Re: [PATCH 4/4] mm: vmscan: Only read new_classzone_idx from pgdat when reclaiming successfully

[Rik van Riel <riel@redhat.com>]

On 06/24/2011 10:44 AM, Mel Gorman wrote:
> During allocator-intensive workloads, kswapd will be woken frequently
> causing free memory to oscillate between the high and min watermark.
> This is expected behaviour.  Unfortunately, if the highest zone is
> small, a problem occurs.
>
> When balance_pgdat() returns, it may be at a lower classzone_idx than
> it started because the highest zone was unreclaimable. Before checking
> if it should go to sleep though, it checks pgdat->classzone_idx which
> when there is no other activity will be MAX_NR_ZONES-1. It interprets
> this as it has been woken up while reclaiming, skips scheduling and
> reclaims again. As there is no useful reclaim work to do, it enters
> into a loop of shrinking slab consuming loads of CPU until the highest
> zone becomes reclaimable for a long period of time.
>
> There are two problems here. 1) If the returned classzone or order is
> lower, it'll continue reclaiming without scheduling. 2) if the highest
> zone was marked unreclaimable but balance_pgdat() returns immediately
> at DEF_PRIORITY, the new lower classzone is not communicated back to
> kswapd() for sleeping.
>
> This patch does two things that are related. If the end_zone is
> unreclaimable, this information is communicated back. Second, if
> the classzone or order was reduced due to failing to reclaim, new
> information is not read from pgdat and instead an attempt is made to go
> to sleep. Due to this, it is also necessary that pgdat->classzone_idx
> be initialised each time to pgdat->nr_zones - 1 to avoid re-reads
> being interpreted as wakeups.
>
> Reported-and-tested-by: Pádraig Brady<P@draigBrady.com>
> Signed-off-by: Mel Gorman<mgorman@suse.de>

Acked-by: Rik van Riel <riel@redhat.com>

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