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[95.203.18.139]) by smtp.gmail.com with ESMTPSA id 2adb3069b0e04-59b6c1792a2sm4250608e87.25.2026.01.12.03.07.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 12 Jan 2026 03:07:33 -0800 (PST) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Mon, 12 Jan 2026 12:07:31 +0100 To: Vishal Chourasia Cc: Uladzislau Rezki , rcu@vger.kernel.org, linux-kernel@vger.kernel.org, paulmck@kernel.org, frederic@kernel.org, neeraj.upadhyay@kernel.org, joelagnelf@nvidia.com, josh@joshtriplett.org, boqun.feng@gmail.com, rostedt@goodmis.org, tglx@linutronix.de, peterz@infradead.org, sshegde@linux.ibm.com, srikar@linux.ibm.com, aboorvad@linux.ibm.com Subject: Re: [PATCH] cpuhp: Expedite synchronize_rcu during CPU hotplug operations Message-ID: References: <20260112094332.66006-2-vishalc@linux.ibm.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Hello, Vishalc! > Hi Uladzislau, > > On 12/01/26 15:38, Uladzislau Rezki wrote: > > On Mon, Jan 12, 2026 at 03:13:33PM +0530, Vishal Chourasia wrote: > > > Performance data on a PPC64 system with 400 CPUs: > > > > > > + ppc64_cpu --smt=1 (SMT8 to SMT1) > > > Before: real 1m14.792s > > > After: real 0m03.205s # ~23x improvement > > > > > > + ppc64_cpu --smt=8 (SMT1 to SMT8) > > > Before: real 2m27.695s > > > After: real 0m02.510s # ~58x improvement > > > > > > Above numbers were collected on Linux 6.19.0-rc4-00310-g755bc1335e3b > > > > > > [1] https://lore.kernel.org/all/5f2ab8a44d685701fe36cdaa8042a1aef215d10d.camel@linux.vnet.ibm.com > > > > > Also you can try: echo 1 > /sys/module/rcutree/parameters/rcu_normal_wake_from_gp > > to speedup regular synchronize_rcu() call. But i am not saying that it would beat > > your "expedited switch" improvement. > > # echo 1 > /sys/module/rcutree/parameters/rcu_normal_wake_from_gp > > After setting, > > # time ppc64_cpu --smt=1; > real    1m10.726s          # Run 1 > real    1m12.530s          # Run 2 > > # time ppc64_cpu --smt=8 > real    0m36.661s          # Run 1 > real    0m41.401s          # Run 2 > Thanks. "ppc64_cpu --smt=1" is the same, i assume it is offlining. "ppc64_cpu --smt=8", whereas, onlining, sees the differences(~5x). But your real "0m02.510s" is hard to beat event by activating the "rcu_normal_wake_from_gp" option. -- Uladzislau Rezki