Re: [RFC PATCH 1/6] mm, compaction: periodically drop lock and restore IRQs in scanners

[David Rientjes <rientjes@google.com>]

On Wed, 4 Jun 2014, Vlastimil Babka wrote:

> diff --git a/mm/compaction.c b/mm/compaction.c
> index ed7102c..f0fd4b5 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -185,47 +185,74 @@ static void update_pageblock_skip(struct compact_control *cc,
>  }
>  #endif /* CONFIG_COMPACTION */
>  
> -static inline bool should_release_lock(spinlock_t *lock)
> +/*
> + * 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 if the process
> + * needs to be scheduled, or the lock cannot be taken immediately. For sync
> + * compaction, schedule and 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 inline bool compact_trylock_irqsave(spinlock_t *lock,
> +			unsigned long *flags, struct compact_control *cc)

Hmm, what tree is this series based on?  It doesn't apply cleanly to 
linux-next, I think you're missing
mm-compaction-properly-signal-and-act-upon-lock-and-need_sched-contention-fix.patch
in your tree.

Is there a performance benefit to doing the inlining here?

>  {
> -	return need_resched() || spin_is_contended(lock);
> +	if (cc->mode == MIGRATE_ASYNC) {
> +		if (need_resched() || !spin_trylock_irqsave(lock, *flags)) {
> +			cc->contended = true;
> +			return false;
> +		}
> +	} else {
> +		cond_resched();

Why do we need this cond_resched() here if there is already a 
cond_resched() in compact_unlock_should_abort() for non-async compaction?

If this is trying to reschedule right before disabling irqs because 
otherwise spinning on the lock causes irq starvation, then that's a very 
delicate balance and I think we're going to get in trouble later.

> +		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, or somebody else
> + * has taken the lock, 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 lock contention or need to schedule
> + * 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 inline bool compact_unlock_should_abort(spinlock_t *lock,
> +		unsigned long flags, bool *locked, struct compact_control *cc)

This inlining is also suspicious and I think keeping both of them 
out-of-line for the freeing and migration scanners is going to be the best 
route unless there's some measurable performance benefit I'm not seeing.

>  {
> -	if (should_release_lock(lock)) {
> -		if (locked) {
> -			spin_unlock_irqrestore(lock, *flags);
> -			locked = false;
> -		}
> +	if (*locked) {
> +		spin_unlock_irqrestore(lock, flags);
> +		*locked = false;
> +	}
>  
> -		/* async aborts if taking too long or contended */
> -		if (cc->mode == MIGRATE_ASYNC) {
> +	if (fatal_signal_pending(current))
> +		return true;
> +
> +	if (cc->mode == MIGRATE_ASYNC) {
> +		if (need_resched() || spin_is_locked(lock)) {
>  			cc->contended = true;
> -			return false;
> +			return true;
>  		}
> -
> +	} else {
>  		cond_resched();
>  	}
>  
> -	if (!locked)
> -		spin_lock_irqsave(lock, *flags);
> -	return true;
> +	return false;
>  }
>  
>  /*
> - * Aside from avoiding lock contention, compaction also periodically checks
> + * Aside from avoiding lock contention, compaction should also periodically checks

Not sure what the purpose of this commentary change is, it's gramatically 
incorrect now.

>   * need_resched() and either schedules in sync compaction, or aborts async
> - * compaction. This is similar to compact_checklock_irqsave() does, but used
> + * compaction. This is similar to compact_unlock_should_abort() does, but used

This was and still is gramatically incorrect :)

>   * where no lock is concerned.
>   *
>   * Returns false when no scheduling was needed, or sync compaction scheduled.
> @@ -285,6 +312,16 @@ 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;
> @@ -302,8 +339,9 @@ 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;
>  
> @@ -523,13 +561,15 @@ 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
> @@ -631,10 +671,11 @@ 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 */

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