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

David Rientjes
2014-06-04T23:39:30Z

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 */