Re: [PATCH 3/3] mm, vmscan: Prevent kswapd sleeping prematurely due to mismatched classzone_idx

[Vlastimil Babka <vbabka@suse.cz>]

On 02/23/2017 04:01 PM, Mel Gorman wrote:
> On Mon, Feb 20, 2017 at 05:42:49PM +0100, Vlastimil Babka wrote:
>>> With this patch on top, all the latencies relative to the baseline are
>>> improved, particularly write latencies. The read latencies are still high
>>> for the number of threads but it's worth noting that this is mostly due
>>> to the IO scheduler and not directly related to reclaim. The vmstats are
>>> a bit of a mix but the relevant ones are as follows;
>>>
>>>                             4.10.0-rc7  4.10.0-rc7  4.10.0-rc7
>>>                           mmots-20170209 clear-v1r25keepawake-v1r25
>>> Swap Ins                             0           0           0
>>> Swap Outs                            0         608           0
>>> Direct pages scanned           6910672     3132699     6357298
>>> Kswapd pages scanned          57036946    82488665    56986286
>>> Kswapd pages reclaimed        55993488    63474329    55939113
>>> Direct pages reclaimed         6905990     2964843     6352115
>>
>> These stats are confusing me. The earlier description suggests that this patch
>> should cause less direct reclaim and more kswapd reclaim, but compared to
>> "clear-v1r25" it does the opposite? Was clear-v1r25 overreclaiming then? (when
>> considering direct + kswapd combined)
>>
> 
> The description is referring to the impact relative to baseline. It is
> true that relative to patch that direct reclaim is higher but there are
> a number of anomalies.
> 
> Note that kswapd is scanning very aggressively in "clear-v1" and overall
> efficiency is down to 76%. It's also not clear in the stats but in
> "clear-v1", pgskip_* is active as the wrong zone is being reclaimed for
> due to the patch "mm, vmscan: fix zone balance check in
> prepare_kswapd_sleep". It's also doing a lot of writing of file-backed
> pages from reclaim context and some swapping due to the aggressiveness
> of the scan.
> 
> While direct reclaim activity might be lower, it's due to kswapd scanning
> aggressively and trying to reclaim the world which is not the right thing
> to do.  With the patches applied, there is still direct reclaim but the fast
> bulk of them are when the workload changes phase from "creating work files"
> to starting multiple threads that allocate a lot of anonymous memory with
> a sudden spike in memory pressure that kswapd does not keep ahead of with
> multiple allocating threads.

Thanks for the explanation.

> 
>>> @@ -3328,6 +3330,22 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
>>>  	return sc.order;
>>>  }
>>>  
>>> +/*
>>> + * pgdat->kswapd_classzone_idx is the highest zone index that a recent
>>> + * allocation request woke kswapd for. When kswapd has not woken recently,
>>> + * the value is MAX_NR_ZONES which is not a valid index. This compares a
>>> + * given classzone and returns it or the highest classzone index kswapd
>>> + * was recently woke for.
>>> + */
>>> +static enum zone_type kswapd_classzone_idx(pg_data_t *pgdat,
>>> +					   enum zone_type classzone_idx)
>>> +{
>>> +	if (pgdat->kswapd_classzone_idx == MAX_NR_ZONES)
>>> +		return classzone_idx;
>>> +
>>> +	return max(pgdat->kswapd_classzone_idx, classzone_idx);
>>
>> A bit paranoid comment: this should probably read pgdat->kswapd_classzone_idx to
>> a local variable with READ_ONCE(), otherwise something can set it to
>> MAX_NR_ZONES between the check and max(), and compiler can decide to reread.
>> Probably not an issue with current callers, but I'd rather future-proof it.
>>
> 
> I'm a little wary of adding READ_ONCE unless there is a definite
> problem. Even if it was an issue, I think it would be better to protect
> thse kswapd_classzone_idx and kswapd_order with a spinlock that is taken
> if an update is required or a read to fully guarantee the ordering.
> 
> The consequences as they are is that kswapd may miss reclaiming at a
> higher order or classzone than it should have although it is very
> unlikely and the update and read are made with a workqueue wake and
> scheduler wakeup which should be sufficient in terms of barriers.

OK then.

Acked-by: Vlastimil Babka <vbabka@suse.cz>