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SubjectRe: [v3 PATCH 2/2] mm: vmscan: correct some vmscan counters for THP swapout
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Yang Shi <yang.shi@linux.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@intel.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Shakeel Butt <shakeelb@google.com>
> Signed-off-by: Yang Shi <yang.shi@linux.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.

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?

Best Regards,
Huang, Ying


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