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

[Andrew Morton]

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.

Seems sensible.

> --- 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.

Nice comment, but I don't understand it ;)

Why would testing gfp_mask prevent us from waking kswapd?

> +		 */
> +		if (unlikely(!order && !(alloc_flags & ALLOC_WMARK_MASK) &&
> +		     (alloc_flags & (ALLOC_HARDER | ALLOC_HIGH)))) {
> +			mark = zone->_watermark[WMARK_MIN];
> +		}

Why is this not implemented for higher-order allocation attempts?

>  		if (!zone_watermark_fast(zone, order, mark,
>  				       ac->highest_zoneidx, alloc_flags)) {
>  			int ret;