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Date: Thu, 23 May 2019 21:55:37 +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: [v4 PATCH 2/2] mm: vmscan: correct some vmscan counters for THP
swapout
On 5/23/19 8:52 PM, 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.
>>
>> nr_dirty, nr_unqueued_dirty, nr_congested and nr_writeback are used by
>> file cache, so they are not impacted by THP swap.
>>
>> 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>
>> ---
>> v4: Fixed the comments from Johannes and Huang Ying
>> 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 | 34 ++++++++++++++++++++++------------
>> 1 file changed, 22 insertions(+), 12 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index b65bc50..1b35a7a 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1118,6 +1118,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>> int may_enter_fs;
>> enum page_references references = PAGEREF_RECLAIM_CLEAN;
>> bool dirty, writeback;
>> + unsigned int nr_pages;
>>
>> cond_resched();
>>
>> @@ -1129,7 +1130,9 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>>
>> VM_BUG_ON_PAGE(PageActive(page), page);
>>
>> - sc->nr_scanned++;
>> + /* Account the number of base pages evne though THP */
> s/evne/even/
>
>> + nr_pages = 1 << compound_order(page);
>> + sc->nr_scanned += nr_pages;
>>
>> if (unlikely(!page_evictable(page)))
>> goto activate_locked;
>> @@ -1250,7 +1253,7 @@ 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 += nr_pages;
>> goto keep_locked;
>> case PAGEREF_RECLAIM:
>> case PAGEREF_RECLAIM_CLEAN:
> If the THP is split, you need
>
> sc->nr_scanned -= nr_pages - 1;
>
> Otherwise the tail pages will be counted twice.
Yes, it looks so.
>
>> @@ -1315,7 +1318,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>> if (unlikely(PageTransHuge(page)))
>> flags |= TTU_SPLIT_HUGE_PMD;
>> if (!try_to_unmap(page, flags)) {
>> - stat->nr_unmap_fail++;
>> + stat->nr_unmap_fail += nr_pages;
>> goto activate_locked;
>> }
>> }
>> @@ -1442,7 +1445,11 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>>
>> unlock_page(page);
>> free_it:
>> - nr_reclaimed++;
>> + /*
>> + * THP may get swapped out in a whole, need account
>> + * all base pages.
>> + */
>> + nr_reclaimed += (1 << compound_order(page));
> Best Regards,
> Huang, Ying
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