Date:	Mon, 9 Jun 2014 16:50:18 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Vlastimil Babka <vbabka@...e.cz>
cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Thelen <gthelen@...gle.com>,
	Minchan Kim <minchan@...nel.org>, Mel Gorman <mgorman@...e.de>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Michal Nazarewicz <mina86@...a86.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 02/10] mm, compaction: report compaction as contended
 only due to lock contention

On Mon, 9 Jun 2014, 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().
> 

s/hugepaged/khugepaged/ on the changelog.

> Reported-by: David Rientjes <rientjes@...gle.com>
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> Cc: Minchan Kim <minchan@...nel.org>
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
> Cc: Michal Nazarewicz <mina86@...a86.com>
> Cc: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> Cc: Christoph Lameter <cl@...ux.com>
> Cc: Rik van Riel <riel@...hat.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;
>  }
>  
>  /*

I think eventually we're going to remove the need_resched() heuristic 
entirely and so enum compact_contended might be overkill, but do we need 
to worry about spin_is_contended(lock) && need_resched() reporting 
COMPACT_CONTENDED_SCHED here instead of COMPACT_CONTENDED_LOCK?

> @@ -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;
>  }
>  

Hmm, since the only thing that matters for cc->contended is 
COMPACT_CONTENDED_LOCK, it may make sense to just leave this as a bool 
within struct compact_control instead of passing the actual reason around 
when it doesn't matter.

> 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
--
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