Re: [PATCH 03/10] mm: vmscan: Flatten kswapd priority loop

[Mel Gorman <mgorman@suse.de>]

On Thu, Mar 21, 2013 at 03:54:58PM +0100, Michal Hocko wrote:
> > Signed-off-by: Mel Gorman <mgorman@suse.de>
> > ---
> >  mm/vmscan.c | 86 ++++++++++++++++++++++++++++++-------------------------------
> >  1 file changed, 42 insertions(+), 44 deletions(-)
> > 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 182ff15..279d0c2 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -2625,8 +2625,11 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining,
> >  /*
> >   * kswapd shrinks the zone by the number of pages required to reach
> >   * the high watermark.
> > + *
> > + * Returns true if kswapd scanned at least the requested number of
> > + * pages to reclaim.
> 
> Maybe move the comment about not rising priority in such case here to be
> clear what the return value means. Without that the return value could
> be misinterpreted that kswapd_shrink_zone succeeded in shrinking might
> be not true.

I moved the comment.

> Or maybe even better, leave the void there and add bool *raise_priority
> argument here so the decision and raise_priority are at the same place.
> 

The priority is raised if kswapd failed to reclaim from any of the unbalanced
zone. If raise_priority is moved inside kswapd_shrink_zone then it can
only take one zone into account.

> >   */
> > -static void kswapd_shrink_zone(struct zone *zone,
> > +static bool kswapd_shrink_zone(struct zone *zone,
> >  			       struct scan_control *sc,
> >  			       unsigned long lru_pages)
> >  {
> > @@ -2646,6 +2649,8 @@ static void kswapd_shrink_zone(struct zone *zone,
> >  
> >  	if (nr_slab == 0 && !zone_reclaimable(zone))
> >  		zone->all_unreclaimable = 1;
> > +
> > +	return sc->nr_scanned >= sc->nr_to_reclaim;
> >  }
> >  
> >  /*
> [...]
> > @@ -2803,8 +2805,16 @@ loop_again:
> >  
> >  			if ((buffer_heads_over_limit && is_highmem_idx(i)) ||
> >  			    !zone_balanced(zone, testorder,
> > -					   balance_gap, end_zone))
> > -				kswapd_shrink_zone(zone, &sc, lru_pages);
> > +					   balance_gap, end_zone)) {
> > +				/*
> > +				 * There should be no need to raise the
> > +				 * scanning priority if enough pages are
> > +				 * already being scanned that that high
> 
> s/that that/that/
> 

Fixed

> > +				 * watermark would be met at 100% efficiency.
> > +				 */
> > +				if (kswapd_shrink_zone(zone, &sc, lru_pages))
> > +					raise_priority = false;
> > +			}
> >  
> >  			/*
> >  			 * If we're getting trouble reclaiming, start doing
> > @@ -2839,46 +2849,33 @@ loop_again:
> >  				pfmemalloc_watermark_ok(pgdat))
> >  			wake_up(&pgdat->pfmemalloc_wait);
> >  
> > -		if (pgdat_balanced(pgdat, order, *classzone_idx)) {
> > -			pgdat_is_balanced = true;
> > -			break;		/* kswapd: all done */
> > -		}
> > -
> >  		/*
> > -		 * We do this so kswapd doesn't build up large priorities for
> > -		 * example when it is freeing in parallel with allocators. It
> > -		 * matches the direct reclaim path behaviour in terms of impact
> > -		 * on zone->*_priority.
> > +		 * Fragmentation may mean that the system cannot be rebalanced
> > +		 * for high-order allocations in all zones. If twice the
> > +		 * allocation size has been reclaimed and the zones are still
> > +		 * not balanced then recheck the watermarks at order-0 to
> > +		 * prevent kswapd reclaiming excessively. Assume that a
> > +		 * process requested a high-order can direct reclaim/compact.
> >  		 */
> > -		if (sc.nr_reclaimed >= SWAP_CLUSTER_MAX)
> > -			break;
> > -	} while (--sc.priority >= 0);
> > +		if (order && sc.nr_reclaimed >= 2UL << order)
> > +			order = sc.order = 0;
> >  
> > -out:
> > -	if (!pgdat_is_balanced) {
> > -		cond_resched();
> > +		/* Check if kswapd should be suspending */
> > +		if (try_to_freeze() || kthread_should_stop())
> > +			break;
> >  
> > -		try_to_freeze();
> > +		/* If no reclaim progress then increase scanning priority */
> > +		if (sc.nr_reclaimed - nr_reclaimed == 0)
> > +			raise_priority = true;
> >  
> >  		/*
> > -		 * Fragmentation may mean that the system cannot be
> > -		 * rebalanced for high-order allocations in all zones.
> > -		 * At this point, if nr_reclaimed < SWAP_CLUSTER_MAX,
> > -		 * it means the zones have been fully scanned and are still
> > -		 * not balanced. For high-order allocations, there is
> > -		 * little point trying all over again as kswapd may
> > -		 * infinite loop.
> > -		 *
> > -		 * Instead, recheck all watermarks at order-0 as they
> > -		 * are the most important. If watermarks are ok, kswapd will go
> > -		 * back to sleep. High-order users can still perform direct
> > -		 * reclaim if they wish.
> > +		 * Raise priority if scanning rate is too low or there was no
> > +		 * progress in reclaiming pages
> >  		 */
> > -		if (sc.nr_reclaimed < SWAP_CLUSTER_MAX)
> > -			order = sc.order = 0;
> > -
> > -		goto loop_again;
> > -	}
> > +		if (raise_priority || sc.nr_reclaimed - nr_reclaimed == 0)
> 
> (sc.nr_reclaimed - nr_reclaimed == 0) is redundant because you already
> set raise_priority above in that case.
> 

I removed the redundant check.

> > +			sc.priority--;
> > +	} while (sc.priority >= 0 &&
> > +		 !pgdat_balanced(pgdat, order, *classzone_idx));
> >  
> >  	/*
> >  	 * If kswapd was reclaiming at a higher order, it has the option of
> > @@ -2907,6 +2904,7 @@ out:
> >  			compact_pgdat(pgdat, order);
> >  	}
> >  
> > +out:
> >  	/*
> >  	 * Return the order we were reclaiming at so prepare_kswapd_sleep()
> >  	 * makes a decision on the order we were last reclaiming at. However,
> 
> It looks OK otherwise but I have to think some more as balance_pgdat is
> still tricky, albeit less then it was before so this is definitely
> progress.
> 

Thanks.

-- 
Mel Gorman
SUSE Labs

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