On Mon, Nov 10, 2014 at 08:53:38AM +0100, Vlastimil Babka wrote: > On 11/10/2014 07:07 AM, Joonsoo Kim wrote: > >On Sat, Nov 08, 2014 at 11:18:37PM +0100, Vlastimil Babka wrote: > >>On 11/08/2014 02:11 PM, P. Christeas wrote: > >> > >>Hi, > >> > >>I think I finally found the cause by staring into the code... CCing > >>people from all 4 separate threads I know about this issue. > >>The problem with finding the cause was that the first report I got from > >>Markus was about isolate_freepages_block() overhead, and later Norbert > >>reported that reverting a patch for isolate_freepages* helped. But the > >>problem seems to be that although the loop in isolate_migratepages exits > >>because the scanners almost meet (they are within same pageblock), they > >>don't truly meet, therefore compact_finished() decides to continue, but > >>isolate_migratepages() exits immediately... boom! But indeed e14c720efdd7 > >>made this situation possible, as free scaner pfn can now point to a > >>middle of pageblock. > > > >Indeed. > > > >> > >>So I hope the attached patch will fix the soft-lockup issues in > >>compact_zone. Please apply on 3.18-rc3 or later without any other reverts, > >>and test. It probably won't help Markus and his isolate_freepages_block() > >>overhead though... > > > >Yes, I found this bug too, but, it can't explain > >isolate_freepages_block() overhead. Anyway, I can't find another bug > >related to isolate_freepages_block(). :/ > > Thanks for checking. > > >>Thanks, > >>Vlastimil > >> > >>------8<------ > >>>From fbf8eb0bcd2897090312e23da6a31bad9cc6b337 Mon Sep 17 00:00:00 2001 > >>From: Vlastimil Babka <vbabka@xxxxxxx> > >>Date: Sat, 8 Nov 2014 22:20:43 +0100 > >>Subject: [PATCH] mm, compaction: prevent endless loop in migrate scanner > >> > >>--- > >> mm/compaction.c | 8 ++++++-- > >> 1 file changed, 6 insertions(+), 2 deletions(-) > >> > >>diff --git a/mm/compaction.c b/mm/compaction.c > >>index ec74cf0..1b7a1be 100644 > >>--- a/mm/compaction.c > >>+++ b/mm/compaction.c > >>@@ -1029,8 +1029,12 @@ static isolate_migrate_t isolate_migratepages(struct zone *zone, > >> } > >> > >> acct_isolated(zone, cc); > >>- /* Record where migration scanner will be restarted */ > >>- cc->migrate_pfn = low_pfn; > >>+ /* > >>+ * Record where migration scanner will be restarted. If we end up in > >>+ * the same pageblock as the free scanner, make the scanners fully > >>+ * meet so that compact_finished() terminates compaction. > >>+ */ > >>+ cc->migrate_pfn = (end_pfn <= cc->free_pfn) ? low_pfn : cc->free_pfn; > >> > >> return cc->nr_migratepages ? ISOLATE_SUCCESS : ISOLATE_NONE; > >> } > > > >IMHO, proper fix is not to change this logic, but, to change decision > >logic in compact_finished() and in compact_zone(). Maybe helper > >function would be good for readability. > > OK but I would think that to fix 3.18 ASAP and not introduce more > regressions, go with the patch above first, as it is the minimal fix > and people already test it. Then we can implement your suggestion > later as a cleanup for 3.19? Yeap. Agreed. Thanks. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>