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From: "Huang\, Ying" <ying.huang@intel.com>
To: Yang Shi <yang.shi@linux.alibaba.com>
Cc: <hannes@cmpxchg.org>, <mhocko@suse.com>,
	<mgorman@techsingularity.net>, <kirill.shutemov@linux.intel.com>,
	<josef@toxicpanda.com>, <hughd@google.com>, <shakeelb@google.com>,
	<hdanton@sina.com>, <akpm@linux-foundation.org>,
	<linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [v7 PATCH 2/2] mm: vmscan: correct some vmscan counters for THP swapout
Date: Wed, 29 May 2019 08:57:40 +0800	[thread overview]
Message-ID: <87sgsy6prv.fsf@yhuang-dev.intel.com> (raw)
In-Reply-To: <1559025859-72759-2-git-send-email-yang.shi@linux.alibaba.com> (Yang Shi's message of "Tue, 28 May 2019 14:44:19 +0800")

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.
>
> 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@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>
> Cc: Hillf Danton <hdanton@sina.com>
> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>

Looks good to me!  Thanks for your effort!

Reviewed-by: "Huang, Ying" <ying.huang@intel.com>

Best Regards,
Huang, Ying

  reply	other threads:[~2019-05-29  0:57 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-28  6:44 [v7 PATCH 1/2] mm: vmscan: remove double slab pressure by inc'ing sc->nr_scanned Yang Shi
2019-05-28  6:44 ` [v7 PATCH 2/2] mm: vmscan: correct some vmscan counters for THP swapout Yang Shi
2019-05-29  0:57   ` Huang, Ying [this message]
2019-06-10 21:36 ` [v7 PATCH 1/2] mm: vmscan: remove double slab pressure by inc'ing sc->nr_scanned Oscar Salvador
2019-06-11 17:12   ` Yang Shi
2019-06-11 17:17     ` Yang Shi

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