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SubjectRe: [PATCH] mm, page_alloc: skip ->watermark_boost for atomic order-0 allocations-fix
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On 6/11/20 2:09 PM, Charan Teja Kalla 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.
>
> Fix-suggested-by: Mel Gorman <mgorman@techsingularity.net>
> Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>

For the patch+fix:

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

The boost and highatomic stuff certainly made the whole thing more subtle.

> ---
>
> Change in linux-next: https://lore.kernel.org/patchwork/patch/1244272/
>
> mm/page_alloc.c | 36 ++++++++++++++++++++----------------
> 1 file changed, 20 insertions(+), 16 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 0c435b2..18f407e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3580,7 +3580,7 @@ bool zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark,
>
> static inline bool zone_watermark_fast(struct zone *z, unsigned int order,
> unsigned long mark, int highest_zoneidx,
> - unsigned int alloc_flags)
> + unsigned int alloc_flags, gfp_t gfp_mask)
> {
> long free_pages = zone_page_state(z, NR_FREE_PAGES);
> long cma_pages = 0;
> @@ -3602,8 +3602,23 @@ static inline bool zone_watermark_fast(struct zone *z, unsigned int order,
> mark + z->lowmem_reserve[highest_zoneidx])
> return true;
>
> - return __zone_watermark_ok(z, order, mark, highest_zoneidx, alloc_flags,
> - free_pages);
> + if (__zone_watermark_ok(z, order, mark, highest_zoneidx, alloc_flags,
> + free_pages))
> + return true;
> + /*
> + * Ignore watermark boosting for GFP_ATOMIC order-0 allocations
> + * when checking the min watermark. The min watermark is the
> + * point where boosting is ignored so that kswapd is woken up
> + * when below the low watermark.
> + */
> + if (unlikely(!order && (gfp_mask & __GFP_ATOMIC) && z->watermark_boost
> + && ((alloc_flags & ALLOC_WMARK_MASK) == WMARK_MIN))) {
> + mark = z->_watermark[WMARK_MIN];
> + return __zone_watermark_ok(z, order, mark, highest_zoneidx,
> + alloc_flags, free_pages);
> + }
> +
> + return false;
> }
>
> bool zone_watermark_ok_safe(struct zone *z, unsigned int order,
> @@ -3746,20 +3761,9 @@ 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 their 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)) {
> + ac->highest_zoneidx, alloc_flags,
> + gfp_mask)) {
> int ret;
>
> #ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT
>

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