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[79.53.78.93]) by smtp.gmail.com with ESMTPSA id g26-20020a170906395a00b00a38103ab710sm1566021eje.125.2024.02.07.22.55.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 07 Feb 2024 22:55:39 -0800 (PST) Date: Thu, 8 Feb 2024 07:55:37 +0100 From: Andrea Righi To: Joel Fernandes Cc: "Paul E. McKenney" , Uladzislau Rezki , Joel Fernandes , rcu@vger.kernel.org, "Cc: Frederic Weisbecker" Subject: Re: Observation on NOHZ_FULL Message-ID: References: <9f9b6885-6d9d-4f30-9635-49443e1615e9@joelfernandes.org> <1fcf7e8a-c245-4c5c-ac86-7f0a1531032d@joelfernandes.org> Precedence: bulk X-Mailing-List: rcu@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1fcf7e8a-c245-4c5c-ac86-7f0a1531032d@joelfernandes.org> On Thu, Feb 08, 2024 at 12:54:58AM -0500, Joel Fernandes wrote: ... > >> Slightly related, but one of the things we are wondering also is how > >> much of the overhead for your nohz-full and lazy-RCU test (on top of > >> baseline - that is just CONFIG_HZ=1000 without nohz-full or nocbs) is > >> because of just using NOCB. Uladsizlau mentioned he might run a test > >> for comparing along those lines as well. > > > > Just to clarify, "lazy rcu on" results are just with rcu_nocb=all and > > lazy RCUs enabled (and HZ=1000), so without nohz_full. > > > > If I enable only nohz_full=all (without rcu_nocb) I see something like > > this: > > Ok. I did want to mention nohz_full implies rcu_nocb on the same CPUs as well. > > Its also mentioned in the boot param docs on the last line of the description: > > nohz_full= [KNL,BOOT,SMP,ISOL] > The argument is a cpu list, as described above. > In kernels built with CONFIG_NO_HZ_FULL=y, set > the specified list of CPUs whose tick will be stopped > whenever possible. The boot CPU will be forced outside > the range to maintain the timekeeping. Any CPUs > in this list will have their RCU callbacks offloaded, > just as if they had also been called out in the > rcu_nocbs= boot parameter. Ah I didn't realize that, it definitely makes sense, thanks for clarifying it. Then basically in the results that I posted the difference is "nohz_full=all+rcu_nocb=all" vs "rcu_nocb=all+lazy_RCU=on". > > (btw, I am a bit slow to respond to email due to being ill). Get well soon! > > thanks, > > - Joel -Andrea