Re: [PATCH 08/10] mm: vmscan: Have kswapd shrink slab only once per priority

[Simon Jeons <simon.jeons@gmail.com>]

Hi Joonsoo,
On 04/09/2013 02:53 PM, Joonsoo Kim wrote:
> Hello, Mel.
> Sorry for too late question.
>
> On Sun, Mar 17, 2013 at 01:04:14PM +0000, Mel Gorman wrote:
>> If kswaps fails to make progress but continues to shrink slab then it'll
>> either discard all of slab or consume CPU uselessly scanning shrinkers.
>> This patch causes kswapd to only call the shrinkers once per priority.
>>
>> Signed-off-by: Mel Gorman <mgorman@suse.de>
>> ---
>>   mm/vmscan.c | 28 +++++++++++++++++++++-------
>>   1 file changed, 21 insertions(+), 7 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 7d5a932..84375b2 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -2661,9 +2661,10 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining,
>>    */
>>   static bool kswapd_shrink_zone(struct zone *zone,
>>   			       struct scan_control *sc,
>> -			       unsigned long lru_pages)
>> +			       unsigned long lru_pages,
>> +			       bool shrinking_slab)
>>   {
>> -	unsigned long nr_slab;
>> +	unsigned long nr_slab = 0;
>>   	struct reclaim_state *reclaim_state = current->reclaim_state;
>>   	struct shrink_control shrink = {
>>   		.gfp_mask = sc->gfp_mask,
>> @@ -2673,9 +2674,15 @@ static bool kswapd_shrink_zone(struct zone *zone,
>>   	sc->nr_to_reclaim = max(SWAP_CLUSTER_MAX, high_wmark_pages(zone));
>>   	shrink_zone(zone, sc);
>>   
>> -	reclaim_state->reclaimed_slab = 0;
>> -	nr_slab = shrink_slab(&shrink, sc->nr_scanned, lru_pages);
>> -	sc->nr_reclaimed += reclaim_state->reclaimed_slab;
>> +	/*
>> +	 * Slabs are shrunk for each zone once per priority or if the zone
>> +	 * being balanced is otherwise unreclaimable
>> +	 */
>> +	if (shrinking_slab || !zone_reclaimable(zone)) {
>> +		reclaim_state->reclaimed_slab = 0;
>> +		nr_slab = shrink_slab(&shrink, sc->nr_scanned, lru_pages);
>> +		sc->nr_reclaimed += reclaim_state->reclaimed_slab;
>> +	}
>>   
>>   	if (nr_slab == 0 && !zone_reclaimable(zone))
>>   		zone->all_unreclaimable = 1;
> Why shrink_slab() is called here?
> I think that outside of zone loop is better place to run shrink_slab(),
> because shrink_slab() is not directly related to a specific zone.

True.

>
> And this is a question not related to this patch.
> Why nr_slab is used here to decide zone->all_unreclaimable?
> nr_slab is not directly related whether a specific zone is reclaimable
> or not, and, moreover, nr_slab is not directly related to number of
> reclaimed pages. It just say some objects in the system are freed.
>
> This question comes from my ignorance, so please enlighten me.

Good question, I also want to know. ;-)

>
> Thanks.
>
>> @@ -2713,6 +2720,7 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>>   	int end_zone = 0;	/* Inclusive.  0 = ZONE_DMA */
>>   	unsigned long nr_soft_reclaimed;
>>   	unsigned long nr_soft_scanned;
>> +	bool shrinking_slab = true;
>>   	struct scan_control sc = {
>>   		.gfp_mask = GFP_KERNEL,
>>   		.priority = DEF_PRIORITY,
>> @@ -2861,7 +2869,8 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>>   				 * already being scanned that that high
>>   				 * watermark would be met at 100% efficiency.
>>   				 */
>> -				if (kswapd_shrink_zone(zone, &sc, lru_pages))
>> +				if (kswapd_shrink_zone(zone, &sc,
>> +						lru_pages, shrinking_slab))
>>   					raise_priority = false;
>>   
>>   				nr_to_reclaim += sc.nr_to_reclaim;
>> @@ -2900,6 +2909,9 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>>   				pfmemalloc_watermark_ok(pgdat))
>>   			wake_up(&pgdat->pfmemalloc_wait);
>>   
>> +		/* Only shrink slab once per priority */
>> +		shrinking_slab = false;
>> +
>>   		/*
>>   		 * Fragmentation may mean that the system cannot be rebalanced
>>   		 * for high-order allocations in all zones. If twice the
>> @@ -2925,8 +2937,10 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>>   		 * Raise priority if scanning rate is too low or there was no
>>   		 * progress in reclaiming pages
>>   		 */
>> -		if (raise_priority || !this_reclaimed)
>> +		if (raise_priority || !this_reclaimed) {
>>   			sc.priority--;
>> +			shrinking_slab = true;
>> +		}
>>   	} while (sc.priority >= 1 &&
>>   		 !pgdat_balanced(pgdat, order, *classzone_idx));
>>   
>> -- 
>> 1.8.1.4
>>
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