Re: [PATCH] mm, page_alloc: skip ->waternark_boost for atomic order-0 allocations

[Charan Teja Kalla]

Adding more people to get additional reviewer inputs.

On 6/5/2020 3:13 AM, Andrew Morton wrote:
> On Tue, 19 May 2020 15:28:04 +0530 Charan Teja Reddy <charante@xxxxxxxxxxxxxx> wrote:
> 
>> When boosting is enabled, it is observed that rate of atomic order-0
>> allocation failures are high due to the fact that free levels in the
>> system are checked with ->watermark_boost offset. This is not a problem
>> for sleepable allocations but for atomic allocations which looks like
>> regression.
>>
>> This problem is seen frequently on system setup of Android kernel
>> running on Snapdragon hardware with 4GB RAM size. When no extfrag event
>> occurred in the system, ->watermark_boost factor is zero, thus the
>> watermark configurations in the system are:
>>    _watermark = (
>>           [WMARK_MIN] = 1272, --> ~5MB
>>           [WMARK_LOW] = 9067, --> ~36MB
>>           [WMARK_HIGH] = 9385), --> ~38MB
>>    watermark_boost = 0
>>
>> After launching some memory hungry applications in Android which can
>> cause extfrag events in the system to an extent that ->watermark_boost
>> can be set to max i.e. default boost factor makes it to 150% of high
>> watermark.
>>    _watermark = (
>>           [WMARK_MIN] = 1272, --> ~5MB
>>           [WMARK_LOW] = 9067, --> ~36MB
>>           [WMARK_HIGH] = 9385), --> ~38MB
>>    watermark_boost = 14077, -->~57MB
>>
>> With default system configuration, for an atomic order-0 allocation to
>> succeed, having free memory of ~2MB will suffice. But boosting makes
>> the min_wmark to ~61MB thus for an atomic order-0 allocation to be
>> successful system should have minimum of ~23MB of free memory(from
>> calculations of zone_watermark_ok(), min = 3/4(min/2)). But failures are
>> observed despite system is having ~20MB of free memory. In the testing,
>> this is reproducible as early as first 300secs since boot and with
>> furtherlowram configurations(<2GB) it is observed as early as first
>> 150secs since boot.
>>
>> These failures can be avoided by excluding the ->watermark_boost in
>> watermark caluculations for atomic order-0 allocations.
> 
> Do we have any additional reviewer input on this one?
> 
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -3709,6 +3709,18 @@ static bool zone_allows_reclaim(struct zone *local_zone, struct zone *zone)
>>  		}
>>  
>>  		mark = wmark_pages(zone, alloc_flags & ALLOC_WMARK_MASK);
>> +		/*
>> +		 * Allow GFP_ATOMIC order-0 allocations to exclude the
>> +		 * zone->watermark_boost in its watermark calculations.
>> +		 * We rely on the ALLOC_ flags set for GFP_ATOMIC
>> +		 * requests in gfp_to_alloc_flags() for this. Reason not to
>> +		 * use the GFP_ATOMIC directly is that we want to fall back
>> +		 * to slow path thus wake up kswapd.
>> +		 */
>> +		if (unlikely(!order && !(alloc_flags & ALLOC_WMARK_MASK) &&
>> +		     (alloc_flags & (ALLOC_HARDER | ALLOC_HIGH)))) {
>> +			mark = zone->_watermark[WMARK_MIN];
>> +		}
>>  		if (!zone_watermark_fast(zone, order, mark,
>>  				       ac->highest_zoneidx, alloc_flags)) {
>>  			int ret;
> 
> It would seem smart to do
> 
> --- a/mm/page_alloc.c~mm-page_alloc-skip-waternark_boost-for-atomic-order-0-allocations-fix
> +++ a/mm/page_alloc.c
> @@ -3745,7 +3745,6 @@ retry:
>  			}
>  		}
>  
> -		mark = wmark_pages(zone, alloc_flags & ALLOC_WMARK_MASK);
>  		/*
>  		 * Allow GFP_ATOMIC order-0 allocations to exclude the
>  		 * zone->watermark_boost in their watermark calculations.
> @@ -3757,6 +3756,8 @@ retry:
>  		if (unlikely(!order && !(alloc_flags & ALLOC_WMARK_MASK) &&
>  		     (alloc_flags & (ALLOC_HARDER | ALLOC_HIGH)))) {
>  			mark = zone->_watermark[WMARK_MIN];
> +		} else {
> +			mark = wmark_pages(zone, alloc_flags & ALLOC_WMARK_MASK);
>  		}
>  		if (!zone_watermark_fast(zone, order, mark,
>  				       ac->highest_zoneidx, alloc_flags)) {
> 
> but that makes page_alloc.o 16 bytes larger, so I guess don't bother.
> 

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