Re: [Qemu-devel] [PATCH 5/9] mm: compaction: Acquire the zone->lru_lock as late as possible

[Minchan Kim <minchan@kernel.org>]

On Tue, Sep 25, 2012 at 08:51:05AM +0100, Mel Gorman wrote:
> On Tue, Sep 25, 2012 at 04:05:17PM +0900, Minchan Kim wrote:
> > Hi Mel,
> > 
> > I have a question below.
> > 
> > On Fri, Sep 21, 2012 at 11:46:19AM +0100, Mel Gorman wrote:
> > > Compactions migrate scanner acquires the zone->lru_lock when scanning a range
> > > of pages looking for LRU pages to acquire. It does this even if there are
> > > no LRU pages in the range. If multiple processes are compacting then this
> > > can cause severe locking contention. To make matters worse commit b2eef8c0
> > > (mm: compaction: minimise the time IRQs are disabled while isolating pages
> > > for migration) releases the lru_lock every SWAP_CLUSTER_MAX pages that are
> > > scanned.
> > > 
> > > This patch makes two changes to how the migrate scanner acquires the LRU
> > > lock. First, it only releases the LRU lock every SWAP_CLUSTER_MAX pages if
> > > the lock is contended. This reduces the number of times it unnecessarily
> > > disables and re-enables IRQs. The second is that it defers acquiring the
> > > LRU lock for as long as possible. If there are no LRU pages or the only
> > > LRU pages are transhuge then the LRU lock will not be acquired at all
> > > which reduces contention on zone->lru_lock.
> > > 
> > > Signed-off-by: Mel Gorman <mgorman@suse.de>
> > > Acked-by: Rik van Riel <riel@redhat.com>
> > > ---
> > >  mm/compaction.c |   63 +++++++++++++++++++++++++++++++++++++------------------
> > >  1 file changed, 43 insertions(+), 20 deletions(-)
> > > 
> > > diff --git a/mm/compaction.c b/mm/compaction.c
> > > index 6b55491..a6068ff 100644
> > > --- a/mm/compaction.c
> > > +++ b/mm/compaction.c
> > > @@ -50,6 +50,11 @@ static inline bool migrate_async_suitable(int migratetype)
> > >  	return is_migrate_cma(migratetype) || migratetype == MIGRATE_MOVABLE;
> > >  }
> > >  
> > > +static inline bool should_release_lock(spinlock_t *lock)
> > > +{
> > > +	return need_resched() || spin_is_contended(lock);
> > > +}
> > > +
> > >  /*
> > >   * Compaction requires the taking of some coarse locks that are potentially
> > >   * very heavily contended. Check if the process needs to be scheduled or
> > > @@ -62,7 +67,7 @@ static inline bool migrate_async_suitable(int migratetype)
> > >  static bool compact_checklock_irqsave(spinlock_t *lock, unsigned long *flags,
> > >  				      bool locked, struct compact_control *cc)
> > >  {
> > > -	if (need_resched() || spin_is_contended(lock)) {
> > > +	if (should_release_lock(lock)) {
> > >  		if (locked) {
> > >  			spin_unlock_irqrestore(lock, *flags);
> > >  			locked = false;
> > > @@ -327,7 +332,7 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
> > >  	isolate_mode_t mode = 0;
> > >  	struct lruvec *lruvec;
> > >  	unsigned long flags;
> > > -	bool locked;
> > > +	bool locked = false;
> > >  
> > >  	/*
> > >  	 * Ensure that there are not too many pages isolated from the LRU
> > > @@ -347,23 +352,17 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
> > >  
> > >  	/* Time to isolate some pages for migration */
> > >  	cond_resched();
> > > -	spin_lock_irqsave(&zone->lru_lock, flags);
> > > -	locked = true;
> > >  	for (; low_pfn < end_pfn; low_pfn++) {
> > >  		struct page *page;
> > >  
> > >  		/* give a chance to irqs before checking need_resched() */
> > > -		if (!((low_pfn+1) % SWAP_CLUSTER_MAX)) {
> > > -			spin_unlock_irqrestore(&zone->lru_lock, flags);
> > > -			locked = false;
> > > +		if (locked && !((low_pfn+1) % SWAP_CLUSTER_MAX)) {
> > > +			if (should_release_lock(&zone->lru_lock)) {
> > > +				spin_unlock_irqrestore(&zone->lru_lock, flags);
> > > +				locked = false;
> > > +			}
> > >  		}
> > >  
> > > -		/* 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))
> > > -			break;
> > > -
> > >  		/*
> > >  		 * migrate_pfn does not necessarily start aligned to a
> > >  		 * pageblock. Ensure that pfn_valid is called when moving
> > > @@ -403,21 +402,38 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
> > >  		pageblock_nr = low_pfn >> pageblock_order;
> > >  		if (!cc->sync && last_pageblock_nr != pageblock_nr &&
> > >  		    !migrate_async_suitable(get_pageblock_migratetype(page))) {
> > > -			low_pfn += pageblock_nr_pages;
> > > -			low_pfn = ALIGN(low_pfn, pageblock_nr_pages) - 1;
> > > -			last_pageblock_nr = pageblock_nr;
> > > -			continue;
> > > +			goto next_pageblock;
> > >  		}
> > >  
> > > +		/* Check may be lockless but that's ok as we recheck later */
> > >  		if (!PageLRU(page))
> > >  			continue;
> > >  
> > >  		/*
> > > -		 * PageLRU is set, and lru_lock excludes isolation,
> > > -		 * splitting and collapsing (collapsing has already
> > > -		 * happened if PageLRU is set).
> > > +		 * PageLRU is set. lru_lock normally excludes isolation
> > > +		 * splitting and collapsing (collapsing has already happened
> > > +		 * if PageLRU is set) but the lock is not necessarily taken
> > > +		 * here and it is wasteful to take it just to check transhuge.
> > > +		 * Check transhuge without lock and skip if it's either a
> > > +		 * transhuge or hugetlbfs page.
> > >  		 */
> > >  		if (PageTransHuge(page)) {
> > > +			if (!locked)
> > > +				goto next_pageblock;
> > 
> > Why skip all pages in a pageblock if !locked?
> > Shouldn't we add some comment?
> > 
> 
> The comment is above the block already. The lru_lock normally excludes
> isolation and splitting. If we do not hold the hold, it's not safe to
> call compound_order so instead we skip the entire pageblock.

I see. To me, your saying is better than current comment.
I hope comment could be more explicit.

diff --git a/mm/compaction.c b/mm/compaction.c
index df01b4e..f1d2cc7 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -542,8 +542,9 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
                 * splitting and collapsing (collapsing has already happened
                 * if PageLRU is set) but the lock is not necessarily taken
                 * here and it is wasteful to take it just to check transhuge.
-                * Check transhuge without lock and skip if it's either a
-                * transhuge or hugetlbfs page.
+                * Check transhuge without lock and *skip* if it's either a
+                * transhuge or hugetlbfs page because it's not safe to call
+                * compound_order.
                 */
                if (PageTransHuge(page)) {
                        if (!locked)


Anyway, it's trivial and if anyone think it's valuable, feel free to apply it, please.

> 
> -- 
> Mel Gorman
> SUSE Labs
> 
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-- 
Kind regards,
Minchan Kim