Re: [PATCH 02/10] mm, compaction: report compaction as contended only due to lock contention

[Minchan Kim <minchan@kernel.org>]

On Wed, Jun 11, 2014 at 02:22:30PM +0200, Vlastimil Babka wrote:
> On 06/11/2014 03:10 AM, Minchan Kim wrote:
> >On Mon, Jun 09, 2014 at 11:26:14AM +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 hugepaged 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
> >>contantion has been detected.
> >>
> >>As a result, hugepaged will proceed with second sync compaction as intended,
> >>when the preceding async compaction aborted due to need_resched().
> >
> >You said "second direct compaction is never attempted, which might be OK
> >for a page fault" and said "hugepagd is intented to attempt a sync compaction"
> >so I feel you want to handle khugepaged so special unlike other direct compact
> >(ex, page fault).
> 
> Well khugepaged is my primary concern, but I imagine there are other
> direct compaction users besides THP page fault and khugepaged.
> 
> >By this patch, direct compaction take care only lock contention, not rescheduling
> >so that pop questions.
> >
> >Is it okay not to consider need_resched in direct compaction really?
> 
> It still considers need_resched() to back of from async compaction.
> It's only about signaling contended_compaction back to
> __alloc_pages_slowpath(). There's this code executed after the
> first, async compaction fails:
> 
> /*
>  * It can become very expensive to allocate transparent hugepages at
>  * fault, so use asynchronous memory compaction for THP unless it is
>  * khugepaged trying to collapse.
>  */
> if (!(gfp_mask & __GFP_NO_KSWAPD) || (current->flags & PF_KTHREAD))
>         migration_mode = MIGRATE_SYNC_LIGHT;
> 
> /*
>  * If compaction is deferred for high-order allocations, it is because
>  * sync compaction recently failed. In this is the case and the caller
>  * requested a movable allocation that does not heavily disrupt the
>  * system then fail the allocation instead of entering direct reclaim.
>  */
> if ((deferred_compaction || contended_compaction) &&
>                                         (gfp_mask & __GFP_NO_KSWAPD))
>         goto nopage;
> 
> Both THP page fault and khugepaged use __GFP_NO_KSWAPD. The first
> if() decides whether the second attempt will be sync (for
> khugepaged) or async (page fault). The second if() decides that if
> compaction was contended, then there won't be any second attempt
> (and reclaim) at all. Counting need_resched() as contended in this
> case is bad for khugepaged. Even for page fault it means no direct

I agree khugepaged shouldn't count on need_resched, even lock contention
because it was a result from admin's decision.
If it hurts system performance, he should adjust knobs for khugepaged.

> reclaim and a second async compaction. David says need_resched()
> occurs so often then it is a poor heuristic to decide this.

But page fault is a bit different. Inherently, high-order allocation
(ie, above PAGE_ALLOC_COSTLY_ORDER) is fragile so all of the caller
shoud keep in mind that and prepare second plan(ex, 4K allocation)
so direct reclaim/compaction should take care of latency rather than
success ratio.

If need_resched in second attempt(ie, synchronous compaction) is almost
true, it means the process consumed his timeslice so it shouldn't be
greedy and gives a CPU resource to others.
I don't mean we should abort but the process could sleep and retry.
The point is that we should give latency pain to the process request
high-order alocation, not another random process.

IMHO, if we want to increase high-order alloc ratio in page fault,
kswapd should be more aggressive than now via feedback loop from
fail rate from direct compaction.

> 
> >We have taken care of it in direct reclaim path so why direct compaction is
> >so special?
> 
> I admit I'm not that familiar with reclaim but I didn't quickly find
> any need_resched() there? There's plenty of cond_resched() but that
> doesn't mean it will abort? Could you explain for me?

I meant cond_resched.

> 
> >Why does khugepaged give up easily if lock contention/need_resched happens?
> >khugepaged is important for success ratio as I read your description so IMO,
> >khugepaged should do synchronously without considering early bail out by
> >lock/rescheduling.
> 
> Well a stupid answer is that's how __alloc_pages_slowpath() works :)
> I don't think it's bad to try using first a more lightweight
> approach before trying the heavyweight one. As long as the
> heavyweight one is not skipped for khugepaged.

I'm not saying current two-stage trying is bad. My stand is that we should
take care of need_resched and shouldn't become a greedy but khugepaged would
be okay.

> 
> >If it causes problems, user should increase scan_sleep_millisecs/alloc_sleep_millisecs,
> >which is exactly the knob for that cases.
> >
> >So, my point is how about making khugepaged doing always dumb synchronous
> >compaction thorough PG_KHUGEPAGED or GFP_SYNC_TRANSHUGE?
> >
> >>
> >>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 | 20 ++++++++++++++------
> >>  mm/internal.h   | 15 +++++++++++----
> >>  2 files changed, 25 insertions(+), 10 deletions(-)
> >>
> >>diff --git a/mm/compaction.c b/mm/compaction.c
> >>index b73b182..d37f4a8 100644
> >>--- a/mm/compaction.c
> >>+++ b/mm/compaction.c
> >>@@ -185,9 +185,14 @@ 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 (need_resched())
> >>+		return COMPACT_CONTENDED_SCHED;
> >>+	else if (spin_is_contended(lock))
> >>+		return COMPACT_CONTENDED_LOCK;
> >>+	else
> >>+		return COMPACT_CONTENDED_NONE;
> >>  }
> >>
> >>  /*
> >>@@ -202,7 +207,9 @@ 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;
> >>@@ -210,7 +217,7 @@ 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;
> >>  		}
> >>
> >>@@ -236,7 +243,7 @@ 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;
> >>  		}
> >>
> >>@@ -1095,7 +1102,8 @@ 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;
> >>  	return ret;
> >>  }
> >>
> >>diff --git a/mm/internal.h b/mm/internal.h
> >>index 7f22a11f..4659e8e 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
> >>--
> >>1.8.4.5
> >>
> >>--
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> >
> 
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-- 
Kind regards,
Minchan Kim

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