mm: compaction: Add scanned and isolated counters for compaction

Compaction already has tracepoints to count scanned and isolated pages
but it requires that ftrace be enabled and if that information has to be
written to disk then it can be disruptive. This patch adds vmstat counters
for compaction called compact_migrate_scanned, compact_free_scanned and
compact_isolated.

With these counters, it is possible to define a basic cost model for
compaction. This approximates of how much work compaction is doing and can
be compared that with an oprofile showing TLB misses and see if the cost of
compaction is being offset by THP for example. Minimally a compaction patch
can be evaluated in terms of whether it increases or decreases cost. The
basic cost model looks like this

Fundamental unit u:	a word	sizeof(void *)

Ca  = cost of struct page access = sizeof(struct page) / u

Cmc = Cost migrate page copy = (Ca + PAGE_SIZE/u) * 2
Cmf = Cost migrate failure   = Ca * 2
Ci  = Cost page isolation    = (Ca + Wi)
	where Wi is a constant that should reflect the approximate
	cost of the locking operation.

Csm = Cost migrate scanning = Ca
Csf = Cost free    scanning = Ca

Overall cost =	(Csm * compact_migrate_scanned) +
	      	(Csf * compact_free_scanned)    +
	      	(Ci  * compact_isolated)	+
		(Cmc * pgmigrate_success)	+
		(Cmf * pgmigrate_failed)

Where the values are read from /proc/vmstat.

This is very basic and ignores certain costs such as the allocation cost
to do a migrate page copy but any improvement to the model would still
use the same vmstat counters.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Rik van Riel <riel@redhat.com>

diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
index 8aa7cb94b5fb50480c44830d31cbe8f553f735f2..a1f750b8e72a7591474ccb78b242bf72a6ff77ab 100644 (file)
--- a/include/linux/vm_event_item.h
+++ b/include/linux/vm_event_item.h
@@ -42,6 +42,8 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
                PGMIGRATE_SUCCESS, PGMIGRATE_FAIL,
 #endif
 #ifdef CONFIG_COMPACTION
+               COMPACTMIGRATE_SCANNED, COMPACTFREE_SCANNED,
+               COMPACTISOLATED,
                COMPACTSTALL, COMPACTFAIL, COMPACTSUCCESS,
 #endif
 #ifdef CONFIG_HUGETLB_PAGE

diff --git a/mm/compaction.c b/mm/compaction.c
index 2c077a78487c61b8d083130b566bf05055227d1e..aee7443a4d5ac0a868af34781dbe9287d6e2c225 100644 (file)
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -356,6 +356,10 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
        if (blockpfn == end_pfn)
                update_pageblock_skip(cc, valid_page, total_isolated, false);
 
+       count_vm_events(COMPACTFREE_SCANNED, nr_scanned);
+       if (total_isolated)
+               count_vm_events(COMPACTISOLATED, total_isolated);
+
        return total_isolated;
 }
 
@@ -646,6 +650,10 @@ next_pageblock:
 
        trace_mm_compaction_isolate_migratepages(nr_scanned, nr_isolated);
 
+       count_vm_events(COMPACTMIGRATE_SCANNED, nr_scanned);
+       if (nr_isolated)
+               count_vm_events(COMPACTISOLATED, nr_isolated);
+
        return low_pfn;
 }
 

diff --git a/mm/vmstat.c b/mm/vmstat.c
index 89a7fd665b320977527a5da3edf893e5ffc282c8..3a067fabe190ba9d3a824cc8746811854c9c58ec 100644 (file)
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -779,6 +779,9 @@ const char * const vmstat_text[] = {
        "pgmigrate_fail",
 #endif
 #ifdef CONFIG_COMPACTION
+       "compact_migrate_scanned",
+       "compact_free_scanned",
+       "compact_isolated",
        "compact_stall",
        "compact_fail",
        "compact_success",