From: Johannes Weiner <hannes@xxxxxxxxxxx> Subject: mm: don't wake kswapd prematurely when watermark boosting is disabled On 2-node NUMA hosts we see bursts of kswapd reclaim and subsequent pressure spikes and stalls from cache refaults while there is plenty of free memory in the system. Usually, kswapd is woken up when all eligible nodes in an allocation are full. But the code related to watermark boosting can wake kswapd on one full node while the other one is mostly empty. This may be justified to fight fragmentation, but is currently unconditionally done whether watermark boosting is occurring or not. In our case, many of our workloads' throughput scales with available memory, and pure utilization is a more tangible concern than trends around longer-term fragmentation. As a result we generally disable watermark boosting. Wake kswapd only woken when watermark boosting is requested. Link: https://lkml.kernel.org/r/20201020175833.397286-1-hannes@xxxxxxxxxxx Fixes: 1c30844d2dfe ("mm: reclaim small amounts of memory when an external fragmentation event occurs") Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> Acked-by: Mel Gorman <mgorman@xxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/page_alloc.c | 13 +++++++------ 1 file changed, 7 insertions(+), 6 deletions(-) --- a/mm/page_alloc.c~mm-dont-wake-kswapd-prematurely-when-watermark-boosting-is-disabled +++ a/mm/page_alloc.c @@ -2482,12 +2482,12 @@ static bool can_steal_fallback(unsigned return false; } -static inline void boost_watermark(struct zone *zone) +static inline bool boost_watermark(struct zone *zone) { unsigned long max_boost; if (!watermark_boost_factor) - return; + return false; /* * Don't bother in zones that are unlikely to produce results. * On small machines, including kdump capture kernels running @@ -2495,7 +2495,7 @@ static inline void boost_watermark(struc * memory situation immediately. */ if ((pageblock_nr_pages * 4) > zone_managed_pages(zone)) - return; + return false; max_boost = mult_frac(zone->_watermark[WMARK_HIGH], watermark_boost_factor, 10000); @@ -2509,12 +2509,14 @@ static inline void boost_watermark(struc * boosted watermark resulting in a hang. */ if (!max_boost) - return; + return false; max_boost = max(pageblock_nr_pages, max_boost); zone->watermark_boost = min(zone->watermark_boost + pageblock_nr_pages, max_boost); + + return true; } /* @@ -2552,8 +2554,7 @@ static void steal_suitable_fallback(stru * likelihood of future fallbacks. Wake kswapd now as the node * may be balanced overall and kswapd will not wake naturally. */ - boost_watermark(zone); - if (alloc_flags & ALLOC_KSWAPD) + if (boost_watermark(zone) && (alloc_flags & ALLOC_KSWAPD)) set_bit(ZONE_BOOSTED_WATERMARK, &zone->flags); /* We are not allowed to try stealing from the whole block */ _
