mm: tune vm_committed_as percpu_counter batching size

Currently the per cpu counter's batch size for memory accounting is
configured as twice the number of cpus in the system.  However, for
system with very large memory, it is more appropriate to make it
proportional to the memory size per cpu in the system.

For example, for a x86_64 system with 64 cpus and 128 GB of memory, the
batch size is only 2*64 pages (0.5 MB).  So any memory accounting
changes of more than 0.5MB will overflow the per cpu counter into the
global counter.  Instead, for the new scheme, the batch size is
configured to be 0.4% of the memory/cpu = 8MB (128 GB/64 /256), which is
more inline with the memory size.

I've done a repeated brk test of 800KB (from will-it-scale test suite)
with 80 concurrent processes on a 4 socket Westmere machine with a total
of 40 cores.  Without the patch, about 80% of cpu is spent on spin-lock
contention within the vm_committed_as counter.  With the patch, there's
a 73x speedup on the benchmark and the lock contention drops off almost
entirely.

[akpm@linux-foundation.org: fix section mismatch]
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 1 deletions

diff --git a/include/linux/mman.h b/include/linux/mman.h
index 9aa863da287f..92dc257251e4 100644
--- a/include/linux/mman.h
+++ b/include/linux/mman.h
@@ -11,11 +11,17 @@ extern int sysctl_overcommit_memory;
 extern int sysctl_overcommit_ratio;
 extern struct percpu_counter vm_committed_as;
 
+#ifdef CONFIG_SMP
+extern s32 vm_committed_as_batch;
+#else
+#define vm_committed_as_batch 0
+#endif
+
 unsigned long vm_memory_committed(void);
 
 static inline void vm_acct_memory(long pages)
 {
-	percpu_counter_add(&vm_committed_as, pages);
+	__percpu_counter_add(&vm_committed_as, pages, vm_committed_as_batch);
 }
 
 static inline void vm_unacct_memory(long pages)