2014-04-23 16:30 GMT+09:00 Vlastimil Babka <vba...@suse.cz>: > On 04/23/2014 04:58 AM, Joonsoo Kim wrote: >> On Tue, Apr 22, 2014 at 03:17:30PM +0200, Vlastimil Babka wrote: >>> On 04/22/2014 08:52 AM, Minchan Kim wrote: >>>> On Tue, Apr 22, 2014 at 08:33:35AM +0200, Vlastimil Babka wrote: >>>>> On 22.4.2014 1:53, Minchan Kim wrote: >>>>>> On Mon, Apr 21, 2014 at 11:43:24PM +0200, Vlastimil Babka wrote: >>>>>>> On 21.4.2014 21:41, Andrew Morton wrote: >>>>>>>> On Thu, 17 Apr 2014 09:07:45 +0900 Minchan Kim <minc...@kernel.org> >>>>>>>> wrote: >>>>>>>> >>>>>>>>> Hi Vlastimil, >>>>>>>>> >>>>>>>>> Below just nitpicks. >>>>>>>> It seems you were ignored ;) >>>>>>> Oops, I managed to miss your e-mail, sorry. >>>>>>> >>>>>>>>>> { >>>>>>>>>> struct page *page; >>>>>>>>>> - unsigned long high_pfn, low_pfn, pfn, z_end_pfn; >>>>>>>>>> + unsigned long pfn, low_pfn, next_free_pfn, z_end_pfn; >>>>>>>>> Could you add comment for each variable? >>>>>>>>> >>>>>>>>> unsigned long pfn; /* scanning cursor */ >>>>>>>>> unsigned long low_pfn; /* lowest pfn free scanner is able to scan */ >>>>>>>>> unsigned long next_free_pfn; /* start pfn for scaning at next truen */ >>>>>>>>> unsigned long z_end_pfn; /* zone's end pfn */ >>>>>>>>> >>>>>>>>> >>>>>>>>>> @@ -688,11 +688,10 @@ static void isolate_freepages(struct zone >>>>>>>>>> *zone, >>>>>>>>>> low_pfn = ALIGN(cc->migrate_pfn + 1, pageblock_nr_pages); >>>>>>>>>> /* >>>>>>>>>> - * Take care that if the migration scanner is at the end of >>>>>>>>>> the zone >>>>>>>>>> - * that the free scanner does not accidentally move to the >>>>>>>>>> next zone >>>>>>>>>> - * in the next isolation cycle. >>>>>>>>>> + * Seed the value for max(next_free_pfn, pfn) updates. If >>>>>>>>>> there are >>>>>>>>>> + * none, the pfn < low_pfn check will kick in. >>>>>>>>> "none" what? I'd like to clear more. >>>>>>> If there are no updates to next_free_pfn within the for cycle. Which >>>>>>> matches Andrew's formulation below. >>>>>>> >>>>>>>> I did this: >>>>>>> Thanks! >>>>>>> >>>>>>>> --- a/mm/compaction.c~mm-compaction-cleanup-isolate_freepages-fix >>>>>>>> +++ a/mm/compaction.c >>>>>>>> @@ -662,7 +662,10 @@ static void isolate_freepages(struct zon >>>>>>>> struct compact_control *cc) >>>>>>>> { >>>>>>>> struct page *page; >>>>>>>> - unsigned long pfn, low_pfn, next_free_pfn, z_end_pfn; >>>>>>>> + unsigned long pfn; /* scanning cursor */ >>>>>>>> + unsigned long low_pfn; /* lowest pfn scanner is able to >>>>>>>> scan */ >>>>>>>> + unsigned long next_free_pfn; /* start pfn for scaning at next >>>>>>>> round */ >>>>>>>> + unsigned long z_end_pfn; /* zone's end pfn */ >>>>>>> Yes that works. >>>>>>> >>>>>>>> int nr_freepages = cc->nr_freepages; >>>>>>>> struct list_head *freelist = &cc->freepages; >>>>>>>> @@ -679,8 +682,8 @@ static void isolate_freepages(struct zon >>>>>>>> low_pfn = ALIGN(cc->migrate_pfn + 1, pageblock_nr_pages); >>>>>>>> /* >>>>>>>> - * Seed the value for max(next_free_pfn, pfn) updates. If >>>>>>>> there are >>>>>>>> - * none, the pfn < low_pfn check will kick in. >>>>>>>> + * Seed the value for max(next_free_pfn, pfn) updates. If no >>>>>>>> pages are >>>>>>>> + * isolated, the pfn < low_pfn check will kick in. >>>>>>> OK. >>>>>>> >>>>>>>> */ >>>>>>>> next_free_pfn = 0; >>>>>>>>>> @@ -766,9 +765,9 @@ static void isolate_freepages(struct zone *zone, >>>>>>>>>> * so that compact_finished() may detect this >>>>>>>>>> */ >>>>>>>>>> if (pfn < low_pfn) >>>>>>>>>> - cc->free_pfn = max(pfn, zone->zone_start_pfn); >>>>>>>>>> - else >>>>>>>>>> - cc->free_pfn = high_pfn; >>>>>>>>>> + next_free_pfn = max(pfn, zone->zone_start_pfn); >>>>>>>>> Why we need max operation? >>>>>>>>> IOW, what's the problem if we do (next_free_pfn = pfn)? >>>>>>>> An answer to this would be useful, thanks. >>>>>>> The idea (originally, not new here) is that the free scanner wants >>>>>>> to remember the highest-pfn >>>>>>> block where it managed to isolate some pages. If the following page >>>>>>> migration fails, these isolated >>>>>>> pages might be put back and would be skipped in further compaction >>>>>>> attempt if we used just >>>>>>> "next_free_pfn = pfn", until the scanners get reset. >>>>>>> >>>>>>> The question of course is if such situations are frequent and makes >>>>>>> any difference to compaction >>>>>>> outcome. And the downsides are potentially useless rescans and code >>>>>>> complexity. Maybe Mel >>>>>>> remembers how important this is? It should probably be profiled >>>>>>> before changes are made. >>>>>> I didn't mean it. What I mean is code snippet you introduced in >>>>>> 7ed695e069c3c. >>>>>> At that time, I didn't Cced so I missed that code so let's ask this time. >>>>>> In that patch, you added this. >>>>>> >>>>>> if (pfn < low_pfn) >>>>>> cc->free_pfn = max(pfn, zone->zone_start_pfn); >>>>>> else >>>>>> cc->free_pfn = high_pfn; >>>>> >>>>> Oh, right, this max(), not the one in the for loop. Sorry, I should >>>>> have read more closely. >>>>> But still maybe it's a good opportunity to kill the other max() as >>>>> well. I'll try some testing. >>>>> >>>>> Anyway, this is what I answered to Mel when he asked the same thing >>>>> when I sent >>>>> that 7ed695069c3c patch: >>>>> >>>>> If a zone starts in a middle of a pageblock and migrate scanner isolates >>>>> enough pages early to stay within that pageblock, low_pfn will be at the >>>>> end of that pageblock and after the for cycle in this function ends, pfn >>>>> might be at the beginning of that pageblock. It might not be an actual >>>>> problem (this compaction will finish at this point, and if someone else >>>>> is racing, he will probably check the boundaries himself), but I played >>>>> it safe. >>>>> >>>>> >>>>>> So the purpose of max(pfn, zone->zone_start_pfn) is to be detected by >>>>>> compact_finished to stop compaction. And your [1/2] patch in this >>>>>> patchset >>>>>> always makes free page scanner start on pageblock boundary so when the >>>>>> loop in isolate_freepages is finished and pfn is lower low_pfn, the pfn >>>>>> would be lower than migration scanner so compact_finished will always >>>>>> detect >>>>>> it so I think you could just do >>>>>> >>>>>> if (pfn < low_pfn) >>>>>> next_free_pfn = pfn; >>>>>> >>>>>> cc->free_pfn = next_free_pfn; >>>>> >>>>> That could work. I was probably wrong about danger of racing in the >>>>> reply to Mel, >>>>> because free_pfn is stored in cc (private), not zone (shared). >>>>> >>>>>> >>>>>> Or, if you want to clear *reset*, >>>>>> if (pfn < lown_pfn) >>>>>> next_free_pfn = zone->zone_start_pfn; >>>>>> >>>>>> cc->free_pfn = next_free_pfn; >>>>> >>>>> That would work as well but is less straightforward I think. Might >>>>> be misleading if >>>>> someone added tracepoints to track the free scanner progress with >>>>> pfn's (which >>>>> might happen soon...) >>>> >>>> My preference is to add following with pair of compact_finished >>>> >>>> static inline void finish_compact(struct compact_control *cc) >>>> { >>>> cc->free_pfn = cc->migrate_pfn; >>>> } >>> >>> Yes setting free_pfn to migrate_pfn is probably the best way, as these >>> are the values compared in compact_finished. But I wouldn't introduce a >>> new function just for one instance of this. Also compact_finished() >>> doesn't test just the scanners to decide whether compaction should >>> continue, so the pairing would be imperfect anyway. >>> So Andrew, if you agree can you please fold in the patch below. >>> >>>> But I don't care. >>>> If you didn't send this patch as clean up, I would never interrupt >>>> on the way but you said it's cleanup patch and the one made me spend a >>>> few minutes to understand the code so it's not a clean up patch. ;-). >>>> So, IMO, it's worth to tidy it up. >>> >>> Yes, I understand and agree. >>> >>> ------8<------ >>> From: Vlastimil Babka <vba...@suse.cz> >>> Date: Tue, 22 Apr 2014 13:55:36 +0200 >>> Subject: mm-compaction-cleanup-isolate_freepages-fix2 >>> >>> Cleanup detection of compaction scanners crossing in isolate_freepages(). >>> To make sure compact_finished() observes scanners crossing, we can just set >>> free_pfn to migrate_pfn instead of confusing max() construct. >>> >>> Suggested-by: Minchan Kim <minc...@kernel.org> >>> Cc: Bartlomiej Zolnierkiewicz <b.zolnier...@samsung.com> >>> Cc: Christoph Lameter <c...@linux.com> >>> Cc: Dongjun Shin <d.j.s...@samsung.com> >>> Cc: Joonsoo Kim <iamjoonsoo....@lge.com> >>> Cc: Mel Gorman <mgor...@suse.de> >>> Cc: Michal Nazarewicz <min...@mina86.com> >>> Cc: Minchan Kim <minc...@kernel.org> >>> Cc: Naoya Horiguchi <n-horigu...@ah.jp.nec.com> >>> Cc: Rik van Riel <r...@redhat.com> >>> Cc: Sunghwan Yun <sunghwan....@samsung.com> >>> Signed-off-by: Vlastimil Babka <vba...@suse.cz> >>> >>> --- >>> mm/compaction.c | 2 +- >>> 1 file changed, 1 insertion(+), 1 deletion(-) >>> >>> diff --git a/mm/compaction.c b/mm/compaction.c >>> index 37c15fe..1c992dc 100644 >>> --- a/mm/compaction.c >>> +++ b/mm/compaction.c >>> @@ -768,7 +768,7 @@ static void isolate_freepages(struct zone *zone, >>> * so that compact_finished() may detect this >>> */ >>> if (pfn < low_pfn) >>> - next_free_pfn = max(pfn, zone->zone_start_pfn); >>> + next_free_pfn = cc->migrate_pfn; >>> >>> cc->free_pfn = next_free_pfn; >>> cc->nr_freepages = nr_freepages; >>> -- >>> 1.8.4.5 >>> >> >> Hello, >> >> How about doing more clean-up at this time? >> >> What I did is that taking end_pfn out of the loop and consider zone >> boundary once. After then, we just subtract pageblock_nr_pages on >> every iteration. With this change, we can remove local variable, z_end_pfn. >> Another things I did are removing max() operation and un-needed >> assignment to isolate variable. >> >> Thanks. >> >> --------->8------------ >> diff --git a/mm/compaction.c b/mm/compaction.c >> index 1c992dc..95a506d 100644 >> --- a/mm/compaction.c >> +++ b/mm/compaction.c >> @@ -671,10 +671,10 @@ static void isolate_freepages(struct zone *zone, >> struct compact_control *cc) >> { >> struct page *page; >> - unsigned long pfn; /* scanning cursor */ >> + unsigned long pfn; /* start of scanning window */ >> + unsigned long end_pfn; /* end of scanning window */ >> unsigned long low_pfn; /* lowest pfn scanner is able to scan */ >> unsigned long next_free_pfn; /* start pfn for scaning at next round */ >> - unsigned long z_end_pfn; /* zone's end pfn */ >> int nr_freepages = cc->nr_freepages; >> struct list_head *freelist = &cc->freepages; >> >> @@ -688,15 +688,16 @@ static void isolate_freepages(struct zone *zone, >> * is using. >> */ >> pfn = cc->free_pfn & ~(pageblock_nr_pages-1); >> - low_pfn = ALIGN(cc->migrate_pfn + 1, pageblock_nr_pages); >> >> /* >> - * Seed the value for max(next_free_pfn, pfn) updates. If no pages are >> - * isolated, the pfn < low_pfn check will kick in. >> + * Take care when isolating in last pageblock of a zone which >> + * ends in the middle of a pageblock. >> */ >> - next_free_pfn = 0; >> + end_pfn = min(pfn + pageblock_nr_pages, zone_end_pfn(zone)); >> + low_pfn = ALIGN(cc->migrate_pfn + 1, pageblock_nr_pages); >> >> - z_end_pfn = zone_end_pfn(zone); >> + /* If no pages are isolated, the pfn < low_pfn check will kick in. */ >> + next_free_pfn = 0; >> >> /* >> * Isolate free pages until enough are available to migrate the >> @@ -704,9 +705,8 @@ static void isolate_freepages(struct zone *zone, >> * and free page scanners meet or enough free pages are isolated. >> */ >> for (; pfn >= low_pfn && cc->nr_migratepages > nr_freepages; >> - pfn -= pageblock_nr_pages) { >> + pfn -= pageblock_nr_pages, end_pfn -= pageblock_nr_pages) { > > If zone_end_pfn was in the middle of a pageblock, then your end_pfn will > always be in the middle of a pageblock and you will not scan half of all > pageblocks. >
Okay. I think a way to fix it. By assigning pfn(start of scanning window) to end_pfn(end of scanning window) for the next loop, we can solve the problem you mentioned. How about below? - pfn -= pageblock_nr_pages, end_pfn -= pageblock_nr_pages) { + end_pfn = pfn, pfn -= pageblock_nr_pages) { Thanks. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/