From a87c75fbcc8dc8466be6c4b18a45bcaf315883ab Mon Sep 17 00:00:00 2001 From: Greg Thelen Date: Wed, 3 May 2017 14:51:47 -0700 Subject: [PATCH] slab: avoid IPIs when creating kmem caches Each slab kmem cache has per cpu array caches. The array caches are created when the kmem_cache is created, either via kmem_cache_create() or lazily when the first object is allocated in context of a kmem enabled memcg. Array caches are replaced by writing to /proc/slabinfo. Array caches are protected by holding slab_mutex or disabling interrupts. Array cache allocation and replacement is done by __do_tune_cpucache() which holds slab_mutex and calls kick_all_cpus_sync() to interrupt all remote processors which confirms there are no references to the old array caches. IPIs are needed when replacing array caches. But when creating a new array cache, there's no need to send IPIs because there cannot be any references to the new cache. Outside of memcg kmem accounting these IPIs occur at boot time, so they're not a problem. But with memcg kmem accounting each container can create kmem caches, so the IPIs are wasteful. Avoid unnecessary IPIs when creating array caches. Test which reports the IPI count of allocating slab in 10000 memcg: import os def ipi_count(): with open("/proc/interrupts") as f: for l in f: if 'Function call interrupts' in l: return int(l.split()[1]) def echo(val, path): with open(path, "w") as f: f.write(val) n = 10000 os.chdir("/mnt/cgroup/memory") pid = str(os.getpid()) a = ipi_count() for i in range(n): os.mkdir(str(i)) echo("1G\n", "%d/memory.limit_in_bytes" % i) echo("1G\n", "%d/memory.kmem.limit_in_bytes" % i) echo(pid, "%d/cgroup.procs" % i) open("/tmp/x", "w").close() os.unlink("/tmp/x") b = ipi_count() print "%d loops: %d => %d (+%d ipis)" % (n, a, b, b-a) echo(pid, "cgroup.procs") for i in range(n): os.rmdir(str(i)) patched: 10000 loops: 1069 => 1170 (+101 ipis) unpatched: 10000 loops: 1192 => 48933 (+47741 ipis) Link: http://lkml.kernel.org/r/20170416214544.109476-1-gthelen@google.com Signed-off-by: Greg Thelen Acked-by: Joonsoo Kim Acked-by: David Rientjes Cc: Christoph Lameter Cc: Pekka Enberg Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/slab.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/mm/slab.c b/mm/slab.c index 807d86c76908..1880d482a0cb 100644 --- a/mm/slab.c +++ b/mm/slab.c @@ -3879,7 +3879,12 @@ static int __do_tune_cpucache(struct kmem_cache *cachep, int limit, prev = cachep->cpu_cache; cachep->cpu_cache = cpu_cache; - kick_all_cpus_sync(); + /* + * Without a previous cpu_cache there's no need to synchronize remote + * cpus, so skip the IPIs. + */ + if (prev) + kick_all_cpus_sync(); check_irq_on(); cachep->batchcount = batchcount; -- 2.20.1