Re: [PATCH 1/4] mm: vmscan: Correct check for kswapd sleeping insleeping_prematurely

[Pádraig Brady]

On 28/06/11 22:49, Andrew Morton wrote:
> On Fri, 24 Jun 2011 15:44:54 +0100
> Mel Gorman <mgorman@xxxxxxx> 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.
>>
>> A problem occurs if the highest zone is small.  balance_pgdat()
>> only considers unreclaimable zones when priority is DEF_PRIORITY
>> but sleeping_prematurely considers all zones. It's possible for this
>> sequence to occur
>>
>>   1. kswapd wakes up and enters balance_pgdat()
>>   2. At DEF_PRIORITY, marks highest zone unreclaimable
>>   3. At DEF_PRIORITY-1, ignores highest zone setting end_zone
>>   4. At DEF_PRIORITY-1, calls shrink_slab freeing memory from
>>         highest zone, clearing all_unreclaimable. Highest zone
>>         is still unbalanced
>>   5. kswapd returns and calls sleeping_prematurely
>>   6. sleeping_prematurely looks at *all* zones, not just the ones
>>      being considered by balance_pgdat. The highest small zone
>>      has all_unreclaimable cleared but but the zone is not
>>      balanced. all_zones_ok is false so kswapd stays awake
>>
>> This patch corrects the behaviour of sleeping_prematurely to check
>> the zones balance_pgdat() checked.
> 
> But kswapd is making progress: it's reclaiming slab.  Eventually that
> won't work any more and all_unreclaimable will not be cleared and the
> condition will fix itself up?
> 
> 
> 
> btw,
> 
> 	if (!sleeping_prematurely(...))
> 		sleep();
> 
> hurts my brain.  My brain would prefer
> 
> 	if (kswapd_should_sleep(...))
> 		sleep();
> 
> no?
> 
>> Reported-and-tested-by: Pádraig Brady <P@xxxxxxxxxxxxxx>
> 
> But what were the before-and-after observations?  I don't understand
> how this can cause a permanent cpuchew by kswapd.

Context:
  http://marc.info/?t=130865025500001&r=1&w=2
  https://bugzilla.redhat.com/show_bug.cgi?id=712019

Summary:

This will spin kswapd0 on my SNB laptop with 3GB RAM (with small normal zone):

    dd bs=1M count=3000 if=/dev/zero of=spin.test

Basically once a certain amount of data is cached,
kswapd0 will start spinning, until the data
is removed from cache (by `rm spin.test` for example).

cheers,
Pádraig.
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