There was a report that a task is waiting at the throttle_direct_reclaim. The pgscan_direct_throttle in vmstat was increasing. This is a bug where zone_watermark_fast returns true even when the free is very low. The commit f27ce0e14088 ("page_alloc: consider highatomic reserve in watermark fast") changed the watermark fast to consider highatomic reserve. But it did not handle a negative value case which can be happened when reserved_highatomic pageblock is bigger than the actual free. If watermark is considered as ok for the negative value, allocating contexts for order-0 will consume all free pages without direct reclaim, and finally free page may become depleted except highatomic free. Then allocating contexts may fall into throttle_direct_reclaim. This symptom may easily happen in a system where wmark min is low and other reclaimers like kswapd does not make free pages quickly. Handle the negative case by using MIN.
Unified · Split
| @@ -3968,11 +3968,15 @@ static inline bool zone_watermark_fast(struct zone *z, unsigned int order, · 3967 unmodified lines | ||
| 3968 | 3968 | * need to be calculated. |
| 3969 | 3969 | */ |
| 3970 | 3970 | if (!order) { |
| 3971 | − long fast_free; | |
| 3971 | + long usable_free; | |
| 3972 | + long reserved; | |
| 3972 | 3973 | |
| 3973 | − fast_free = free_pages; | |
| 3974 | − fast_free -= __zone_watermark_unusable_free(z, 0, alloc_flags); | |
| 3975 | − if (fast_free > mark + z->lowmem_reserve[highest_zoneidx]) | |
| 3974 | + usable_free = free_pages; | |
| 3975 | + reserved = __zone_watermark_unusable_free(z, 0, alloc_flags); | |
| 3976 | + | |
| 3977 | + /* reserved may over estimate high-atomic reserves. */ | |
| 3978 | + usable_free -= min(usable_free, reserved); | |
| 3979 | + if (usable_free > mark + z->lowmem_reserve[highest_zoneidx]) | |
| 3976 | 3980 | return true; |
| 3977 | 3981 | } |
| 3978 | 3982 | |