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[37.188.249.36]) by smtp.gmail.com with ESMTPSA id f5sm2207946wrp.70.2020.05.15.00.23.07 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 15 May 2020 00:23:08 -0700 (PDT) Date: Fri, 15 May 2020 09:23:07 +0200 From: Michal Hocko To: Feng Tang Cc: Andrew Morton , Johannes Weiner , Matthew Wilcox , Mel Gorman , Kees Cook , Luis Chamberlain , Iurii Zaikin , 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 Subject: Re: [PATCH 2/3] mm/util.c: make vm_memory_committed() more accurate Message-ID: <20200515072307.GG29153@dhcp22.suse.cz> References: <1588922717-63697-1-git-send-email-feng.tang@intel.com> <1588922717-63697-3-git-send-email-feng.tang@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1588922717-63697-3-git-send-email-feng.tang@intel.com> X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri 08-05-20 15:25:16, Feng Tang wrote: > percpu_counter_sum_positive() will provide more accurate info. Why do we need that? > Its time cost is about 800 nanoseconds on a 2C/4T platform and > 2~3 microseconds on a 2S/36C/72T 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 sever, > which should be fine for its two users: /proc/meminfo and HV ballon > driver's status trace per second. OK, this explains that the additional overhead is not terrible but there is no actual information on why the imprecision is a problem. > Signed-off-by: Feng Tang > --- > mm/util.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/util.c b/mm/util.c > index 988d11e..3de78e9 100644 > --- a/mm/util.c > +++ b/mm/util.c > @@ -774,7 +774,7 @@ struct percpu_counter vm_committed_as ____cacheline_aligned_in_smp; > */ > 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 -- Michal Hocko SUSE Labs