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

Mike Kravetz
2021-09-08T21:00:41Z

On 9/7/21 1:50 AM, Hillf Danton wrote:
> On Mon, 6 Sep 2021 16:40:28 +0200 Vlastimil Babka wrote:
>> On 9/2/21 20:17, Mike Kravetz wrote:
>>>
>>> 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).
>
> And/or clamp reclaim retries for costly orders
>
> reclaim retries = MAX_RECLAIM_RETRIES - order;
>
> to pull down the chance for stall as low as possible.

Thanks, and sorry for not replying quickly. I only get back to this as
time allows.

We could clamp the number of compaction and reclaim retries in
__alloc_pages_slowpath as suggested. However, I noticed that a single
reclaim call could take a bunch of time. As a result, I instrumented
shrink_node to see what might be happening. Here is some information
from a long stall. Note that I only dump stats when jiffies > 100000.

[ 8136.874706] shrink_node: 507654 total jiffies, 3557110 tries
[ 8136.881130] 130596341 reclaimed, 32 nr_to_reclaim
[ 8136.887643] compaction_suitable results:
[ 8136.893276] idx COMPACT_SKIPPED, 3557109
[ 8672.399839] shrink_node: 522076 total jiffies, 3466228 tries
[ 8672.406268] 124427720 reclaimed, 32 nr_to_reclaim
[ 8672.412782] compaction_suitable results:
[ 8672.418421] idx COMPACT_SKIPPED, 3466227
[ 8908.099592] __alloc_pages_slowpath: jiffies 2939938 tries 17068 cpu_tries 1 node 0 success
[ 8908.109120] r_tries 11 c_tries 17056 reclaim 2939865 compact 9

In this case, clamping the number of retries from should_compact_retry
and should_reclaim_retry could help. Mostly because we will not be
calling back into the reclaim code? Notice the long amount of time spent
in shrink_node. The 'tries' in shrink_node come about from that:

if (should_continue_reclaim(pgdat, sc->nr_reclaimed - nr_reclaimed,
sc))
goto again;

compaction_suitable results is the values returned from calls to
should_continue_reclaim -> compaction_suitable.

Trying to think if there might be an intelligent way to quit early.
--
Mike Kravetz