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[2a01:cb00:f8b:9700:598d:a2e1:679e:8383]) by smtp.gmail.com with ESMTPSA id l13-20020a5d410d000000b0031766e99429sm6702219wrp.115.2023.07.26.03.44.18 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 26 Jul 2023 03:44:19 -0700 (PDT) Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Date: Wed, 26 Jul 2023 12:44:18 +0200 Message-Id: From: "Robin Jarry" To: "Waiman Long" , "Peter Zijlstra" , "Thomas Gleixner" , "Ingo Molnar" , "Borislav Petkov" , "Dave Hansen" , "H. Peter Anvin" , "Josh Poimboeuf" , "Pawan Gupta" , "Jacob Pan" , "Len Brown" , "Jonathan Corbet" , "Rafael J . Wysocki" Subject: Re: [PATCH v5 4/4] intel_idle: Add ibrs_off module parameter to force disable IBRS Cc: , , , , "Joe Mario" , "Randy Dunlap" X-Mailer: aerc/0.15.2-111-gf7a4c37c303c-dirty References: <20230710194857.2898284-1-longman@redhat.com> <20230710194857.2898284-5-longman@redhat.com> In-Reply-To: <20230710194857.2898284-5-longman@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org Waiman Long, Jul 10, 2023 at 21:48: > Commit bf5835bcdb96 ("intel_idle: Disable IBRS during long idle") > disables IBRS when the cstate is 6 or lower. However, there are > some use cases where a customer may want to use max_cstate=3D1 to > lower latency. Such use cases will suffer from the performance > degradation caused by the enabling of IBRS in the sibling idle thread. > Add a "ibrs_off" module parameter to force disable IBRS and the > CPUIDLE_FLAG_IRQ_ENABLE flag if set. > > In the case of a Skylake server with max_cstate=3D1, this new ibrs_off > option will likely increase the IRQ response latency as IRQ will now > be disabled. > > When running SPECjbb2015 with cstates set to C1 on a Skylake system. > > First test when the kernel is booted with: "intel_idle.ibrs_off" > max-jOPS =3D 117828, critical-jOPS =3D 66047 > > Then retest when the kernel is booted without the "intel_idle.ibrs_off" > added. > max-jOPS =3D 116408, critical-jOPS =3D 58958 > > That means booting with "intel_idle.ibrs_off" improves performance by: > max-jOPS: 1.2%, which could be considered noise range. > critical-jOPS: 12%, which is definitely a solid improvement. > > The admin-guide/pm/intel_idle.rst file is updated to add a description > about the new "ibrs_off" module parameter. > > Signed-off-by: Waiman Long > Acked-by: Rafael J. Wysocki Hi Longman, Adding back my tag that I had set in v1. Thanks! Tested-by: Robin Jarry