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emacs 26.3 From: Valentin Schneider To: peterz@infradead.org Subject: Re: [RFC PATCH] arm64: defconfig: Disable fine-grained task level IRQ time accounting In-reply-to: <20200805134002.GQ2674@hirez.programming.kicks-ass.net> Date: Wed, 05 Aug 2020 14:56:49 +0100 Message-ID: MIME-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200805_095701_099172_D95555A1 X-CRM114-Status: GOOD ( 18.82 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: mw@semihalf.com, paulmck@kernel.org, Anna-Maria Gleixner , catalin.marinas@arm.com, Alison Wang , linux-kernel@vger.kernel.org, leoyang.li@nxp.com, vladimir.oltean@nxp.com, Kurt Kanzenbach , Thomas Gleixner , Vladimir Oltean , will@kernel.org, linux-arm-kernel@lists.infradead.org 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 05/08/20 14:40, peterz@infradead.org wrote: > On Mon, Aug 03, 2020 at 09:22:53PM +0200, Thomas Gleixner wrote: > >> totaltime = irqtime + tasktime >> >> Ignoring irqtime and pretending that totaltime is what the scheduler >> can control and deal with is naive at best. > > Well no, that's what we call system overhead and is assumed to be > included in the 'error margin'. > > The way things are set up is that we say that, by default, RT tasks can > consume 95% of cputime and the remaining 5% is sufficient to keep the > system alive. > > Those 5% include all system overhead, IRQs, RCU, !RT workqueues etc.. > > Obviously IRQ_TIME accounting changes the balance a bit, but that's what > it is. We can't really do anything better. > I'm starting to think that as well. I tried some fugly hack of injecting avg_irq into sched_rt_runtime_exceeded() with something along the lines of: irq_time = (rq->avg_irq.util_avg * sched_rt_period(rt_rq)) >> SCHED_CAPACITY_SHIFT; It's pretty bad for a few reasons; one is that avg_irq already has its own period (PELT-based). Another is that it is, as Dietmar pointed out, CPU and freq invariant, so falls over on big.LITTLE. Making update_curr_rt() use rq_clock() rather than rq_clock_task() makes it "work" but goes against all the good reasons there were to introduce rq_clock_task() in the first place. > Apparently this SoC has significant IRQ time for some reason. Also, > relying on RT throttling for 'correct' behaviour is also wrong. What > needs to be done is find who is using all this RT time and why, that > isn't right. I've been tempted to say the test case is a bit bogus, but am not familiar enough with the RT throttling details to stand that ground. That said, from both looking at the execution and the stress-ng source code, it seems to unconditionally spawn 32 FIFO-50 tasks (there's even an option to make these FIFO-99!!!), which is quite a crowd on monoCPU systems. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel