Re: [PATCH] mm/compaction: do not count migratepages when unnecessary

[Vlastimil Babka <vbabka@suse.cz>]

On 05/06/2014 11:18 PM, Naoya Horiguchi wrote:
> On Fri, May 02, 2014 at 05:26:18PM +0200, Vlastimil Babka wrote:
>> During compaction, update_nr_listpages() has been used to count remaining
>> non-migrated and free pages after a call to migrage_pages(). The freepages
>> counting has become unneccessary, and it turns out that migratepages counting
>> is also unnecessary in most cases.
>>
>> The only situation when it's needed to count cc->migratepages is when
>> migrate_pages() returns with a negative error code. Otherwise, the non-negative
>> return value is the number of pages that were not migrated, which is exactly
>> the count of remaining pages in the cc->migratepages list.
>>
>> Furthermore, any non-zero count is only interesting for the tracepoint of
>> mm_compaction_migratepages events, because after that all remaining unmigrated
>> pages are put back and their count is set to 0.
>>
>> This patch therefore removes update_nr_listpages() completely, and changes the
>> tracepoint definition so that the manual counting is done only when the
>> tracepoint is enabled, and only when migrate_pages() returns a negative error
>> code.
>>
>> Furthermore, migrate_pages() and the tracepoints won't be called when there's
>> nothing to migrate. This potentially avoids some wasted cycles and reduces the
>> volume of uninteresting mm_compaction_migratepages events where "nr_migrated=0
>> nr_failed=0". In the stress-highalloc mmtest, this was about 75% of the events.
>> The mm_compaction_isolate_migratepages event is better for determining that
>> nothing was isolated for migration, and this one was just duplicating the info.
>
> I agree with this patch.
> Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>

Thanks.

> A few nitpicks below...
>
>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
>> Cc: Minchan Kim <minchan@kernel.org>
>> Cc: Mel Gorman <mgorman@suse.de>
>> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
>> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
>> Cc: Christoph Lameter <cl@linux.com>
>> Cc: Rik van Riel <riel@redhat.com>
>> ---
>>   include/trace/events/compaction.h | 26 ++++++++++++++++++++++----
>>   mm/compaction.c                   | 30 ++++++++----------------------
>>   2 files changed, 30 insertions(+), 26 deletions(-)
>>
>> diff --git a/include/trace/events/compaction.h b/include/trace/events/compaction.h
>> index 06f544e..bbd5e1f 100644
>> --- a/include/trace/events/compaction.h
>> +++ b/include/trace/events/compaction.h
>> @@ -5,7 +5,9 @@
>>   #define _TRACE_COMPACTION_H
>>
>>   #include <linux/types.h>
>> +#include <linux/list.h>
>>   #include <linux/tracepoint.h>
>> +#include <linux/mm_types.h>
>>   #include <trace/events/gfpflags.h>
>>
>>   DECLARE_EVENT_CLASS(mm_compaction_isolate_template,
>> @@ -47,10 +49,11 @@ DEFINE_EVENT(mm_compaction_isolate_template, mm_compaction_isolate_freepages,
>>
>>   TRACE_EVENT(mm_compaction_migratepages,
>>
>> -	TP_PROTO(unsigned long nr_migrated,
>> -		unsigned long nr_failed),
>> +	TP_PROTO(unsigned long nr_all,
>> +		int migrate_rc,
>> +		struct list_head * migratepages),
>
> checkpatch.pl shows code violation message for this line.
> (ERROR: "foo * bar" should be "foo *bar")
>
>>
>> -	TP_ARGS(nr_migrated, nr_failed),
>> +	TP_ARGS(nr_all, migrate_rc, migratepages),
>>
>>   	TP_STRUCT__entry(
>>   		__field(unsigned long, nr_migrated)
>> @@ -58,7 +61,22 @@ TRACE_EVENT(mm_compaction_migratepages,
>>   	),
>>
>>   	TP_fast_assign(
>> -		__entry->nr_migrated = nr_migrated;
>> +		unsigned long nr_failed = 0;
>> +		struct page *page;
>> +
>> +		/*
>> +		 * migrate_pages() returns either a non-negative number
>> +		 * with the number of pages that failed migration, or an
>> +		 * error code, in which case we need to count the remaining
>> +		 * pages manually
>> +		 */
>> +		if (migrate_rc >= 0)
>> +			nr_failed = migrate_rc;
>> +		else
>> +		        list_for_each_entry(page, migratepages, lru)
>
> This line contains whitespace indent.

Oops, will fix those checkpatchs...

>> +				nr_failed++;
>> +
>> +		__entry->nr_migrated = nr_all - nr_failed;
>>   		__entry->nr_failed = nr_failed;
>>   	),
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index ae1d0ae..873d7de 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -799,22 +799,6 @@ static void compaction_free(struct page *page, unsigned long data)
>>   	cc->nr_freepages++;
>>   }
>>
>> -/*
>> - * We cannot control nr_migratepages fully when migration is running as
>> - * migrate_pages() has no knowledge of of compact_control.  When migration is
>> - * complete, we count the number of pages on the list by hand.
>> - */
>> -static void update_nr_listpages(struct compact_control *cc)
>> -{
>> -	int nr_migratepages = 0;
>> -	struct page *page;
>> -
>> -	list_for_each_entry(page, &cc->migratepages, lru)
>> -		nr_migratepages++;
>> -
>> -	cc->nr_migratepages = nr_migratepages;
>> -}
>> -
>>   /* possible outcome of isolate_migratepages */
>>   typedef enum {
>>   	ISOLATE_ABORT,		/* Abort compaction now */
>> @@ -1006,7 +990,6 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
>>   	migrate_prep_local();
>>
>>   	while ((ret = compact_finished(zone, cc)) == COMPACT_CONTINUE) {
>> -		unsigned long nr_migrate, nr_remaining;
>>   		int err;
>>
>>   		switch (isolate_migratepages(zone, cc)) {
>> @@ -1021,16 +1004,16 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
>>   			;
>>   		}
>>
>> -		nr_migrate = cc->nr_migratepages;
>> +		if (!cc->nr_migratepages)
>> +			continue;
>> +
>>   		err = migrate_pages(&cc->migratepages, compaction_alloc,
>>   				compaction_free, (unsigned long)cc,
>>   				cc->sync ? MIGRATE_SYNC_LIGHT : MIGRATE_ASYNC,
>>   				MR_COMPACTION);
>> -		update_nr_listpages(cc);
>> -		nr_remaining = cc->nr_migratepages;
>>
>> -		trace_mm_compaction_migratepages(nr_migrate - nr_remaining,
>> -						nr_remaining);
>> +		trace_mm_compaction_migratepages(cc->nr_migratepages, err,
>> +							&cc->migratepages);
>>
>>   		/* Release isolated pages not migrated */
>>   		if (err) {
>> @@ -1044,6 +1027,9 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
>>   				ret = COMPACT_PARTIAL;
>>   				goto out;
>>   			}
>> +		} else {
>> +			/* All pages were successfully migrated */
>> +			cc->nr_migratepages = 0;
>
> cc->nr_migratepages = 0 is also done in err != 0, so can it be done in common path?

Well it would have to be done before 'if (err)', thus also before 
putback_movable_pages(), which is a bit awkward. But avoiding 'else' is 
also good I guess so I'll do that for next version.

> Thanks,
> Naoya Horiguchi
>

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