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Tue, 27 Oct 2020 12:06:05 +0000 MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Date: Tue, 27 Oct 2020 12:06:05 +0000 From: Marc Zyngier To: Vincent Guittot Cc: LAK , linux-kernel , Will Deacon , Catalin Marinas , Russell King , Thomas Gleixner , Jason Cooper , Sumit Garg , Valentin Schneider , Florian Fainelli , Gregory Clement , Andrew Lunn , Saravana Kannan , Android Kernel Team Subject: Re: [PATCH v3 03/16] arm64: Allow IPIs to be handled as normal interrupts In-Reply-To: References: <20200901144324.1071694-1-maz@kernel.org> <20200901144324.1071694-4-maz@kernel.org> <353f13b0dcc6c7ea1b44012d9632a0cc@kernel.org> <7b754dfe579044902d3468b9f864bd37@kernel.org> User-Agent: Roundcube Webmail/1.4.9 Message-ID: <11951c39bc676b83def46150e58e8df3@kernel.org> X-Sender: maz@kernel.org X-SA-Exim-Connect-IP: 51.254.78.96 X-SA-Exim-Rcpt-To: vincent.guittot@linaro.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, will@kernel.org, catalin.marinas@arm.com, linux@arm.linux.org.uk, tglx@linutronix.de, jason@lakedaemon.net, sumit.garg@linaro.org, Valentin.Schneider@arm.com, f.fainelli@gmail.com, gregory.clement@bootlin.com, andrew@lunn.ch, saravanak@google.com, kernel-team@android.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2020-10-27 11:21, Vincent Guittot wrote: > On Tue, 27 Oct 2020 at 11:50, Vincent Guittot > wrote: >> >> On Tue, 27 Oct 2020 at 11:37, Marc Zyngier wrote: >> > >> > On 2020-10-27 10:12, Vincent Guittot wrote: >> > > HI Marc, >> > > >> > > On Mon, 19 Oct 2020 at 17:43, Vincent Guittot >> > > wrote: >> > >> >> > >> On Mon, 19 Oct 2020 at 15:04, Marc Zyngier wrote: >> > >> > >> > > >> > > ... >> > > >> > >> > >> >> > >> > >> One of the major difference is that we end up, in some cases >> > >> > >> (such as when performing IRQ time accounting on the scheduler >> > >> > >> IPI), end up with nested irq_enter()/irq_exit() pairs. >> > >> > >> Other than the (relatively small) overhead, there should be >> > >> > >> no consequences to it (these pairs are designed to nest >> > >> > >> correctly, and the accounting shouldn't be off). >> > >> > > >> > >> > > While rebasing on mainline, I have faced a performance regression for >> > >> > > the benchmark: >> > >> > > perf bench sched pipe >> > >> > > on my arm64 dual quad core (hikey) and my 2 nodes x 112 CPUS (thx2) >> > >> > > >> > >> > > The regression comes from: >> > >> > > commit: d3afc7f12987 ("arm64: Allow IPIs to be handled as normal >> > >> > > interrupts") >> > >> > >> > >> > That's interesting, as this patch doesn't really change anything (most >> > >> > of the potential overhead comes in later). The only potential overhead >> > >> > I can see is that the scheduler_ipi() call is now wrapped around >> > >> > irq_enter()/irq_exit(). >> > >> > >> > >> > > >> > >> > > v5.9 + this patch >> > >> > > hikey : 48818(+/- 0.31) 37503(+/- 0.15%) -23.2% >> > >> > > thx2 : 132410(+/- 1.72) 122646(+/- 1.92%) -7.4% >> > >> > > >> > >> > > By + this patch, I mean merging branch from this patch. Whereas >> > >> > > merging the previous: >> > >> > > commit: 83cfac95c018 ("genirq: Allow interrupts to be excluded from >> > >> > > /proc/interrupts") >> > >> > > It doesn't show any regression >> > >> > >> > >> > Since you are running perf, can you spot where the overhead occurs? >> > > >> > > Any idea about the root cause of the regression ? >> > > I have faced it on more arm64 platforms in the meantime >> > >> > two possible causes: >> > >> > (1) irq_enter/exit on the rescheduling IPI means we reschedule much more >> > often >> > (2) irq_domain lookups add some overhead. >> > >> > For (1), I have this series[1] which is ugly as sin and needs much more >> > testing. >> >> Ok, I'm going to test this series to see if it fixes the perf >> regression > > You have spotted the root cause of the regression. We are back to ~1% > performance diff on the hikey Yeah. Only thing is that I can't look at this hack without vomiting... M. -- Jazz is not dead. It just smells funny...