From: Vlastimil Babka <vbabka@suse.cz>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: Linux-MM <linux-mm@kvack.org>,
David Rientjes <rientjes@google.com>,
Andrea Arcangeli <aarcange@redhat.com>,
ying.huang@intel.com, kirill@shutemov.name,
Andrew Morton <akpm@linux-foundation.org>,
Linux List Kernel Mailing <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 15/25] mm, compaction: Finish pageblock scanning on contention
Date: Fri, 18 Jan 2019 09:57:38 +0100 [thread overview]
Message-ID: <aa34f8da-a592-48b2-9e86-32a9433d55ce@suse.cz> (raw)
In-Reply-To: <20190117171122.GK27437@techsingularity.net>
On 1/17/19 6:11 PM, Mel Gorman wrote:
> On Thu, Jan 17, 2019 at 05:38:36PM +0100, Vlastimil Babka wrote:
>> > rate but also by the fact that the scanners do not meet for longer when
>> > pageblocks are actually used. Overall this is justified and completing
>> > a pageblock scan is very important for later patches.
>> >
>> > Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
>>
>> Acked-by: Vlastimil Babka <vbabka@suse.cz>
>>
>> Some comments below.
>>
>
> Thanks
>
>> > @@ -538,18 +535,8 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
>> > * recheck as well.
>> > */
>> > if (!locked) {
>> > - /*
>> > - * The zone lock must be held to isolate freepages.
>> > - * Unfortunately this is a very coarse lock and can be
>> > - * heavily contended if there are parallel allocations
>> > - * or parallel compactions. For async compaction do not
>> > - * spin on the lock and we acquire the lock as late as
>> > - * possible.
>> > - */
>> > - locked = compact_trylock_irqsave(&cc->zone->lock,
>> > + locked = compact_lock_irqsave(&cc->zone->lock,
>> > &flags, cc);
>> > - if (!locked)
>> > - break;
>>
>> Seems a bit dangerous to continue compact_lock_irqsave() to return bool that
>> however now always returns true, and remove the safety checks that test the
>> result. Easy for somebody in the future to reintroduce some 'return false'
>> condition (even though the name now says lock and not trylock) and start
>> crashing. I would either change it to return void, or leave the checks in place.
>>
>
> I considered changing it from bool at the same time as "Rework
> compact_should_abort as compact_check_resched". It turned out to be a
> bit clumsy because the locked state must be explicitly updated in the
> caller then. e.g.
>
> locked = compact_lock_irqsave(...)
>
> becomes
>
> compact_lock_irqsave(...)
> locked = true
>
> I didn't think the result looked that great to be honest but maybe it's
> worth revisiting as a cleanup patch like "Rework compact_should_abort as
> compact_check_resched" on top.
>
>> >
>> > @@ -1411,12 +1395,8 @@ static void isolate_freepages(struct compact_control *cc)
>> > isolate_freepages_block(cc, &isolate_start_pfn, block_end_pfn,
>> > freelist, false);
>> >
>> > - /*
>> > - * If we isolated enough freepages, or aborted due to lock
>> > - * contention, terminate.
>> > - */
>> > - if ((cc->nr_freepages >= cc->nr_migratepages)
>> > - || cc->contended) {
>>
>> Does it really make sense to continue in the case of free scanner, when we know
>> we will just return back the extra pages in the end? release_freepages() will
>> update the cached pfns, but the pageblock skip bit will stay, so we just leave
>> those pages behind. Unless finishing the block is important for the later
>> patches (as changelog mentions) even in the case of free scanner, but then we
>> can just skip the rest of it, as truly scanning it can't really help anything?
>>
>
> Finishing is important for later patches is one factor but not the only
> factor. While we eventually return all pages, we do not know at this
> point in time how many free pages are needed. Remember the migration
> source isolates COMPACT_CLUSTER_MAX pages and then looks for migration
> targets. If the source isolates 32 pages, free might isolate more from
> one pageblock but that's ok as the migration source may need more free
> pages in the immediate future. It's less wasteful than it looks at first
> glance (or second or even third glance).
>
> However, if we isolated exactly enough targets, and the pageblock gets
> marked skipped, then each COMPACT_CLUSTER_MAX isolation from the target
> could potentially marge one new pageblock unnecessarily and increase
> scanning+resets overall. That would be bad.
>
> There still can be waste because we do not know in advance exactly how
> many migration sources there will be -- sure, we could calculate it but
> that involves scanning the source pageblock twice which is wasteful.
> I did try estimating it based on the remaining number of pages in the
> pageblock but the additional complexity did not appear to help.
>
> Does that make sense?
OK, thanks.
next prev parent reply other threads:[~2019-01-18 8:57 UTC|newest]
Thread overview: 75+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-04 12:49 [PATCH 00/25] Increase success rates and reduce latency of compaction v2 Mel Gorman
2019-01-04 12:49 ` [PATCH 01/25] mm, compaction: Shrink compact_control Mel Gorman
2019-01-04 12:49 ` [PATCH 02/25] mm, compaction: Rearrange compact_control Mel Gorman
2019-01-04 12:49 ` [PATCH 03/25] mm, compaction: Remove last_migrated_pfn from compact_control Mel Gorman
2019-01-04 12:49 ` [PATCH 04/25] mm, compaction: Remove unnecessary zone parameter in some instances Mel Gorman
2019-01-15 11:43 ` Vlastimil Babka
2019-01-04 12:49 ` [PATCH 05/25] mm, compaction: Rename map_pages to split_map_pages Mel Gorman
2019-01-15 11:59 ` Vlastimil Babka
2019-01-04 12:49 ` [PATCH 06/25] mm, compaction: Skip pageblocks with reserved pages Mel Gorman
2019-01-15 12:10 ` Vlastimil Babka
2019-01-15 12:50 ` Mel Gorman
2019-01-16 9:42 ` Mel Gorman
2019-01-04 12:49 ` [PATCH 07/25] mm, migrate: Immediately fail migration of a page with no migration handler Mel Gorman
2019-01-04 12:49 ` [PATCH 08/25] mm, compaction: Always finish scanning of a full pageblock Mel Gorman
2019-01-04 12:49 ` [PATCH 09/25] mm, compaction: Use the page allocator bulk-free helper for lists of pages Mel Gorman
2019-01-15 12:39 ` Vlastimil Babka
2019-01-16 9:46 ` Mel Gorman
2019-01-04 12:49 ` [PATCH 10/25] mm, compaction: Ignore the fragmentation avoidance boost for isolation and compaction Mel Gorman
2019-01-15 13:18 ` Vlastimil Babka
2019-01-04 12:49 ` [PATCH 11/25] mm, compaction: Use free lists to quickly locate a migration source Mel Gorman
2019-01-16 13:15 ` Vlastimil Babka
2019-01-16 14:33 ` Mel Gorman
2019-01-16 15:00 ` Vlastimil Babka
2019-01-16 15:43 ` Mel Gorman
2019-01-04 12:49 ` [PATCH 12/25] mm, compaction: Keep migration source private to a single compaction instance Mel Gorman
2019-01-16 15:45 ` Vlastimil Babka
2019-01-16 16:15 ` Mel Gorman
2019-01-17 9:29 ` Vlastimil Babka
2019-01-17 9:40 ` Vlastimil Babka
2019-01-04 12:49 ` [PATCH 13/25] mm, compaction: Use free lists to quickly locate a migration target Mel Gorman
2019-01-17 14:36 ` Vlastimil Babka
2019-01-17 15:51 ` Mel Gorman
2019-01-04 12:50 ` [PATCH 14/25] mm, compaction: Avoid rescanning the same pageblock multiple times Mel Gorman
2019-01-17 15:16 ` Vlastimil Babka
2019-01-17 16:00 ` Mel Gorman
2019-01-04 12:50 ` [PATCH 15/25] mm, compaction: Finish pageblock scanning on contention Mel Gorman
2019-01-17 16:38 ` Vlastimil Babka
2019-01-17 17:11 ` Mel Gorman
2019-01-18 8:57 ` Vlastimil Babka [this message]
2019-01-04 12:50 ` [PATCH 16/25] mm, compaction: Check early for huge pages encountered by the migration scanner Mel Gorman
2019-01-17 17:01 ` Vlastimil Babka
2019-01-17 17:35 ` Mel Gorman
2019-01-04 12:50 ` [PATCH 17/25] mm, compaction: Keep cached migration PFNs synced for unusable pageblocks Mel Gorman
2019-01-17 17:17 ` Vlastimil Babka
2019-01-17 17:37 ` Mel Gorman
2019-01-04 12:50 ` [PATCH 18/25] mm, compaction: Rework compact_should_abort as compact_check_resched Mel Gorman
2019-01-17 17:27 ` Vlastimil Babka
2019-01-04 12:50 ` [PATCH 19/25] mm, compaction: Do not consider a need to reschedule as contention Mel Gorman
2019-01-17 17:33 ` Vlastimil Babka
2019-01-17 18:05 ` Mel Gorman
2019-01-04 12:50 ` [PATCH 20/25] mm, compaction: Reduce unnecessary skipping of migration target scanner Mel Gorman
2019-01-17 17:58 ` Vlastimil Babka
2019-01-17 19:39 ` Mel Gorman
2019-01-18 9:09 ` Vlastimil Babka
2019-01-04 12:50 ` [PATCH 21/25] mm, compaction: Round-robin the order while searching the free lists for a target Mel Gorman
2019-01-18 9:17 ` Vlastimil Babka
2019-01-04 12:50 ` [PATCH 22/25] mm, compaction: Sample pageblocks for free pages Mel Gorman
2019-01-18 10:38 ` Vlastimil Babka
2019-01-18 13:44 ` Mel Gorman
2019-01-04 12:50 ` [PATCH 23/25] mm, compaction: Be selective about what pageblocks to clear skip hints Mel Gorman
2019-01-18 12:55 ` Vlastimil Babka
2019-01-18 14:10 ` Mel Gorman
2019-01-04 12:50 ` [PATCH 24/25] mm, compaction: Capture a page under direct compaction Mel Gorman
2019-01-18 13:40 ` Vlastimil Babka
2019-01-18 14:39 ` Mel Gorman
2019-01-04 12:50 ` [PATCH 25/25] mm, compaction: Do not direct compact remote memory Mel Gorman
2019-01-18 13:51 ` Vlastimil Babka
2019-01-18 14:46 ` Mel Gorman
2019-01-07 23:43 ` [PATCH 00/25] Increase success rates and reduce latency of compaction v2 Andrew Morton
2019-01-08 9:12 ` Mel Gorman
2019-01-09 11:13 ` [PATCH] mm, compaction: Use free lists to quickly locate a migration target -fix Mel Gorman
2019-01-09 19:27 ` Andrew Morton
2019-01-09 21:26 ` Mel Gorman
2019-01-09 11:15 ` [PATCH] mm, compaction: Finish pageblock scanning on contention -fix Mel Gorman
2019-01-09 11:16 ` [PATCH] mm, compaction: Round-robin the order while searching the free lists for a target -fix Mel Gorman
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