Re: [RESEND v5 PATCH 2/2] mm: vmscan: correct some vmscan counters for THP swapout

[Yang Shi <yang.shi@linux.alibaba.com>]

On 5/27/19 10:58 AM, Huang, Ying wrote:
> Yang Shi <yang.shi@linux.alibaba.com> writes:
>
>> On 5/27/19 10:11 AM, Huang, Ying wrote:
>>> 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>
>>>> Signed-off-by: Yang Shi <yang.shi@linux.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.
> Add keep_locked label.
>
>> 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.
> I think we can do that in one place too.  Just before try_to_unmap() via
> checking nr_pages and page order.  And we need to update nr_pages if
> the THP is split anyway.

Yes, I agree.

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