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SubjectRe: [PATCH v3 06/13] mm, compaction: periodically drop lock and restore IRQs in scanners
On 06/23/2014 04:53 AM, Minchan Kim wrote:
> 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@suse.cz>
>> Cc: Minchan Kim <minchan@kernel.org>
>> Cc: Mel Gorman <mgorman@suse.de>
>> 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>
>> Cc: David Rientjes <rientjes@google.com>
>> ---
>> 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?

Hm right, try_to_compact_pages should check it before trying another
zone. Then it will be ok.

> 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?

A good place to fix it is the previous patch. Thanks.

> If you want to handle it later, please put the XXX for TODO.
> Anyway,
>
> Acked-by: Minchan Kim <minchan@kernel.org>
>
>>
>> - /* 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
>>
>> --
>> To unsubscribe, send a message with 'unsubscribe linux-mm' in
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>



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