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Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1kQoma-0001dI-Vf; Fri, 09 Oct 2020 09:33:49 +0000 Received: from mail.kernel.org ([198.145.29.99]) by merlin.infradead.org with esmtps (Exim 4.92.3 #3 (Red Hat Linux)) id 1kQomY-0001cZ-Dw for linux-arm-kernel@lists.infradead.org; Fri, 09 Oct 2020 09:33:47 +0000 Received: from gaia (unknown [95.149.105.49]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 8804C22258; Fri, 9 Oct 2020 09:33:43 +0000 (UTC) Date: Fri, 9 Oct 2020 10:33:41 +0100 From: Catalin Marinas To: Will Deacon Subject: Re: [RFC PATCH 3/3] arm64: Handle AArch32 tasks running on non AArch32 cpu Message-ID: <20201009093340.GC23638@gaia> References: <20201008181641.32767-1-qais.yousef@arm.com> <20201008181641.32767-4-qais.yousef@arm.com> <20201009072943.GD2628@hirez.programming.kicks-ass.net> <20201009081312.GA8004@e123083-lin> <20201009083146.GA29594@willie-the-truck> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20201009083146.GA29594@willie-the-truck> User-Agent: Mutt/1.10.1 (2018-07-13) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20201009_053346_654558_9D5BAE31 X-CRM114-Status: GOOD ( 36.26 ) 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: linux-arch@vger.kernel.org, Peter Zijlstra , Greg Kroah-Hartman , Qais Yousef , Marc Zyngier , Linus Torvalds , Morten Rasmussen , 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 Fri, Oct 09, 2020 at 09:31:47AM +0100, Will Deacon wrote: > On Fri, Oct 09, 2020 at 10:13:12AM +0200, Morten Rasmussen wrote: > > On Fri, Oct 09, 2020 at 09:29:43AM +0200, Peter Zijlstra wrote: > > > On Thu, Oct 08, 2020 at 07:16:41PM +0100, Qais Yousef wrote: > > > > diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c > > > > index cf94cc248fbe..7e97f1589f33 100644 > > > > --- a/arch/arm64/kernel/signal.c > > > > +++ b/arch/arm64/kernel/signal.c > > > > @@ -908,13 +908,28 @@ static void do_signal(struct pt_regs *regs) > > > > restore_saved_sigmask(); > > > > } > > > > > > > > +static void set_32bit_cpus_allowed(void) > > > > { > > > > + cpumask_var_t cpus_allowed; > > > > + int ret = 0; > > > > + > > > > + if (cpumask_subset(current->cpus_ptr, &aarch32_el0_mask)) > > > > + return; > > > > + > > > > /* > > > > + * On asym aarch32 systems, if the task has invalid cpus in its mask, > > > > + * we try to fix it by removing the invalid ones. > > > > */ > > > > + if (!alloc_cpumask_var(&cpus_allowed, GFP_ATOMIC)) { > > > > + ret = -ENOMEM; > > > > + } else { > > > > + cpumask_and(cpus_allowed, current->cpus_ptr, &aarch32_el0_mask); > > > > + ret = set_cpus_allowed_ptr(current, cpus_allowed); > > > > + free_cpumask_var(cpus_allowed); > > > > + } > > > > + > > > > + if (ret) { > > > > + pr_warn_once("Failed to fixup affinity of running 32-bit task\n"); > > > > force_sig(SIGKILL); > > > > } > > > > } > > > > > > Yeah, no. Not going to happen. > > > > > > Fundamentally, you're not supposed to change the userspace provided > > > affinity mask. If we want to do something like this, we'll have to teach > > > the scheduler about this second mask such that it can compute an > > > effective mask as the intersection between the 'feature' and user mask. > > > > I agree that we shouldn't mess wit the user-space mask directly. Would it > > be unthinkable to go down the route of maintaining a new mask which is > > the intersection of the feature mask (controlled and updated by arch > > code) and the user-space mask? > > > > It shouldn't add overhead in the scheduler as it would use the > > intersection mask instead of the user-space mask, the main complexity > > would be around making sure the intersection mask is updated correctly > > (cpusets, hotplug, ...). > > > > Like the above tweak, this won't help if the intersection mask is empty, > > task will still get killed but it will allow tasks to survive > > user-space masks including some non-compatible CPUs. If we want to > > prevent task killing in all cases (ignoring hotplug) it gets more ugly > > as we would have to ignore the user-space mask in some cases. > > Honestly, I don't understand why we're trying to hide this asymmetry from > userspace by playing games with affinity masks in the kernel. Userspace > is likely to want to move things about _anyway_ because even amongst the > 32-bit capable cores, you may well have different clock frequencies to > contend with. > > So I'd be *much* happier to let the schesduler do its thing, and if one > of these 32-bit tasks ends up on a core that can't deal with it, then > tough, it gets killed. Give userspace the information it needs to avoid > that happening in the first place, rather than implicitly limit the mask. > > That way, the kernel support really boils down to two parts: > > 1. Remove the sanity checks we have to prevent 32-bit applications running > on asymmetric systems > > 2. Tell userspace about the problem This works for me as well as long as it is default off with a knob to turn it on. I'd prefer a sysctl (which can be driven from the command line in recent kernels IIRC) so that one can play with it a run-time. This way it's also a userspace choice and not an admin or whoever controls the cmdline (well, that's rather theoretical since the target is Android). -- Catalin _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel