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Tue, 20 Sep 2022 03:04:36 -0400 (EDT) X-Mailer: MessagingEngine.com Webmail Interface User-Agent: Cyrus-JMAP/3.7.0-alpha0-935-ge4ccd4c47b-fm-20220914.001-ge4ccd4c4 Mime-Version: 1.0 Message-Id: <45d1a6b4-eb28-4392-84a3-d30a2f8e150a@www.fastmail.com> In-Reply-To: <183588fd5a7.1074ac6241209386.2442883306267550831@linux.beauty> References: <183588fd5a7.1074ac6241209386.2442883306267550831@linux.beauty> Date: Tue, 20 Sep 2022 09:04:16 +0200 From: "Arnd Bergmann" To: "Li Chen" Cc: linux-arm-kernel Subject: Re: Why GICD_ITARGETSR is not used by Linux X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20220920_000444_356120_549E09B8 X-CRM114-Status: GOOD ( 19.66 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Sep 20, 2022, at 3:42 AM, Li Chen wrote: > Hi Arnd, > > I noticed GIC has GICD_ITARGETSR to distribute IRQ to different CPUs, > but currently, it is not used by Linux. > > There was a patchset from MTK people: > http://archive.lwn.net:8080/linux-kernel/1606486531-25719-1-git-send-email-hanks.chen@mediatek.com/T/#t > which implements GIC-level IRQ distributor using GICD_ITARGETSR, but it > is > not accepted because the maintainer thinks it will break existing codes > and not provide benefits compared with the existing affinity mechanism. > > IIUC, Linux only relies on affinity/irqbalance to distribute IRQ > instead of architecture-specific solutions like GIC's distributor. > > Maybe latency can somewhat get improved, but there is no benchmark yet. > > I have two questions here: > 1. Now that Linux doesn't use GICD_ITARGETSR, where does it set CPU 0 > to be the only IRQ distributor core? > 2. Do you know any other reasons that GICD_ITARGETSR is not used by > Linux? Hi Li, It looks like the original submitter never followed up with a new version of the patch that addresses the issues found in review. I would assume they gave up either because it did not show any real-world advantage, or they could not address all of the concerns. Arnd _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel