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

Mike Kravetz
2021-09-02T18:17:39Z

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