu(cpu) {
> > +		pcp = &per_cpu_ptr(zone->pageset, cpu)->pcp;
> > +		pcp->batch = batch;
> > +	}
> > +}
> > +
> > +/*
> > + * pageset_get_values_by_high() gets the high water mark for
> > + * hot per_cpu_pagelist to the value high for the pageset p.
> > + */
> > +static void pageset_get_values_by_high(int input_high,
> > +				int *output_high, int *output_batch)
> 
> You don't use output_high so we could make it as follows,

Yeah, it could be. But I want to make pageset_get_values_xxx variants
consistent. I will remove output_high and leave output_batch as is.

> 
> int pageset_batch(int high);
> 
> > +{
> > +	*output_batch = max(1, input_high / 4);
> > +	if ((input_high / 4) > (PAGE_SHIFT * 8))
> > +		*output_batch = PAGE_SHIFT * 8;
> > +}
> >  
> > -	pcp->batch = batch;
> > +/* a companion to pageset_get_values_by_high() */
> > +static void pageset_get_values_by_batch(int input_batch,
> > +				int *output_high, int *output_batch)
> > +{
> > +	*output_high = 6 * input_batch;
> > +	*output_batch = max(1, 1 * input_batch);
> >  }
> >  
> > -/* a companion to pageset_set_high() */
> > -static void pageset_set_batch(struct per_cpu_pageset *p, unsigned long batch)
> > +static void pageset_get_values(struct zone *zone, int *high, int *batch)
> >  {
> > -	pageset_update(&p->pcp, 6 * batch, max(1UL, 1 * batch));
> > +	if (percpu_pagelist_fraction) {
> > +		pageset_get_values_by_high(
> > +			(zone->managed_pages / percpu_pagelist_fraction),
> > +			high, batch);
> > +	} else
> > +		pageset_get_values_by_batch(zone_batchsize(zone), high, batch);
> >  }
> >  
> >  static void pageset_init(struct per_cpu_pageset *p)
> > @@ -4260,51 +4295,38 @@ static void pageset_init(struct per_cpu_pageset *p)
> >  		INIT_LIST_HEAD(&pcp->lists[migratetype]);
> >  }
> >  
> > -static void setup_pageset(struct per_cpu_pageset *p, unsigned long batch)
> > +/* Use this only in boot time, because it doesn't do any synchronization */
> > +static void setup_pageset(struct per_cpu_pageset __percpu *pcp)
> 
> If we can use it with only boot_pages in boot time, let's make it more clear.
> 
> static void boot_setup_pageset(void)
> {
> 	boot_pageset;
> 	XXX;
> }

Okay.

Thanks.

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