linux-kernel - Re: [PATCH RESEND 0/8] hugetlb: add demote/split page functionality

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Date:   Mon, 6 Sep 2021 16:40:28 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        David Hildenbrand <david@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Oscar Salvador <osalvador@...e.de>, Zi Yan <ziy@...dia.com>,
        Muchun Song <songmuchun@...edance.com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        David Rientjes <rientjes@...gle.com>,
        Hillf Danton <hdanton@...a.com>
Subject: Re: [PATCH RESEND 0/8] hugetlb: add demote/split page functionality

On 9/2/21 20:17, Mike Kravetz wrote:
> On 8/30/21 3:11 AM, Vlastimil Babka wrote:
>> On 8/28/21 01:04, Mike Kravetz wrote:
>>> On 8/27/21 10:22 AM, Vlastimil Babka wrote:
>>> I 'may' have been over stressing the system with all CPUs doing file
>>> reads to fill the page cache with clean pages.  I certainly need to
>>> spend some more debug/analysis time on this.
>>
>> Hm that *could* play a role, as these will allow reclaim to make progress, but
>> also the reclaimed pages might be stolen immediately and compaction will return
>> COMPACT_SKIPPED and in should_compact_retry() we might go through this code path:
>>
>>         /*      
>>          * compaction was skipped because there are not enough order-0 pages
>>          * to work with, so we retry only if it looks like reclaim can help.
>>          */
>>         if (compaction_needs_reclaim(compact_result)) {
>>                 ret = compaction_zonelist_suitable(ac, order, alloc_flags);
>>                 goto out;
>>         }       
>>
>> where compaction_zonelist_suitable() will return true because it appears
>> reclaim can free pages to allow progress. And there are no max retries
>> applied for this case.
>> With the reclaim and compaction tracepoints it should be possible to
>> confirm this scenario.
> 
> Here is some very high level information from a long stall that was
> interrupted.  This was an order 9 allocation from alloc_buddy_huge_page().
> 
> 55269.530564] __alloc_pages_slowpath: jiffies 47329325 tries 609673 cpu_tries 1   node 0 FAIL
> [55269.539893]     r_tries 25       c_tries 609647   reclaim 47325161 compact 607     
> 
> Yes, in __alloc_pages_slowpath for 47329325 jiffies before being interrupted.
> should_reclaim_retry returned true 25 times and should_compact_retry returned
> true 609647 times.
> Almost all time (47325161 jiffies) spent in __alloc_pages_direct_reclaim, and
> 607 jiffies spent in __alloc_pages_direct_compact.
> 
> Looks like both
> reclaim retries > MAX_RECLAIM_RETRIES
> and
> compaction retries > MAX_COMPACT_RETRIES
> 

Yeah AFAICS that's only possible with the scenario I suspected. I guess
we should put a limit on compact retries (maybe some multiple of
MAX_COMPACT_RETRIES) even if it thinks that reclaim could help, while
clearly it doesn't (i.e. because somebody else is stealing the page like
in your test case).