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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