linux-kernel - Re: [PATCH v2] shrink_all_memory() use sc.nr

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Date:	Thu, 12 Feb 2009 22:11:04 +0900
From:	MinChan Kim <minchan.kim@...il.com>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH v2] shrink_all_memory() use sc.nr_reclaimed

On Thu, Feb 12, 2009 at 8:25 PM, Johannes Weiner <hannes@...xchg.org> wrote:
> On Thu, Feb 12, 2009 at 04:33:10PM +0900, MinChan Kim wrote:
>>
>> Impact: cleanup
>>
>> Commit a79311c14eae4bb946a97af25f3e1b17d625985d "vmscan: bail out of
>> direct reclaim after swap_cluster_max pages" moved the nr_reclaimed
>> counter into the scan control to accumulate the number of all
>> reclaimed pages in a reclaim invocation.
>>
>> The shrink_all_memory() can use the same mechanism. it increases code
>> consistency and readability.
>>
>> It's based on mmtom 2009-02-11-17-15.
>>
>> Signed-off-by: MinChan Kim <minchan.kim@...il.com>
>> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
>> Cc: Johannes Weiner <hannes@...xchg.org>
>> Cc: "Rafael J. Wysocki" <rjw@...k.pl>
>> Cc: Rik van Riel <riel@...hat.com>
>>
>>
>> ---
>>  mm/vmscan.c |   51 ++++++++++++++++++++++++++++++---------------------
>>  1 files changed, 30 insertions(+), 21 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index ae4202b..caa2de5 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -2055,16 +2055,15 @@ unsigned long global_lru_pages(void)
>>  #ifdef CONFIG_PM
>>  /*
>>   * Helper function for shrink_all_memory().  Tries to reclaim 'nr_pages' pages
>> - * from LRU lists system-wide, for given pass and priority, and returns the
>> - * number of reclaimed pages
>> + * from LRU lists system-wide, for given pass and priority.
>>   *
>>   * For pass > 3 we also try to shrink the LRU lists that contain a few pages
>>   */
>> -static unsigned long shrink_all_zones(unsigned long nr_pages, int prio,
>> +static void shrink_all_zones(unsigned long nr_pages, int prio,
>>                                     int pass, struct scan_control *sc)
>>  {
>>       struct zone *zone;
>> -     unsigned long ret = 0;
>> +     unsigned long nr_reclaimed = 0;
>
> Why this extra variable?  You could use sc->nr_reclaimed throughout,
> like you do in shrink_all_memory().

It's just for matching shrink_zone style in order to code consistency.
But, I have no objection to remove extra variable.

>
>>       for_each_populated_zone(zone) {
>>               enum lru_list l;
>> @@ -2087,14 +2086,16 @@ static unsigned long shrink_all_zones(unsigned long nr_pages, int prio,
>>
>>                               zone->lru[l].nr_scan = 0;
>>                               nr_to_scan = min(nr_pages, lru_pages);
>> -                             ret += shrink_list(l, nr_to_scan, zone,
>> +                             nr_reclaimed += shrink_list(l, nr_to_scan, zone,
>>                                                               sc, prio);
>> -                             if (ret >= nr_pages)
>> -                                     return ret;
>> +                             if (nr_reclaimed >= nr_pages) {
>> +                                     sc->nr_reclaimed = nr_reclaimed;
>> +                                     return;
>> +                             }
>>                       }
>>               }
>>       }
>> -     return ret;
>> +     sc->nr_reclaimed = nr_reclaimed;
>>  }
>>
>>  /*
>> @@ -2126,13 +2127,15 @@ unsigned long shrink_all_memory(unsigned long nr_pages)
>>       /* If slab caches are huge, it's better to hit them first */
>>       while (nr_slab >= lru_pages) {
>>               reclaim_state.reclaimed_slab = 0;
>> -             shrink_slab(nr_pages, sc.gfp_mask, lru_pages);
>> +             shrink_slab(sc.swap_cluster_max, sc.gfp_mask, lru_pages);
>>               if (!reclaim_state.reclaimed_slab)
>>                       break;
>>
>> -             ret += reclaim_state.reclaimed_slab;
>> -             if (ret >= nr_pages)
>> +             sc.nr_reclaimed += reclaim_state.reclaimed_slab;
>> +             if (sc.nr_reclaimed >= sc.swap_cluster_max) {
>> +                     ret = sc.nr_reclaimed;
>
> Why do you still maintain `ret'?  Just return sc.nr_reclaimed at the
> end and get rid of ret alltogether.

It' just for emphasis on return variable.
Of course, I have no objection to remove 'ret'. ;

> Using sc.swap_cluster_max here seems to be a good idea at first sight
> but really it is not.
>
> Usually, swap_cluster_max is smaller than the reclaim goal and reclaim
> code uses it combined with other conditions to bail out BEFORE the
> original reclaim goal is met.  But sc.swap_cluster_max IS our original
> reclaim goal, so it means something different.
>
> It's btw buggy, we never decrease swap_cluster_max which leads to
> funky overreclaim in shrink_inactive_list().  I will send the original
> patch from Kosaki-san for using sc->nr_reclaimed and a patch for the
> overreclaim problem.
>
>        Hannes
>



-- 
Kinds regards,
MinChan Kim
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