Re: [PATCH] Revert "mm: vmscan: fix misused nr_reclaimed in shrink_mem_cgroup_zone()"

[Ying Han <yinghan@google.com>]

On Mon, Apr 9, 2012 at 2:23 PM, Ying Han <yinghan@google.com> wrote:
> On Mon, Apr 9, 2012 at 12:50 PM, Andrew Morton
> <akpm@linux-foundation.org> wrote:
>> On Mon,  9 Apr 2012 12:42:04 -0700
>> Ying Han <yinghan@google.com> wrote:
>>
>>> This reverts commit c38446cc65e1f2b3eb8630c53943b94c4f65f670.
>>>
>>> Before the commit, the code makes senses to me but not after the commit. The
>>> "nr_reclaimed" is the number of pages reclaimed by scanning through the memcg's
>>> lru lists. The "nr_to_reclaim" is the target value for the whole function. For
>>> example, we like to early break the reclaim if reclaimed 32 pages under direct
>>> reclaim (not DEF_PRIORITY).
>>>
>>> After the reverted commit, the target "nr_to_reclaim" is decremented each time
>>> by "nr_reclaimed" but we still use it to compare the "nr_reclaimed". It just
>>> doesn't make sense to me...
>>>
>>> Signed-off-by: Ying Han <yinghan@google.com>
>>> ---
>>>  mm/vmscan.c |    7 +------
>>>  1 files changed, 1 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>> index 33c332b..1a51868 100644
>>> --- a/mm/vmscan.c
>>> +++ b/mm/vmscan.c
>>> @@ -2107,12 +2107,7 @@ restart:
>>>                * with multiple processes reclaiming pages, the total
>>>                * freeing target can get unreasonably large.
>>>                */
>>> -             if (nr_reclaimed >= nr_to_reclaim)
>>> -                     nr_to_reclaim = 0;
>>> -             else
>>> -                     nr_to_reclaim -= nr_reclaimed;
>>> -
>>> -             if (!nr_to_reclaim && priority < DEF_PRIORITY)
>>> +             if (nr_reclaimed >= nr_to_reclaim && priority < DEF_PRIORITY)
>>>                       break;
>>>       }
>>>       blk_finish_plug(&plug);
>>
>> This code is all within a loop: the "goto restart" thing.  We reset
>> nr_reclaimed to zero each time around that loop.  nr_to_reclaim is (or
>> rather, was) constant throughout the entire function.
>>
>> Comparing nr_reclaimed (whcih is reset each time around the loop) to
>> nr_to_reclaim made no sense.
>>
>> I think the code as it stands is ugly.  It would be better to make
>> nr_to_reclaim a const and to add another local total_reclaimed, and
>> compare that with nr_to_reclaim.
>
> Ok, I will resend the patch w/ the "total_reclaimed" change.
>
> --Ying


I have the patch ready but I am not sure if that is what we want. If
we use total_reclaimed to compare w/ nr_to_reclaim, we end up reducing
the amount of work to reclaim before
compaction(should_continue_reclaim() is true case).

--Ying

>
> Or just stop resetting nr_reclaimed
>> each time around the loop.
>>

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