Date:   Mon, 27 May 2019 10:59:55 +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, hdanton@...a.com,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RESEND v5 PATCH 2/2] mm: vmscan: correct some vmscan counters
 for THP swapout



On 5/27/19 10:47 AM, Yang Shi wrote:
>
>
> On 5/27/19 10:11 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.
>>>
>>> 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>
>>> ---
>>> v5: Fixed sc->nr_scanned double accounting per Huang Ying
>>>      Added some comments to address the concern about premature OOM 
>>> per Hillf Danton
>>> 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 | 42 +++++++++++++++++++++++++++++++-----------
>>>   1 file changed, 31 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>> index b65bc50..f4f4d57 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();
>>>   @@ -1129,6 +1130,13 @@ static unsigned long 
>>> shrink_page_list(struct list_head *page_list,
>>>             VM_BUG_ON_PAGE(PageActive(page), page);
>>>   +        nr_pages = 1 << compound_order(page);
>>> +
>>> +        /*
>>> +         * Accounted one page for THP for now.  If THP gets swapped
>>> +         * out in a whole, will account all tail pages later to
>>> +         * avoid accounting tail pages twice.
>>> +         */
>>>           sc->nr_scanned++;
>>>             if (unlikely(!page_evictable(page)))
>>> @@ -1250,7 +1258,7 @@ 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 "Accessed bit" of a THP is set in the page table that maps it, it
>> will go PAGEREF_ACTIVATE path here.  And the sc->nr_scanned should
>> increase 512 instead of 1.  Otherwise sc->nr_activate may be larger than
>> sc->nr_scanned.
>
> Yes, it looks so. It seems the easiest way is to add "nr_pages - 1" in 
> activate_locked label if the page is still a THP.
>
> If we add all tail pages at the very beginning, then we have to minus 
> tail pages when THP gets split, there are a few places do this.

Reiterating all the goto, it seems minus tail pages may be easier since 
there are a lot places which goto activate_locked, keep_locked and keep 
after THP has been added into swap cache. We can't tell if the tail 
pages are accounted or not unless we introduce a new flag, otherwise 
adding tails pages in those labels may account tail pages twice.

>
>>
>> Best Regards,
>> Huang, Ying
>

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