On 9 Apr 2024, at 5:31, Baolin Wang wrote:
> On 2024/4/8 22:23, Johannes Weiner wrote:
>> On Mon, Apr 08, 2024 at 09:38:20AM +0200, Vlastimil Babka wrote:
>>> On 4/7/24 12:19 PM, Baolin Wang wrote:
>>>> On 2024/3/21 02:02, Johannes Weiner wrote:
>>>>> + account_freepages(page, zone, 1 << order, migratetype);
>>>>> +
>>>>> while (order < MAX_PAGE_ORDER) {
>>>>> - if (compaction_capture(capc, page, order, migratetype)) {
>>>>> - __mod_zone_freepage_state(zone, -(1 << order),
>>>>> - migratetype);
>>>>> + int buddy_mt = migratetype;
>>>>> +
>>>>> + if (compaction_capture(capc, page, order, migratetype))
>>>>> return;
>>>>> - }
>>>>
>>>> IIUC, if the released page is captured by compaction, then the
>>>> statistics for free pages should be correspondingly decreased,
>>>> otherwise, there will be a slight regression for my thpcompact benchmark.
>>>>
>>>> thpcompact Percentage Faults Huge
>>>> k6.9-rc2-base base + patch10 + 2 fixes
>>>> Percentage huge-1 78.18 ( 0.00%) 71.92 ( -8.01%)
>>>> Percentage huge-3 86.70 ( 0.00%) 86.07 ( -0.73%)
>>>> Percentage huge-5 90.26 ( 0.00%) 78.02 ( -13.57%)
>>>> Percentage huge-7 92.34 ( 0.00%) 78.67 ( -14.81%)
>>>> Percentage huge-12 91.18 ( 0.00%) 81.04 ( -11.12%)
>>>> Percentage huge-18 89.00 ( 0.00%) 79.57 ( -10.60%)
>>>> Percentage huge-24 90.52 ( 0.00%) 80.07 ( -11.54%)
>>>> Percentage huge-30 94.44 ( 0.00%) 96.28 ( 1.95%)
>>>> Percentage huge-32 93.09 ( 0.00%) 99.39 ( 6.77%)
>>>>
>>>> I add below fix based on your fix 2, then the thpcompact Percentage
>>>> looks good. How do you think for the fix?
>>>
>>> Yeah another well spotted, thanks. "slight regression" is an understatement,
>>> this affects not just a "statistics" but very important counter
>>> NR_FREE_PAGES which IIUC would eventually become larger than reality, make
>>> the watermark checks false positive and result in depleted reserves etc etc.
>>> Actually wondering why we're not seeing -next failures already (or maybe I
>>> just haven't noticed).
>>
>> Good catch indeed.
>>
>> Trying to understand why I didn't notice this during testing, and I
>> think it's because I had order-10 pageblocks in my config. There is
>> this in compaction_capture():
>>
>> if (order < pageblock_order && migratetype == MIGRATE_MOVABLE)
>> return false;
>>
>> Most compaction is for order-9 THPs on movable blocks, so I didn't get
>> much capturing in practice in order for that leak to be noticable.
>
> This makes me wonder why not use 'cc->migratetype' for migratetype comparison, so that low-order (like mTHP) compaction can directly get the released pages, which could avoid some compaction scans without mixing the migratetype?
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 2facf844ef84..7a64020f8222 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -622,7 +622,7 @@ compaction_capture(struct capture_control *capc, struct page *page,
> * and vice-versa but no more than normal fallback logic which can
> * have trouble finding a high-order free page.
> */
> - if (order < pageblock_order && migratetype == MIGRATE_MOVABLE)
> + if (order < pageblock_order && capc->cc->migratetype != migratetype)
> return false;
>
> capc->page = page;
It is worth trying, since at the original patch time mTHP was not present and
not capturing any MIGRATE_MOVABLE makes sense. But with your change, the capture
will lose the opportunity of letting an unmovable request use a reclaimable
pageblock and vice-versa, like the comment says. Please change the comment
as well and we should monitor potential unmovable and reclaimable regression.
--
Best Regards,
Yan, Zi
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