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

[Mel Gorman <mgorman@suse.de>]

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.

-- 
Mel Gorman
SUSE Labs