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From:   Feng Tang <feng.tang@intel.com>
To:     Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@suse.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Matthew Wilcox <willy@infradead.org>,
        Mel Gorman <mgorman@suse.de>,
        Kees Cook <keescook@chromium.org>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Iurii Zaikin <yzaikin@google.com>, andi.kleen@intel.com,
        tim.c.chen@intel.com, dave.hansen@intel.com, ying.huang@intel.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Cc:     Feng Tang <feng.tang@intel.com>,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>
Subject: [PATCH v5 2/3] mm/util.c: make vm_memory_committed() more accurate
Date:   Sun, 21 Jun 2020 15:36:39 +0800
Message-Id: <1592725000-73486-3-git-send-email-feng.tang@intel.com>
In-Reply-To: <1592725000-73486-1-git-send-email-feng.tang@intel.com>
References: <1592725000-73486-1-git-send-email-feng.tang@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.

Signed-off-by: Feng Tang <feng.tang@intel.com>
Acked-by: Michal Hocko <mhocko@suse.com> # for /proc/meminfo
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Andi Kleen <andi.kleen@intel.com>
Cc: Tim Chen <tim.c.chen@intel.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Huang Ying <ying.huang@intel.com>

---
 mm/util.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/util.c b/mm/util.c
index c63c8e4..1c9d097 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