linux-kernel - [PATCH v6 2/4] mm/util.c: make vm_memory

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Date:   Fri, 10 Jul 2020 22:01:46 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Kees Cook <keescook@...omium.org>, Qian Cai <cai@....pw>,
        Dennis Zhou <dennis@...nel.org>, andi.kleen@...el.com,
        tim.c.chen@...el.com, dave.hansen@...el.com, ying.huang@...el.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     Feng Tang <feng.tang@...el.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>
Subject: [PATCH v6 2/4] mm/util.c: make vm_memory_committed() more accurate

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.tang@intel.com
Signed-off-by: Feng Tang <feng.tang@...el.com>
Acked-by: Michal Hocko <mhocko@...e.com> # for /proc/meminfo
Cc: "K. Y. Srinivasan" <kys@...rosoft.com>
Cc: Haiyang Zhang <haiyangz@...rosoft.com>
Cc: Matthew Wilcox (Oracle) <willy@...radead.org>
Cc: Johannes Weiner <hannes@...xchg.org>
Cc: Mel Gorman <mgorman@...e.de>
Cc: Qian Cai <cai@....pw>
Cc: Andi Kleen <andi.kleen@...el.com>
Cc: Tim Chen <tim.c.chen@...el.com>
Cc: Dave Hansen <dave.hansen@...el.com>
Cc: Huang Ying <ying.huang@...el.com>
Signed-off-by: Andrew Morton <akpm@...ux-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