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SubjectRe: [PATCH v3 05/13] mm, compaction: report compaction as contended only due to lock contention
On 07/11/2014 10:28 AM, Vlastimil Babka wrote:
> On 06/23/2014 03:39 AM, Minchan Kim wrote:
>> Hello Vlastimil,
>>
>> On Fri, Jun 20, 2014 at 05:49:35PM +0200, Vlastimil Babka wrote:
>>> Async compaction aborts when it detects zone lock contention or need_resched()
>>> is true. David Rientjes has reported that in practice, most direct async
>>> compactions for THP allocation abort due to need_resched(). This means that a
>>> second direct compaction is never attempted, which might be OK for a page
>>> fault, but khugepaged is intended to attempt a sync compaction in such case and
>>> in these cases it won't.
>>>
>>> This patch replaces "bool contended" in compact_control with an enum that
>>> distinguieshes between aborting due to need_resched() and aborting due to lock
>>> contention. This allows propagating the abort through all compaction functions
>>> as before, but declaring the direct compaction as contended only when lock
>>> contention has been detected.
>>>
>>> A second problem is that try_to_compact_pages() did not act upon the reported
>>> contention (both need_resched() or lock contention) and could proceed with
>>> another zone from the zonelist. When need_resched() is true, that means
>>> initializing another zone compaction, only to check again need_resched() in
>>> isolate_migratepages() and aborting. For zone lock contention, the unintended
>>> consequence is that the contended status reported back to the allocator
>>> is decided from the last zone where compaction was attempted, which is rather
>>> arbitrary.
>>>
>>> This patch fixes the problem in the following way:
>>> - need_resched() being true after async compaction returned from a zone means
>>> that further zones should not be tried. We do a cond_resched() so that we
>>> do not hog the CPU, and abort. "contended" is reported as false, since we
>>> did not fail due to lock contention.
>>> - aborting zone compaction due to lock contention means we can still try
>>> another zone, since it has different locks. We report back "contended" as
>>> true only if *all* zones where compaction was attempted, it aborted due to
>>> lock contention.
>>>
>>> As a result of these fixes, khugepaged will proceed with second sync compaction
>>> as intended, when the preceding async compaction aborted due to need_resched().
>>> Page fault compactions aborting due to need_resched() will spare some cycles
>>> previously wasted by initializing another zone compaction only to abort again.
>>> Lock contention will be reported only when compaction in all zones aborted due
>>> to lock contention, and therefore it's not a good idea to try again after
>>> reclaim.
>>>
>>> Reported-by: David Rientjes <rientjes@google.com>
>>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
>>> Cc: Minchan Kim <minchan@kernel.org>
>>> Cc: Mel Gorman <mgorman@suse.de>
>>> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
>>> Cc: Michal Nazarewicz <mina86@mina86.com>
>>> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
>>> Cc: Christoph Lameter <cl@linux.com>
>>> Cc: Rik van Riel <riel@redhat.com>
>>> ---
>>> mm/compaction.c | 48 +++++++++++++++++++++++++++++++++++++++---------
>>> mm/internal.h | 15 +++++++++++----
>>> 2 files changed, 50 insertions(+), 13 deletions(-)
>>>
>>> diff --git a/mm/compaction.c b/mm/compaction.c
>>> index ebe30c9..e8cfac9 100644
>>> --- a/mm/compaction.c
>>> +++ b/mm/compaction.c
>>> @@ -180,9 +180,14 @@ static void update_pageblock_skip(struct compact_control *cc,
>>> }
>>> #endif /* CONFIG_COMPACTION */
>>>
>>> -static inline bool should_release_lock(spinlock_t *lock)
>>> +enum compact_contended should_release_lock(spinlock_t *lock)
>>> {
>>> - return need_resched() || spin_is_contended(lock);
>>> + if (spin_is_contended(lock))
>>> + return COMPACT_CONTENDED_LOCK;
>>> + else if (need_resched())
>>> + return COMPACT_CONTENDED_SCHED;
>>> + else
>>> + return COMPACT_CONTENDED_NONE;
>>
>> If you want to raise priority of lock contention than need_resched
>> intentionally, please write it down on comment.
>>
>>> }
>>>
>>> /*
>>> @@ -197,7 +202,9 @@ static inline bool should_release_lock(spinlock_t *lock)
>>> static bool compact_checklock_irqsave(spinlock_t *lock, unsigned long *flags,
>>> bool locked, struct compact_control *cc)
>>> {
>>> - if (should_release_lock(lock)) {
>>> + enum compact_contended contended = should_release_lock(lock);
>>> +
>>> + if (contended) {
>>> if (locked) {
>>> spin_unlock_irqrestore(lock, *flags);
>>> locked = false;
>>> @@ -205,7 +212,7 @@ static bool compact_checklock_irqsave(spinlock_t *lock, unsigned long *flags,
>>>
>>> /* async aborts if taking too long or contended */
>>> if (cc->mode == MIGRATE_ASYNC) {
>>> - cc->contended = true;
>>> + cc->contended = contended;
>>> return false;
>>> }
>>
>>
>>>
>>> @@ -231,7 +238,7 @@ static inline bool compact_should_abort(struct compact_control *cc)
>>> /* async compaction aborts if contended */
>>> if (need_resched()) {
>>> if (cc->mode == MIGRATE_ASYNC) {
>>> - cc->contended = true;
>>> + cc->contended = COMPACT_CONTENDED_SCHED;
>>> return true;
>>> }
>>>
>>> @@ -1101,7 +1108,8 @@ static unsigned long compact_zone_order(struct zone *zone, int order,
>>> VM_BUG_ON(!list_empty(&cc.freepages));
>>> VM_BUG_ON(!list_empty(&cc.migratepages));
>>>
>>> - *contended = cc.contended;
>>> + /* We only signal lock contention back to the allocator */
>>> + *contended = cc.contended == COMPACT_CONTENDED_LOCK;
>>
>> Please write down *WHY* as well as your intention we can know by looking at code.
>>
>>> return ret;
>>> }
>>>
>>> @@ -1132,6 +1140,7 @@ unsigned long try_to_compact_pages(struct zonelist *zonelist,
>>> struct zone *zone;
>>> int rc = COMPACT_SKIPPED;
>>> int alloc_flags = 0;
>>> + bool all_zones_contended = true;
>>>
>>> /* Check if the GFP flags allow compaction */
>>> if (!order || !may_enter_fs || !may_perform_io)
>>> @@ -1146,6 +1155,7 @@ unsigned long try_to_compact_pages(struct zonelist *zonelist,
>>> for_each_zone_zonelist_nodemask(zone, z, zonelist, high_zoneidx,
>>> nodemask) {
>>> int status;
>>> + bool zone_contended;
>>>
>>> if (compaction_deferred(zone, order))
>>> continue;
>>> @@ -1153,8 +1163,9 @@ unsigned long try_to_compact_pages(struct zonelist *zonelist,
>>> *deferred = false;
>>>
>>> status = compact_zone_order(zone, order, gfp_mask, mode,
>>> - contended);
>>> + &zone_contended);
>>> rc = max(status, rc);
>>> + all_zones_contended &= zone_contended;
>>>
>>> /* If a normal allocation would succeed, stop compacting */
>>> if (zone_watermark_ok(zone, order, low_wmark_pages(zone), 0,
>>> @@ -1168,12 +1179,31 @@ unsigned long try_to_compact_pages(struct zonelist *zonelist,
>>> * succeeding after all, it will be reset.
>>> */
>>> defer_compaction(zone, order);
>>> + /*
>>> + * If we stopped compacting due to need_resched(), do
>>> + * not try further zones and yield the CPU.
>>> + */
>>
>> For what? It would make your claim more clear.
>>
>>> + if (need_resched()) {
>>
>> compact_zone_order returns true state of contended only if it was lock contention
>> so it couldn't return true state of contended by need_resched so you made
>> need_resched check in here. It's fragile to me because it could be not a result
>> from ahead compact_zone_order call. More clear thing is compact_zone_order
>> should return zone_contended as enum, not bool and in here, you could check it.
>>
>> It means you could return enum in compact_zone_order and make the result bool
>> in try_to_compact_pages.
>>
>>> + /*
>>> + * We might not have tried all the zones, so
>>> + * be conservative and assume they are not
>>> + * all lock contended.
>>> + */
>>> + all_zones_contended = false;
>>> + cond_resched();
>>> + break;
>>> + }
>>> }
>>> }
>>>
>>> - /* If at least one zone wasn't deferred, we count a compaction stall */
>>> - if (!*deferred)
>>> + /*
>>> + * If at least one zone wasn't deferred, we count a compaction stall
>>> + * and we report if all zones that were tried were contended.
>>> + */
>>> + if (!*deferred) {
>>> count_compact_event(COMPACTSTALL);
>>> + *contended = all_zones_contended;
>>
>> Why don't you initialize contended as *false* in function's intro?
>>
>>> + }
>>>
>>> return rc;
>>> }
>>> diff --git a/mm/internal.h b/mm/internal.h
>>> index a1b651b..2c187d2 100644
>>> --- a/mm/internal.h
>>> +++ b/mm/internal.h
>>> @@ -117,6 +117,13 @@ extern int user_min_free_kbytes;
>>>
>>> #if defined CONFIG_COMPACTION || defined CONFIG_CMA
>>>
>>> +/* Used to signal whether compaction detected need_sched() or lock contention */
>>> +enum compact_contended {
>>> + COMPACT_CONTENDED_NONE = 0, /* no contention detected */
>>> + COMPACT_CONTENDED_SCHED, /* need_sched() was true */
>>> + COMPACT_CONTENDED_LOCK, /* zone lock or lru_lock was contended */
>>> +};
>>> +
>>> /*
>>> * in mm/compaction.c
>>> */
>>> @@ -144,10 +151,10 @@ struct compact_control {
>>> int order; /* order a direct compactor needs */
>>> int migratetype; /* MOVABLE, RECLAIMABLE etc */
>>> struct zone *zone;
>>> - bool contended; /* True if a lock was contended, or
>>> - * need_resched() true during async
>>> - * compaction
>>> - */
>>> + enum compact_contended contended; /* Signal need_sched() or lock
>>> + * contention detected during
>>> + * compaction
>>> + */
>>> };
>>>
>>> unsigned long
>>> --
>>
>> Anyway, most big concern is that you are changing current behavior as
>> I said earlier.
>
> Well that behavior is pretty new, introduced recently by David's patches
> and now he himself is saying that it makes page fault THP allocations
> very unsuccessful and the need_resched() is not a good termination
> criterion. But OK, I can restrict this to khugepaged...
>
>> Old behavior in THP page fault when it consumes own timeslot was just
>> abort and fallback 4K page but with your patch, new behavior is
>> take a rest when it founds need_resched and goes to another round with
>> async, not sync compaction. I'm not sure we need another round with
>> async compaction at the cost of increasing latency rather than fallback
>> 4 page.
> >
>> It might be okay if the VMA has MADV_HUGEPAGE which is good hint to
>> indicate non-temporal VMA so latency would be trade-off but it's not
>> for temporal big memory allocation in HUGEPAGE_ALWAYS system.
>
> Yeah that could be useful hint when we eventually move away from
> need_resched() to a fixed/tunable work per compaction attempt. But I'll
> rather not introduce it in the series.
>
>> If you really want to go this, could you show us numbers?
>>
>> 1. How many could we can be successful in direct compaction by this patch?
>
> Doesn't seem to help. I'll drop it.

Actually the patch was wrong as the need_resched() check occured under
if (mode != MIGRATE_ASYNC) branch. But need_resched() contention is only
considered in MIGRATE_ASYNC mode. Oops.

>> 2. How long could we increase latency for temporal allocation
>> for HUGEPAGE_ALWAYS system?
>
> Don't have the data.
>



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