Re: [PATCH 08/10] mm, pcp: decrease PCP high if free pages < high watermark

["Huang, Ying" <ying.huang@intel.com>]

Mel Gorman <mgorman@techsingularity.net> writes:

> On Wed, Sep 20, 2023 at 02:18:54PM +0800, Huang Ying wrote:
>> One target of PCP is to minimize pages in PCP if the system free pages
>> is too few.  To reach that target, when page reclaiming is active for
>> the zone (ZONE_RECLAIM_ACTIVE), we will stop increasing PCP high in
>> allocating path, decrease PCP high and free some pages in freeing
>> path.  But this may be too late because the background page reclaiming
>> may introduce latency for some workloads.  So, in this patch, during
>> page allocation we will detect whether the number of free pages of the
>> zone is below high watermark.  If so, we will stop increasing PCP high
>> in allocating path, decrease PCP high and free some pages in freeing
>> path.  With this, we can reduce the possibility of the premature
>> background page reclaiming caused by too large PCP.
>> 
>> The high watermark checking is done in allocating path to reduce the
>> overhead in hotter freeing path.
>> 
>> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Mel Gorman <mgorman@techsingularity.net>
>> Cc: Vlastimil Babka <vbabka@suse.cz>
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: Johannes Weiner <jweiner@redhat.com>
>> Cc: Dave Hansen <dave.hansen@linux.intel.com>
>> Cc: Michal Hocko <mhocko@suse.com>
>> Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
>> Cc: Matthew Wilcox <willy@infradead.org>
>> Cc: Christoph Lameter <cl@linux.com>
>> ---
>>  include/linux/mmzone.h |  1 +
>>  mm/page_alloc.c        | 22 ++++++++++++++++++++--
>>  2 files changed, 21 insertions(+), 2 deletions(-)
>> 
>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>> index d6cfb5023f3e..8a19e2af89df 100644
>> --- a/include/linux/mmzone.h
>> +++ b/include/linux/mmzone.h
>> @@ -1006,6 +1006,7 @@ enum zone_flags {
>>  					 * Cleared when kswapd is woken.
>>  					 */
>>  	ZONE_RECLAIM_ACTIVE,		/* kswapd may be scanning the zone. */
>> +	ZONE_BELOW_HIGH,		/* zone is below high watermark. */
>>  };
>>  
>>  static inline unsigned long zone_managed_pages(struct zone *zone)
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 225abe56752c..3f8c7dfeed23 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -2409,7 +2409,13 @@ static int nr_pcp_high(struct per_cpu_pages *pcp, struct zone *zone,
>>  		return min(batch << 2, pcp->high);
>>  	}
>>  
>> -	if (pcp->count >= high && high_min != high_max) {
>> +	if (high_min == high_max)
>> +		return high;
>> +
>> +	if (test_bit(ZONE_BELOW_HIGH, &zone->flags)) {
>> +		pcp->high = max(high - (batch << pcp->free_factor), high_min);
>> +		high = max(pcp->count, high_min);
>> +	} else if (pcp->count >= high) {
>>  		int need_high = (batch << pcp->free_factor) + batch;
>>  
>>  		/* pcp->high should be large enough to hold batch freed pages */
>> @@ -2459,6 +2465,10 @@ static void free_unref_page_commit(struct zone *zone, struct per_cpu_pages *pcp,
>>  	if (pcp->count >= high) {
>>  		free_pcppages_bulk(zone, nr_pcp_free(pcp, batch, high, free_high),
>>  				   pcp, pindex);
>> +		if (test_bit(ZONE_BELOW_HIGH, &zone->flags) &&
>> +		    zone_watermark_ok(zone, 0, high_wmark_pages(zone),
>> +				      ZONE_MOVABLE, 0))
>> +			clear_bit(ZONE_BELOW_HIGH, &zone->flags);
>>  	}
>>  }
>>  
>> @@ -2765,7 +2775,7 @@ static int nr_pcp_alloc(struct per_cpu_pages *pcp, struct zone *zone, int order)
>>  	 * If we had larger pcp->high, we could avoid to allocate from
>>  	 * zone.
>>  	 */
>> -	if (high_min != high_max && !test_bit(ZONE_RECLAIM_ACTIVE, &zone->flags))
>> +	if (high_min != high_max && !test_bit(ZONE_BELOW_HIGH, &zone->flags))
>>  		high = pcp->high = min(high + batch, high_max);
>>  
>>  	if (!order) {
>> @@ -3226,6 +3236,14 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,
>>  			}
>>  		}
>>  
>> +		mark = high_wmark_pages(zone);
>> +		if (zone_watermark_fast(zone, order, mark,
>> +					ac->highest_zoneidx, alloc_flags,
>> +					gfp_mask))
>> +			goto try_this_zone;
>> +		else if (!test_bit(ZONE_BELOW_HIGH, &zone->flags))
>> +			set_bit(ZONE_BELOW_HIGH, &zone->flags);
>> +
>
> This absolutely needs a comment explaning why because superficially a
> consequence of this is that allocator performance is slightly degraded
> when below the high watermark. Being below the high watermark is
> completely harmless and can persist indefinitely until something wakes
> kswapd.

Sure.  Will add some comments here.

--
Best Regards,
Huang, Ying