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

Vlastimil Babka
2021-09-06T14:40:31Z

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).