linux-kernel - Re: [PATCH v4 0/4] percpu: partial chunk depopulation

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Date:   Tue, 20 Apr 2021 16:37:02 +0530
From:   Pratik Sampat <psampat@...ux.ibm.com>
To:     Dennis Zhou <dennis@...nel.org>
Cc:     Roman Gushchin <guro@...com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 0/4] percpu: partial chunk depopulation


On 20/04/21 4:27 am, Dennis Zhou wrote:
> On Mon, Apr 19, 2021 at 10:50:43PM +0000, Dennis Zhou wrote:
>> Hello,
>>
>> This series is a continuation of Roman's series in [1]. It aims to solve
>> chunks holding onto free pages by adding a reclaim process to the percpu
>> balance work item.
>>
>> The main difference is that the nr_empty_pop_pages is now managed at
>> time of isolation instead of intermixed. This helps with deciding which
>> chunks to free instead of having to interleave returning chunks to
>> active duty.
>>
>> The allocation priority is as follows:
>>    1) appropriate chunk slot increasing until fit
>>    2) sidelined chunks
>>    3) full free chunks
>>
>> The last slot for to_depopulate is never used for allocations.
>>
>> A big thanks to Roman for initiating the work and being available for
>> iterating on these ideas.
>>
>> This patchset contains the following 4 patches:
>>    0001-percpu-factor-out-pcpu_check_block_hint.patch
>>    0002-percpu-use-pcpu_free_slot-instead-of-pcpu_nr_slots-1.patch
>>    0003-percpu-implement-partial-chunk-depopulation.patch
>>    0004-percpu-use-reclaim-threshold-instead-of-running-for-.patch
>>
>> 0001 and 0002 are clean ups. 0003 implement partial chunk depopulation
>> initially from Roman. 0004 adds a reclaim threshold so we do not need to
>> schedule for every page freed.
>>
>> This series is on top of percpu$for-5.14 67c2669d69fb.
>>
>> diffstats below:
>>
>> Dennis Zhou (2):
>>    percpu: use pcpu_free_slot instead of pcpu_nr_slots - 1
>>    percpu: use reclaim threshold instead of running for every page
>>
>> Roman Gushchin (2):
>>    percpu: factor out pcpu_check_block_hint()
>>    percpu: implement partial chunk depopulation
>>
>>   mm/percpu-internal.h |   5 +
>>   mm/percpu-km.c       |   5 +
>>   mm/percpu-stats.c    |  20 ++--
>>   mm/percpu-vm.c       |  30 ++++++
>>   mm/percpu.c          | 252 ++++++++++++++++++++++++++++++++++++++-----
>>   5 files changed, 278 insertions(+), 34 deletions(-)
>>
>> Thanks,
>> Dennis
> Hello Pratik,
>
> Do you mind testing this series again on POWER9? The base is available
> here:
> https://git.kernel.org/pub/scm/linux/kernel/git/dennis/percpu.git/log/?h=for-5.14
>
> Thanks,
> Dennis

Hello Dennis, I have tested this patchset on POWER9.

I have tried variations of the percpu_test in the top level and nested cgroups
creation as the test with 1000:10 didn't show any benefits.

The following example shows more consistent benefits with the de-allocation
strategy.
Outer: 1000
Inner: 50
# ./percpu_test.sh
Percpu:             6912 kB
Percpu:           532736 kB
Percpu:           278784 kB

I believe it could be a result of bulk freeing within "free_unref_page_commit",
where pages are only free'd if pcp->count >= pcp->high. As POWER has a larger
page size it would end up creating lesser number of pages but with the
effects of fragmentation.

Having said that, the patchset and its behavior does look good to me.

Thanks!
Pratik