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

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

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>

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.

> +				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?

Thanks,
Naoya Horiguchi

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