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

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Date:   Thu, 2 Sep 2021 11:17:06 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Vlastimil Babka <vbabka@...e.cz>,
        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 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
-- 
Mike Kravetz