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authorMel Gorman <mgorman@techsingularity.net>2021-06-28 19:41:38 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2021-06-29 10:53:54 -0700
commit28f836b6777b6f42dce068a40d83a891deaaca37 (patch)
treeb974adb29f921786ded4cf3d56adac6a685fb58a /include/linux
parenta0b8200d06ad6450c179407baa5f0f52f8cfcc97 (diff)
mm/page_alloc: split per cpu page lists and zone stats
The PCP (per-cpu page allocator in page_alloc.c) shares locking requirements with vmstat and the zone lock which is inconvenient and causes some issues. For example, the PCP list and vmstat share the same per-cpu space meaning that it's possible that vmstat updates dirty cache lines holding per-cpu lists across CPUs unless padding is used. Second, PREEMPT_RT does not want to disable IRQs for too long in the page allocator. This series splits the locking requirements and uses locks types more suitable for PREEMPT_RT, reduces the time when special locking is required for stats and reduces the time when IRQs need to be disabled on !PREEMPT_RT kernels. Why local_lock? PREEMPT_RT considers the following sequence to be unsafe as documented in Documentation/locking/locktypes.rst local_irq_disable(); spin_lock(&lock); The pcp allocator has this sequence for rmqueue_pcplist (local_irq_save) -> __rmqueue_pcplist -> rmqueue_bulk (spin_lock). While it's possible to separate this out, it generally means there are points where we enable IRQs and reenable them again immediately. To prevent a migration and the per-cpu pointer going stale, migrate_disable is also needed. That is a custom lock that is similar, but worse, than local_lock. Furthermore, on PREEMPT_RT, it's undesirable to leave IRQs disabled for too long. By converting to local_lock which disables migration on PREEMPT_RT, the locking requirements can be separated and start moving the protections for PCP, stats and the zone lock to PREEMPT_RT-safe equivalent locking. As a bonus, local_lock also means that PROVE_LOCKING does something useful. After that, it's obvious that zone_statistics incurs too much overhead and leaves IRQs disabled for longer than necessary on !PREEMPT_RT kernels. zone_statistics uses perfectly accurate counters requiring IRQs be disabled for parallel RMW sequences when inaccurate ones like vm_events would do. The series makes the NUMA statistics (NUMA_HIT and friends) inaccurate counters that then require no special protection on !PREEMPT_RT. The bulk page allocator can then do stat updates in bulk with IRQs enabled which should improve the efficiency. Technically, this could have been done without the local_lock and vmstat conversion work and the order simply reflects the timing of when different series were implemented. Finally, there are places where we conflate IRQs being disabled for the PCP with the IRQ-safe zone spinlock. The remainder of the series reduces the scope of what is protected by disabled IRQs on !PREEMPT_RT kernels. By the end of the series, page_alloc.c does not call local_irq_save so the locking scope is a bit clearer. The one exception is that modifying NR_FREE_PAGES still happens in places where it's known the IRQs are disabled as it's harmless for PREEMPT_RT and would be expensive to split the locking there. No performance data is included because despite the overhead of the stats, it's within the noise for most workloads on !PREEMPT_RT. However, Jesper Dangaard Brouer ran a page allocation microbenchmark on a E5-1650 v4 @ 3.60GHz CPU on the first version of this series. Focusing on the array variant of the bulk page allocator reveals the following. (CPU: Intel(R) Xeon(R) CPU E5-1650 v4 @ 3.60GHz) ARRAY variant: time_bulk_page_alloc_free_array: step=bulk size Baseline Patched 1 56.383 54.225 (+3.83%) 2 40.047 35.492 (+11.38%) 3 37.339 32.643 (+12.58%) 4 35.578 30.992 (+12.89%) 8 33.592 29.606 (+11.87%) 16 32.362 28.532 (+11.85%) 32 31.476 27.728 (+11.91%) 64 30.633 27.252 (+11.04%) 128 30.596 27.090 (+11.46%) While this is a positive outcome, the series is more likely to be interesting to the RT people in terms of getting parts of the PREEMPT_RT tree into mainline. This patch (of 9): The per-cpu page allocator lists and the per-cpu vmstat deltas are stored in the same struct per_cpu_pages even though vmstats have no direct impact on the per-cpu page lists. This is inconsistent because the vmstats for a node are stored on a dedicated structure. The bigger issue is that the per_cpu_pages structure is not cache-aligned and stat updates either cache conflict with adjacent per-cpu lists incurring a runtime cost or padding is required incurring a memory cost. This patch splits the per-cpu pagelists and the vmstat deltas into separate structures. It's mostly a mechanical conversion but some variable renaming is done to clearly distinguish the per-cpu pages structure (pcp) from the vmstats (pzstats). Superficially, this appears to increase the size of the per_cpu_pages structure but the movement of expire fills a structure hole so there is no impact overall. [mgorman@techsingularity.net: make it W=1 cleaner] Link: https://lkml.kernel.org/r/20210514144622.GA3735@techsingularity.net [mgorman@techsingularity.net: make it W=1 even cleaner] Link: https://lkml.kernel.org/r/20210516140705.GB3735@techsingularity.net [lkp@intel.com: check struct per_cpu_zonestat has a non-zero size] [vbabka@suse.cz: Init zone->per_cpu_zonestats properly] Link: https://lkml.kernel.org/r/20210512095458.30632-1-mgorman@techsingularity.net Link: https://lkml.kernel.org/r/20210512095458.30632-2-mgorman@techsingularity.net Signed-off-by: Mel Gorman <mgorman@techsingularity.net> Acked-by: Vlastimil Babka <vbabka@suse.cz> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Chuck Lever <chuck.lever@oracle.com> Cc: Jesper Dangaard Brouer <brouer@redhat.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Cc: Ingo Molnar <mingo@kernel.org> Cc: Michal Hocko <mhocko@kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'include/linux')
-rw-r--r--include/linux/mmzone.h18
-rw-r--r--include/linux/vmstat.h8
2 files changed, 14 insertions, 12 deletions
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index c2bfefd34b59..a50b123ab7ae 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -341,20 +341,21 @@ struct per_cpu_pages {
int count; /* number of pages in the list */
int high; /* high watermark, emptying needed */
int batch; /* chunk size for buddy add/remove */
+#ifdef CONFIG_NUMA
+ int expire; /* When 0, remote pagesets are drained */
+#endif
/* Lists of pages, one per migrate type stored on the pcp-lists */
struct list_head lists[MIGRATE_PCPTYPES];
};
-struct per_cpu_pageset {
- struct per_cpu_pages pcp;
-#ifdef CONFIG_NUMA
- s8 expire;
- u16 vm_numa_stat_diff[NR_VM_NUMA_STAT_ITEMS];
-#endif
+struct per_cpu_zonestat {
#ifdef CONFIG_SMP
- s8 stat_threshold;
s8 vm_stat_diff[NR_VM_ZONE_STAT_ITEMS];
+ s8 stat_threshold;
+#endif
+#ifdef CONFIG_NUMA
+ u16 vm_numa_stat_diff[NR_VM_NUMA_STAT_ITEMS];
#endif
};
@@ -484,7 +485,8 @@ struct zone {
int node;
#endif
struct pglist_data *zone_pgdat;
- struct per_cpu_pageset __percpu *pageset;
+ struct per_cpu_pages __percpu *per_cpu_pageset;
+ struct per_cpu_zonestat __percpu *per_cpu_zonestats;
/*
* the high and batch values are copied to individual pagesets for
* faster access
diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h
index 3299cd69e4ca..0c5f36504613 100644
--- a/include/linux/vmstat.h
+++ b/include/linux/vmstat.h
@@ -163,7 +163,7 @@ static inline unsigned long zone_numa_state_snapshot(struct zone *zone,
int cpu;
for_each_online_cpu(cpu)
- x += per_cpu_ptr(zone->pageset, cpu)->vm_numa_stat_diff[item];
+ x += per_cpu_ptr(zone->per_cpu_zonestats, cpu)->vm_numa_stat_diff[item];
return x;
}
@@ -236,7 +236,7 @@ static inline unsigned long zone_page_state_snapshot(struct zone *zone,
#ifdef CONFIG_SMP
int cpu;
for_each_online_cpu(cpu)
- x += per_cpu_ptr(zone->pageset, cpu)->vm_stat_diff[item];
+ x += per_cpu_ptr(zone->per_cpu_zonestats, cpu)->vm_stat_diff[item];
if (x < 0)
x = 0;
@@ -291,7 +291,7 @@ struct ctl_table;
int vmstat_refresh(struct ctl_table *, int write, void *buffer, size_t *lenp,
loff_t *ppos);
-void drain_zonestat(struct zone *zone, struct per_cpu_pageset *);
+void drain_zonestat(struct zone *zone, struct per_cpu_zonestat *);
int calculate_pressure_threshold(struct zone *zone);
int calculate_normal_threshold(struct zone *zone);
@@ -399,7 +399,7 @@ static inline void cpu_vm_stats_fold(int cpu) { }
static inline void quiet_vmstat(void) { }
static inline void drain_zonestat(struct zone *zone,
- struct per_cpu_pageset *pset) { }
+ struct per_cpu_zonestat *pzstats) { }
#endif /* CONFIG_SMP */
static inline void __mod_zone_freepage_state(struct zone *zone, int nr_pages,