Re: [PATCH v3 06/13] mm, compaction: periodically drop lock and restore IRQs in scanners

[Minchan Kim]

On Fri, Jun 20, 2014 at 05:49:36PM +0200, Vlastimil Babka wrote:
> Compaction scanners regularly check for lock contention and need_resched()
> through the compact_checklock_irqsave() function. However, if there is no
> contention, the lock can be held and IRQ disabled for potentially long time.
> 
> This has been addressed by commit b2eef8c0d0 ("mm: compaction: minimise the
> time IRQs are disabled while isolating pages for migration") for the migration
> scanner. However, the refactoring done by commit 748446bb6b ("mm: compaction:
> acquire the zone->lru_lock as late as possible") has changed the conditions so
> that the lock is dropped only when there's contention on the lock or
> need_resched() is true. Also, need_resched() is checked only when the lock is
> already held. The comment "give a chance to irqs before checking need_resched"
> is therefore misleading, as IRQs remain disabled when the check is done.
> 
> This patch restores the behavior intended by commit b2eef8c0d0 and also tries
> to better balance and make more deterministic the time spent by checking for
> contention vs the time the scanners might run between the checks. It also
> avoids situations where checking has not been done often enough before. The
> result should be avoiding both too frequent and too infrequent contention
> checking, and especially the potentially long-running scans with IRQs disabled
> and no checking of need_resched() or for fatal signal pending, which can happen
> when many consecutive pages or pageblocks fail the preliminary tests and do not
> reach the later call site to compact_checklock_irqsave(), as explained below.
> 
> Before the patch:
> 
> In the migration scanner, compact_checklock_irqsave() was called each loop, if
> reached. If not reached, some lower-frequency checking could still be done if
> the lock was already held, but this would not result in aborting contended
> async compaction until reaching compact_checklock_irqsave() or end of
> pageblock. In the free scanner, it was similar but completely without the
> periodical checking, so lock can be potentially held until reaching the end of
> pageblock.
> 
> After the patch, in both scanners:
> 
> The periodical check is done as the first thing in the loop on each
> SWAP_CLUSTER_MAX aligned pfn, using the new compact_unlock_should_abort()
> function, which always unlocks the lock (if locked) and aborts async compaction
> if scheduling is needed. It also aborts any type of compaction when a fatal
> signal is pending.
> 
> The compact_checklock_irqsave() function is replaced with a slightly different
> compact_trylock_irqsave(). The biggest difference is that the function is not
> called at all if the lock is already held. The periodical need_resched()
> checking is left solely to compact_unlock_should_abort(). The lock contention
> avoidance for async compaction is achieved by the periodical unlock by
> compact_unlock_should_abort() and by using trylock in compact_trylock_irqsave()
> and aborting when trylock fails. Sync compaction does not use trylock.
> 
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
> Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> ---
>  mm/compaction.c | 114 ++++++++++++++++++++++++++++++++++++--------------------
>  1 file changed, 73 insertions(+), 41 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index e8cfac9..40da812 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -180,54 +180,72 @@ static void update_pageblock_skip(struct compact_control *cc,
>  }
>  #endif /* CONFIG_COMPACTION */
>  
> -enum compact_contended should_release_lock(spinlock_t *lock)
> +/*
> + * Compaction requires the taking of some coarse locks that are potentially
> + * very heavily contended. For async compaction, back out if the lock cannot
> + * be taken immediately. For sync compaction, spin on the lock if needed.
> + *
> + * Returns true if the lock is held
> + * Returns false if the lock is not held and compaction should abort
> + */
> +static bool compact_trylock_irqsave(spinlock_t *lock,
> +			unsigned long *flags, struct compact_control *cc)
>  {
> -	if (spin_is_contended(lock))
> -		return COMPACT_CONTENDED_LOCK;
> -	else if (need_resched())
> -		return COMPACT_CONTENDED_SCHED;
> -	else
> -		return COMPACT_CONTENDED_NONE;
> +	if (cc->mode == MIGRATE_ASYNC) {
> +		if (!spin_trylock_irqsave(lock, *flags)) {
> +			cc->contended = COMPACT_CONTENDED_LOCK;
> +			return false;
> +		}
> +	} else {
> +		spin_lock_irqsave(lock, *flags);
> +	}
> +
> +	return true;
>  }
>  
>  /*
>   * Compaction requires the taking of some coarse locks that are potentially
> - * very heavily contended. Check if the process needs to be scheduled or
> - * if the lock is contended. For async compaction, back out in the event
> - * if contention is severe. For sync compaction, schedule.
> + * very heavily contended. The lock should be periodically unlocked to avoid
> + * having disabled IRQs for a long time, even when there is nobody waiting on
> + * the lock. It might also be that allowing the IRQs will result in
> + * need_resched() becoming true. If scheduling is needed, async compaction
> + * aborts. Sync compaction schedules.
> + * Either compaction type will also abort if a fatal signal is pending.
> + * In either case if the lock was locked, it is dropped and not regained.
>   *
> - * Returns true if the lock is held.
> - * Returns false if the lock is released and compaction should abort
> + * Returns true if compaction should abort due to fatal signal pending, or
> + *		async compaction due to need_resched()
> + * Returns false when compaction can continue (sync compaction might have
> + *		scheduled)
>   */
> -static bool compact_checklock_irqsave(spinlock_t *lock, unsigned long *flags,
> -				      bool locked, struct compact_control *cc)
> +static bool compact_unlock_should_abort(spinlock_t *lock,
> +		unsigned long flags, bool *locked, struct compact_control *cc)
>  {
> -	enum compact_contended contended = should_release_lock(lock);
> +	if (*locked) {
> +		spin_unlock_irqrestore(lock, flags);
> +		*locked = false;
> +	}
>  
> -	if (contended) {
> -		if (locked) {
> -			spin_unlock_irqrestore(lock, *flags);
> -			locked = false;
> -		}
> +	if (fatal_signal_pending(current)) {
> +		cc->contended = COMPACT_CONTENDED_SCHED;
> +		return true;
> +	}


Generally, this patch is really good for me but I doubt what happens
if we bail out by fatal_signal? All the path is going to handle it
rightly to bail out direct compaction path?
I don't think so but anyway, it would be another patch so do you
handle it later or include it in this patchset series?

If you want to handle it later, please put the XXX for TODO.
Anyway,

Acked-by: Minchan Kim <minchan@xxxxxxxxxx>

>  
> -		/* async aborts if taking too long or contended */
> +	if (need_resched()) {
>  		if (cc->mode == MIGRATE_ASYNC) {
> -			cc->contended = contended;
> -			return false;
> +			cc->contended = COMPACT_CONTENDED_SCHED;
> +			return true;
>  		}
> -
>  		cond_resched();
>  	}
>  
> -	if (!locked)
> -		spin_lock_irqsave(lock, *flags);
> -	return true;
> +	return false;
>  }
>  
>  /*
>   * Aside from avoiding lock contention, compaction also periodically checks
>   * need_resched() and either schedules in sync compaction or aborts async
> - * compaction. This is similar to what compact_checklock_irqsave() does, but
> + * compaction. This is similar to what compact_unlock_should_abort() does, but
>   * is used where no lock is concerned.
>   *
>   * Returns false when no scheduling was needed, or sync compaction scheduled.
> @@ -286,6 +304,16 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
>  		int isolated, i;
>  		struct page *page = cursor;
>  
> +		/*
> +		 * Periodically drop the lock (if held) regardless of its
> +		 * contention, to give chance to IRQs. Abort async compaction
> +		 * if contended.
> +		 */
> +		if (!(blockpfn % SWAP_CLUSTER_MAX)
> +		    && compact_unlock_should_abort(&cc->zone->lock, flags,
> +								&locked, cc))
> +			break;
> +
>  		nr_scanned++;
>  		if (!pfn_valid_within(blockpfn))
>  			goto isolate_fail;
> @@ -303,8 +331,9 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
>  		 * spin on the lock and we acquire the lock as late as
>  		 * possible.
>  		 */
> -		locked = compact_checklock_irqsave(&cc->zone->lock, &flags,
> -								locked, cc);
> +		if (!locked)
> +			locked = compact_trylock_irqsave(&cc->zone->lock,
> +								&flags, cc);
>  		if (!locked)
>  			break;
>  
> @@ -506,13 +535,15 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>  
>  	/* Time to isolate some pages for migration */
>  	for (; low_pfn < end_pfn; low_pfn++) {
> -		/* give a chance to irqs before checking need_resched() */
> -		if (locked && !(low_pfn % SWAP_CLUSTER_MAX)) {
> -			if (should_release_lock(&zone->lru_lock)) {
> -				spin_unlock_irqrestore(&zone->lru_lock, flags);
> -				locked = false;
> -			}
> -		}
> +		/*
> +		 * Periodically drop the lock (if held) regardless of its
> +		 * contention, to give chance to IRQs. Abort async compaction
> +		 * if contended.
> +		 */
> +		if (!(low_pfn % SWAP_CLUSTER_MAX)
> +		    && compact_unlock_should_abort(&zone->lru_lock, flags,
> +								&locked, cc))
> +			break;
>  
>  		/*
>  		 * migrate_pfn does not necessarily start aligned to a
> @@ -592,10 +623,11 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>  		    page_count(page) > page_mapcount(page))
>  			continue;
>  
> -		/* Check if it is ok to still hold the lock */
> -		locked = compact_checklock_irqsave(&zone->lru_lock, &flags,
> -								locked, cc);
> -		if (!locked || fatal_signal_pending(current))
> +		/* If the lock is not held, try to take it */
> +		if (!locked)
> +			locked = compact_trylock_irqsave(&zone->lru_lock,
> +								&flags, cc);
> +		if (!locked)
>  			break;
>  
>  		/* Recheck PageLRU and PageTransHuge under lock */
> -- 
> 1.8.4.5
> 
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-- 
Kind regards,
Minchan Kim
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