percpu_counter_sum_positive() will provide more accurate info. As with percpu_counter_read_positive(), in worst case the deviation could be 'batch * nr_cpus', which is totalram_pages/256 for now, and will be more when the batch gets enlarged.
Its time cost is about 800 nanoseconds on a 2C/4T platform and 2~3 microseconds on a 2S/36C/72T Skylake server in normal case, and in worst case where vm_committed_as's spinlock is under severe contention, it costs 30~40 microseconds for the 2S/36C/72T Skylake sever, which should be fine for its only two users: /proc/meminfo and HyperV balloon driver's status trace per second. Link: http://lkml.kernel.org/r/1592725000-73486-3-git-send-email-feng.t...@intel.com Signed-off-by: Feng Tang <feng.t...@intel.com> Acked-by: Michal Hocko <mho...@suse.com> # for /proc/meminfo Cc: "K. Y. Srinivasan" <k...@microsoft.com> Cc: Haiyang Zhang <haiya...@microsoft.com> Cc: Matthew Wilcox (Oracle) <wi...@infradead.org> Cc: Johannes Weiner <han...@cmpxchg.org> Cc: Mel Gorman <mgor...@suse.de> Cc: Qian Cai <c...@lca.pw> Cc: Andi Kleen <andi.kl...@intel.com> Cc: Tim Chen <tim.c.c...@intel.com> Cc: Dave Hansen <dave.han...@intel.com> Cc: Huang Ying <ying.hu...@intel.com> Signed-off-by: Andrew Morton <a...@linux-foundation.org> --- mm/util.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/mm/util.c b/mm/util.c index c856f5f..d076218 100644 --- a/mm/util.c +++ b/mm/util.c @@ -787,10 +787,15 @@ struct percpu_counter vm_committed_as ____cacheline_aligned_in_smp; * balancing memory across competing virtual machines that are hosted. * Several metrics drive this policy engine including the guest reported * memory commitment. + * + * The time cost of this is very low for small platforms, and for big + * platform like a 2S/36C/72T Skylake server, in worst case where + * vm_committed_as's spinlock is under severe contention, the time cost + * could be about 30~40 microseconds. */ unsigned long vm_memory_committed(void) { - return percpu_counter_read_positive(&vm_committed_as); + return percpu_counter_sum_positive(&vm_committed_as); } EXPORT_SYMBOL_GPL(vm_memory_committed); -- 2.7.4
