Re: [PATCH 2/6] mm/page_alloc: Disassociate the pcp->high from pcp->batch
From: Vlastimil Babka <hidden>
Date: 2021-05-26 18:14:17
Also in:
lkml
On 5/25/21 10:01 AM, Mel Gorman wrote:
The pcp high watermark is based on the batch size but there is no
relationship between them other than it is convenient to use early in
boot.
This patch takes the first step and bases pcp->high on the zone low
watermark split across the number of CPUs local to a zone while the batch
size remains the same to avoid increasing allocation latencies. The intent
behind the default pcp->high is "set the number of PCP pages such that
if they are all full that background reclaim is not started prematurely".
Note that in this patch the pcp->high values are adjusted after memory
hotplug events, min_free_kbytes adjustments and watermark scale factor
adjustments but not CPU hotplug events which is handled later in the
series.
On a test KVM instance;
Before grep -E "high:|batch" /proc/zoneinfo | tail -2
high: 378
batch: 63
After grep -E "high:|batch" /proc/zoneinfo | tail -2
high: 649
batch: 63
Signed-off-by: Mel Gorman <redacted>...
quoted hunk ↗ jump to hunk
@@ -6637,6 +6628,34 @@ static int zone_batchsize(struct zone *zone) #endif } +static int zone_highsize(struct zone *zone, int batch) +{ +#ifdef CONFIG_MMU + int high; + int nr_local_cpus; + + /* + * The high value of the pcp is based on the zone low watermark + * so that if they are full then background reclaim will not be + * started prematurely. The value is split across all online CPUs + * local to the zone. Note that early in boot that CPUs may not be + * online yet. + */ + nr_local_cpus = max(1U, cpumask_weight(cpumask_of_node(zone_to_nid(zone)))); + high = low_wmark_pages(zone) / nr_local_cpus; + + /* + * Ensure high is at least batch*4. The multiple is based on the + * historical relationship between high and batch. + */ + high = max(high, batch << 2); + + return high; +#else + return 0; +#endif +} + /* * pcp->high and pcp->batch values are related and generally batch is lower * than high. They are also related to pcp->count such that count is lower@@ -6698,11 +6717,10 @@ static void __zone_set_pageset_high_and_batch(struct zone *zone, unsigned long h */ static void zone_set_pageset_high_and_batch(struct zone *zone) { - unsigned long new_high, new_batch; + int new_high, new_batch; - new_batch = zone_batchsize(zone); - new_high = 6 * new_batch; - new_batch = max(1UL, 1 * new_batch); + new_batch = max(1, zone_batchsize(zone)); + new_high = zone_highsize(zone, new_batch); if (zone->pageset_high == new_high && zone->pageset_batch == new_batch)@@ -8170,6 +8188,12 @@ static void __setup_per_zone_wmarks(void) zone->_watermark[WMARK_LOW] = min_wmark_pages(zone) + tmp; zone->_watermark[WMARK_HIGH] = min_wmark_pages(zone) + tmp * 2; + /* + * The watermark size have changed so update the pcpu batch + * and high limits or the limits may be inappropriate. + */ + zone_set_pageset_high_and_batch(zone);
Hm so this puts the call in the path of various watermark related sysctl handlers, but it's not protected by pcp_batch_high_lock. The zone lock won't help against zone_pcp_update() from a hotplug handler. On the other hand, since hotplug handlers also call __setup_per_zone_wmarks(), the zone_pcp_update() calls there are now redundant and could be removed, no? But later there will be a new sysctl in patch 6/6 using pcp_batch_high_lock, thus that one will not be protected against the watermark related sysctl handlers that reach here. To solve all this, seems like the static lock in setup_per_zone_wmarks() could become a top-level visible lock and pcp high/batch updates could switch to that one instead of own pcp_batch_high_lock. And zone_pcp_update() calls from hotplug handlers could be removed.
+ spin_unlock_irqrestore(&zone->lock, flags); }