From mboxrd@z Thu Jan 1 00:00:00 1970 From: sudeep.holla@arm.com (Sudeep Holla) Date: Fri, 23 May 2014 13:51:31 +0100 Subject: [PATCH] arm: use cpu_online_mask when using forced irq_set_affinity In-Reply-To: <20140523121032.GV3693@n2100.arm.linux.org.uk> References: <1399653640-21559-1-git-send-email-sudeep.holla@arm.com> <20140523121032.GV3693@n2100.arm.linux.org.uk> Message-ID: <537F4453.5000709@arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 23/05/14 13:10, Russell King - ARM Linux wrote: > On Fri, May 09, 2014 at 05:40:40PM +0100, Sudeep Holla wrote: >> From: Sudeep Holla >> >> Commit 01f8fa4f01d8("genirq: Allow forcing cpu affinity of interrupts") >> enabled the forced irq_set_affinity which previously refused to route an >> interrupt to an offline cpu. >> >> Commit ffde1de64012("irqchip: Gic: Support forced affinity setting") >> implements this force logic and disables the cpu online check for GIC >> interrupt controller. >> >> When __cpu_disable calls migrate_irqs, it disables the current cpu in >> cpu_online_mask and uses forced irq_set_affinity to migrate the IRQs >> away from the cpu but passes affinity mask with the cpu being offlined >> also included in it. >> >> When calling irq_set_affinity with force == true in a cpu hotplug path, >> the caller must ensure that the cpu being offlined is not present in the >> affinity mask or it may be selected as the target CPU, leading to the >> interrupt not being migrated. >> >> This patch uses cpu_online_mask when using forced irq_set_affinity so >> that the IRQs are properly migrated away. >> >> Tested on TC2 hotpluging CPU0 in and out. Without this patch the system >> locks up as the IRQs are not migrated away from CPU0. > > You don't explain /how/ this happens, and I'm not convinced that you've > properly diagnosed this bug. > Sorry for not being elaborate enough. - On boot by default all the irqs have cpu_online_mask as affinity - Now if CPU0 is being hotplugged out, CPU0 is removed from cpu_online_mask and migrate_irqs is called - In migrate_one_irq, when affinity is read from the irq_desc, it still contains CPU0 which is expected. - irq_set_affinity is called with affinity with CPU0 set and force = true, which chooses CPU0 resulting in not migrating the IRQ. >> @@ -155,11 +155,15 @@ static bool migrate_one_irq(struct irq_desc *desc) >> if (irqd_is_per_cpu(d) || !cpumask_test_cpu(smp_processor_id(), affinity)) >> return false; >> >> - if (cpumask_any_and(affinity, cpu_online_mask) >= nr_cpu_ids) { >> - affinity = cpu_online_mask; >> + if (cpumask_any_and(affinity, cpu_online_mask) >= nr_cpu_ids) >> ret = true; >> - } > > The idea here with the original code is: > > - if the current CPU (which is the one being offlined) is not in the > affinity mask, do nothing. > - if "affinity & cpu_online_mask" indicates that there's no CPUs in the > new set (cpu_online_mask must have been updated to indicate that the > current CPU is offline) then re-set the affinity mask and report that > we forced a change. > - otherwise, re-set the existing affinity (which will force the IRQ > controller to re-evaluate it's routing given the affinity and online > CPUs.) > I completely understand the above idea, except that the new feature added to allow forced affinity setting(as mentioned in the commit log by 2 commits), changes the behaviour of last step. IRQ controller now re-evaluates it's routing based on the given affinity alone and doesn't consider online CPUs when force = true is set. This will result in the CPU being offlined chosen as the target if it happens to be the first in the affinity mask. > This code is correct. In fact, changing it as you have, you /always/ > reset the affinity mask whether or not the CPU being offlined is the > last CPU in the affinity set. > > If you are finding that CPU0 is left with interrupts afterwards, the > bug lies elsewhere - probably in the IRQ controller code. > Since the IRQ controller code is changed to provide that feature, either - we have to choose not to use forced option, or - we need to make sure we pass valid affinity mask with force = true option I chose latter in this patch. Let me know your opinion. Regards, Sudeep