Re: [PATCH 15/25] mm, compaction: Finish pageblock scanning on contention

From: Vlastimil Babka
Date: Thu Jan 17 2019 - 11:38:41 EST

On 1/4/19 1:50 PM, Mel Gorman wrote:
> Async migration aborts on spinlock contention but contention can be high
> when there are multiple compaction attempts and kswapd is active. The
> consequence is that the migration scanners move forward uselessly while
> still contending on locks for longer while leaving suitable migration
> sources behind.
>
> This patch will acquire the lock but track when contention occurs. When
> it does, the current pageblock will finish as compaction may succeed for
> that block and then abort. This will have a variable impact on latency as
> in some cases useless scanning is avoided (reduces latency) but a lock
> will be contended (increase latency) or a single contended pageblock is
> scanned that would otherwise have been skipped (increase latency).
>
> 4.20.0 4.20.0
> norescan-v2r15 finishcontend-v2r15
> Amean fault-both-1 0.00 ( 0.00%) 0.00 * 0.00%*
> Amean fault-both-3 2872.13 ( 0.00%) 2973.08 ( -3.51%)
> Amean fault-both-5 4330.56 ( 0.00%) 3870.19 ( 10.63%)
> Amean fault-both-7 6496.63 ( 0.00%) 6580.50 ( -1.29%)
> Amean fault-both-12 10280.59 ( 0.00%) 9527.40 ( 7.33%)
> Amean fault-both-18 11079.19 ( 0.00%) 13395.86 * -20.91%*
> Amean fault-both-24 17207.80 ( 0.00%) 14936.94 * 13.20%*
> Amean fault-both-30 17736.13 ( 0.00%) 16748.46 ( 5.57%)
> Amean fault-both-32 18509.41 ( 0.00%) 18521.30 ( -0.06%)
>
> 4.20.0 4.20.0
> norescan-v2r15 finishcontend-v2r15
> Percentage huge-1 0.00 ( 0.00%) 0.00 ( 0.00%)
> Percentage huge-3 96.87 ( 0.00%) 97.57 ( 0.72%)
> Percentage huge-5 94.63 ( 0.00%) 96.88 ( 2.39%)
> Percentage huge-7 93.83 ( 0.00%) 95.47 ( 1.74%)
> Percentage huge-12 92.65 ( 0.00%) 98.64 ( 6.47%)
> Percentage huge-18 93.66 ( 0.00%) 98.33 ( 4.98%)
> Percentage huge-24 93.15 ( 0.00%) 98.88 ( 6.15%)
> Percentage huge-30 93.16 ( 0.00%) 97.09 ( 4.21%)
> Percentage huge-32 92.58 ( 0.00%) 96.20 ( 3.92%)
>
> As expected, a variable impact on latency while allocation success
> rates are slightly higher. System CPU usage is reduced by about 10%
> but scan rate impact is mixed
>
> Compaction migrate scanned 31772603 19980216
> Compaction free scanned 63267928 120381828
>
> Migration scan rates are reduced 37% which is expected as a pageblock
> is used by the async scanner instead of skipped but the free scanning is
> increased. This can be partially accounted for by the increased success
> 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@xxxxxxxxxxxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

Some comments below.

> @@ -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.

>
> /* Recheck this is a buddy page under lock */
> if (!PageBuddy(page))
> @@ -910,15 +897,9 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>
> /* If we already hold the lock, we can skip some rechecking */
> if (!locked) {
> - locked = compact_trylock_irqsave(zone_lru_lock(zone),
> + locked = compact_lock_irqsave(zone_lru_lock(zone),
> &flags, cc);
>
> - /* Allow future scanning if the lock is contended */
> - if (!locked) {
> - clear_pageblock_skip(page);
> - break;
> - }

Ditto.

> -
> /* Try get exclusive access under lock */
> if (!skip_updated) {
> skip_updated = true;
> @@ -961,9 +942,12 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>
> /*
> * Avoid isolating too much unless this block is being
> - * rescanned (e.g. dirty/writeback pages, parallel allocation).
> + * rescanned (e.g. dirty/writeback pages, parallel allocation)
> + * or a lock is contended. For contention, isolate quickly to
> + * potentially remove one source of contention.
> */
> - if (cc->nr_migratepages == COMPACT_CLUSTER_MAX && !cc->rescan) {
> + if (cc->nr_migratepages == COMPACT_CLUSTER_MAX &&
> + !cc->rescan && !cc->contended) {
> ++low_pfn;
> break;
> }
> @@ -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?

> + /* Are enough freepages isolated? */
> + if (cc->nr_freepages >= cc->nr_migratepages) {
> if (isolate_start_pfn >= block_end_pfn) {
> /*
> * Restart at previous pageblock if more