linux-kernel - Re: [v3 PATCH 2/2] mm: vmscan: correct some vmscan counters for THP swapout

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Date:   Wed, 22 May 2019 11:26:17 +0800
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     hannes@...xchg.org, mhocko@...e.com, mgorman@...hsingularity.net,
        kirill.shutemov@...ux.intel.com, josef@...icpanda.com,
        hughd@...gle.com, shakeelb@...gle.com, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [v3 PATCH 2/2] mm: vmscan: correct some vmscan counters for THP
 swapout



On 5/22/19 9:23 AM, Huang, Ying wrote:
> Yang Shi <yang.shi@...ux.alibaba.com> writes:
>
>> Since commit bd4c82c22c36 ("mm, THP, swap: delay splitting THP after
>> swapped out"), THP can be swapped out in a whole.  But, nr_reclaimed
>> and some other vm counters still get inc'ed by one even though a whole
>> THP (512 pages) gets swapped out.
>>
>> This doesn't make too much sense to memory reclaim.  For example, direct
>> reclaim may just need reclaim SWAP_CLUSTER_MAX pages, reclaiming one THP
>> could fulfill it.  But, if nr_reclaimed is not increased correctly,
>> direct reclaim may just waste time to reclaim more pages,
>> SWAP_CLUSTER_MAX * 512 pages in worst case.
>>
>> And, it may cause pgsteal_{kswapd|direct} is greater than
>> pgscan_{kswapd|direct}, like the below:
>>
>> pgsteal_kswapd 122933
>> pgsteal_direct 26600225
>> pgscan_kswapd 174153
>> pgscan_direct 14678312
>>
>> nr_reclaimed and nr_scanned must be fixed in parallel otherwise it would
>> break some page reclaim logic, e.g.
>>
>> vmpressure: this looks at the scanned/reclaimed ratio so it won't
>> change semantics as long as scanned & reclaimed are fixed in parallel.
>>
>> compaction/reclaim: compaction wants a certain number of physical pages
>> freed up before going back to compacting.
>>
>> kswapd priority raising: kswapd raises priority if we scan fewer pages
>> than the reclaim target (which itself is obviously expressed in order-0
>> pages). As a result, kswapd can falsely raise its aggressiveness even
>> when it's making great progress.
>>
>> Other than nr_scanned and nr_reclaimed, some other counters, e.g.
>> pgactivate, nr_skipped, nr_ref_keep and nr_unmap_fail need to be fixed
>> too since they are user visible via cgroup, /proc/vmstat or trace
>> points, otherwise they would be underreported.
>>
>> When isolating pages from LRUs, nr_taken has been accounted in base
>> page, but nr_scanned and nr_skipped are still accounted in THP.  It
>> doesn't make too much sense too since this may cause trace point
>> underreport the numbers as well.
>>
>> So accounting those counters in base page instead of accounting THP as
>> one page.
>>
>> This change may result in lower steal/scan ratio in some cases since
>> THP may get split during page reclaim, then a part of tail pages get
>> reclaimed instead of the whole 512 pages, but nr_scanned is accounted
>> by 512, particularly for direct reclaim.  But, this should be not a
>> significant issue.
>>
>> Cc: "Huang, Ying" <ying.huang@...el.com>
>> Cc: Johannes Weiner <hannes@...xchg.org>
>> Cc: Michal Hocko <mhocko@...e.com>
>> Cc: Mel Gorman <mgorman@...hsingularity.net>
>> Cc: "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
>> Cc: Hugh Dickins <hughd@...gle.com>
>> Cc: Shakeel Butt <shakeelb@...gle.com>
>> Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>
>> ---
>> v3: Removed Shakeel's Reviewed-by since the patch has been changed significantly
>>      Switched back to use compound_order per Matthew
>>      Fixed more counters per Johannes
>> v2: Added Shakeel's Reviewed-by
>>      Use hpage_nr_pages instead of compound_order per Huang Ying and William Kucharski
>>
>>   mm/vmscan.c | 40 ++++++++++++++++++++++++++++------------
>>   1 file changed, 28 insertions(+), 12 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index b65bc50..1044834 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1250,7 +1250,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>>   		case PAGEREF_ACTIVATE:
>>   			goto activate_locked;
>>   		case PAGEREF_KEEP:
>> -			stat->nr_ref_keep++;
>> +			stat->nr_ref_keep += (1 << compound_order(page));
>>   			goto keep_locked;
>>   		case PAGEREF_RECLAIM:
>>   		case PAGEREF_RECLAIM_CLEAN:
>> @@ -1294,6 +1294,17 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>>   						goto activate_locked;
>>   				}
>>   
>> +				/*
>> +				 * Account all tail pages when THP is added
>> +				 * into swap cache successfully.
>> +				 * The head page has been accounted at the
>> +				 * first place.
>> +				 */
>> +				if (PageTransHuge(page))
>> +					sc->nr_scanned +=
>> +						((1 << compound_order(page)) -
>> +							1);
>> +
> The "if" here could be changed to "else if" because if add_to_swap()
> fails we don't need to call PageTransHuge() here.  But this isn't a big
> deal.

This could be moved to the beginning according to Johannes.

>
> You have analyzed the code and found that nr_dirty, nr_unqueued_dirty,
> nr_congested and nr_writeback are file cache related and not impacted by
> THP swap out.  How about add your findings in the patch description?

Yes, sure. Will add in v4.

>
> Best Regards,
> Huang, Ying
>