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Date: Mon, 27 May 2019 15:55:41 +0800
From: "Huang\, Ying" <ying.huang@...el.com>
To: Yang Shi <yang.shi@...ux.alibaba.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: [v6 PATCH 2/2] mm: vmscan: correct some vmscan counters for THP swapout
Yang Shi <yang.shi@...ux.alibaba.com> writes:
> On 5/27/19 3:06 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>
>>> Cc: Hillf Danton <hdanton@...a.com>
>>> Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>
>>> ---
>>> v6: Fixed the other double account issue introduced by v5 per Huang Ying
>>> 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 | 47 +++++++++++++++++++++++++++++++++++------------
>>> 1 file changed, 35 insertions(+), 12 deletions(-)
>>>
>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>> index b65bc50..378edff 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 +1130,10 @@ static unsigned long
>>> shrink_page_list(struct list_head *page_list,
>>> VM_BUG_ON_PAGE(PageActive(page), page);
>>> - sc->nr_scanned++;
>>> + nr_pages = 1 << compound_order(page);
>>> +
>>> + /* Account the number of base pages even though THP */
>>> + sc->nr_scanned += nr_pages;
>>> if (unlikely(!page_evictable(page)))
>>> goto activate_locked;
>>> @@ -1250,7 +1254,7 @@ 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:
>>> @@ -1306,6 +1310,15 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>>> }
>>> /*
>>> + * THP may get split above, need minus tail pages and update
>>> + * nr_pages to avoid accounting tail pages twice.
>>> + */
>>> + if ((nr_pages > 1) && !PageTransHuge(page)) {
>>> + sc->nr_scanned -= (nr_pages - 1);
>>> + nr_pages = 1;
>>> + }
>> After checking the code again, it appears there's another hole in the
>> code. In the following code snippet.
>>
>> if (!add_to_swap(page)) {
>> if (!PageTransHuge(page))
>> goto activate_locked;
>> /* Fallback to swap normal pages */
>> if (split_huge_page_to_list(page,
>> page_list))
>> goto activate_locked;
>> #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>> count_vm_event(THP_SWPOUT_FALLBACK);
>> #endif
>> if (!add_to_swap(page))
>> goto activate_locked;
>> }
>>
>>
>> If the THP is split, but the first or the second add_to_swap() fails, we
>> still need to deal with sc->nr_scanned and nr_pages.
>>
>> How about add a new label before "activate_locked" to deal with that?
>
> It sounds not correct. If swapout fails it jumps to activate_locked
> too, it has to be handled in if (!add_to_swap(page)). The below fix
> should be good enough since only THP can reach here:
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 378edff..fff3937 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1294,8 +1294,15 @@ static unsigned long shrink_page_list(struct
> list_head *page_list,
> #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> count_vm_event(THP_SWPOUT_FALLBACK);
> #endif
> - if (!add_to_swap(page))
> + if (!add_to_swap(page)) {
> + /*
> + * Minus tail pages
> and reset
> + * nr_pages.
> + */
> + sc->nr_scanned -=
> (nr_pages - 1);
> + nr_pages = 1;
> goto activate_locked;
> + }
> }
I think you need to add similar logic for the first add_to_swap() in the
original code snippet.
>> if (!add_to_swap(page)) {
>> if (!PageTransHuge(page))
To reduce code duplication, I suggest to add another label to deal with
it.
activate_locked_split:
if (nr_pages > 1) {
sc->nr_scanned -= nr_pages - 1;
nr_pages = 1;
}
activate_locked:
And use "goto active_locked_split" if add_to_swap() failed.
Best Regards,
Huang, Ying
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