Messages in this thread Patch in this message |  | | Date | Fri, 20 Dec 2019 00:11:15 +0800 | From | Ming Lei <> | Subject | Re: Kernel-managed IRQ affinity (cont) |
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On Thu, Dec 19, 2019 at 09:32:14AM -0500, Peter Xu wrote: > On Thu, Dec 19, 2019 at 04:28:19PM +0800, Ming Lei wrote: > > Hi Peter, > > Hi, Ming, > > > > > On Mon, Dec 16, 2019 at 02:57:12PM -0500, Peter Xu wrote: > > > Hi, Thomas, > > > > > > (Sorry I must have lost the discussion during an email migration, so > > > I'll start with a new one) > > > > > > This is a continued discussion of previous one on kernel managed IRQ > > > affinity [1]. I think at that time the conclusion is that we don't > > > have a usage scenario to change current policy [2]. However recently > > > I noticed that it is probably a very fundamental requirement for some > > > real-time scenarios, even when there's no multi-queue involved. > > > > > > In my test case, it was a very common realtime guest with 10 vcpus, > > > 0-1 are housekeeping vcpus, 2-9 are realtime vcpus. The guest has one > > > virtio-blk device as boot disk. With a distribution very close to > > > latest upstream, we can observe high spikes, probably due to the IRQs. > > > > > > To guarantee realtime responsiveness, we need to make sure the IRQs > > > will be managable, say, when I run a real-time workload on vcpu9, we > > > should be able to move all the IRQs from vcpu9 to the other vcpus > > > (most probably vcpu0 and vcpu1). However with the kernel managed IRQs > > > we can't echo to /proc/irq/N/smp_affinity. Here, vcpu9 gets IRQ 38 > > > from the virtio-blk device: > > > > > > # cat /proc/interrupts | grep -w 38 > > > 38: 0 0 0 0 0 0 0 0 0 15206 PCI-MSI 2621441-edge virtio2-req.0 > > > # cat /proc/irq/38/smp_affinity > > > 3ff > > > # cat /proc/irq/38/effective_affinity > > > 200 > > > > > > Meanwhile, I don't think there's anything special for VMs, so this > > > issue should exist even for hosts as long as the IRQ is managed in the > > > same way here as the virtio-blk device. > > > > > > As Ming has mentioned in previous discussions [3], I think it would be > > > at least good if the kernel IRQ system can respect "irqaffinity=" when > > > assigning IRQs to the cores. Currently it's not. What would you > > > suggest in this case? Do you think this is a valid user scenario? > > > > > > Thanks, > > > > > > [1] https://lkml.org/lkml/2019/3/18/15 > > > [2] https://lkml.org/lkml/2019/3/25/562 > > > [3] https://lkml.org/lkml/2019/3/25/308 > > > > The following patch supposes to implementation the requirement for you, > > can you test it by passing 'isolcpus=managed_irq,X-Y'? > > I really appreciate your patch! I'll keep this version, while before > I start to test it... > > > > > With this kind of change, you can't run any IO from any isolated > > CPU core, otherwise, unpredictable error may be triggered, either oops or > > IO hang. > > ... I'm not sure whether this can be acceptable for a production > environment. > > In our case, the IRQ should come from virtio-blk which is the root > disk, so I assume even the RT core could use it at least when loading > the executable into RAM. So... > > > > > Another conservative approach is to only select effective CPU from > > non-isolated cpus, however, the assigned CPUs may not be balanced among > > interrupt vectors. But it is safer, since the system still works even if > > someone submits IO from any isolated cpu core. > > ... this one seems to be more appealing at least to me.
OK, please try the following patch:
diff --git a/include/linux/sched/isolation.h b/include/linux/sched/isolation.h index 6c8512d3be88..0fbcbacd1b29 100644 --- a/include/linux/sched/isolation.h +++ b/include/linux/sched/isolation.h @@ -13,6 +13,7 @@ enum hk_flags { HK_FLAG_TICK = (1 << 4), HK_FLAG_DOMAIN = (1 << 5), HK_FLAG_WQ = (1 << 6), + HK_FLAG_MANAGED_IRQ = (1 << 7), }; #ifdef CONFIG_CPU_ISOLATION diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c index 1753486b440c..0a75a09cc4e8 100644 --- a/kernel/irq/manage.c +++ b/kernel/irq/manage.c @@ -20,6 +20,7 @@ #include <linux/sched/task.h> #include <uapi/linux/sched/types.h> #include <linux/task_work.h> +#include <linux/sched/isolation.h> #include "internals.h" @@ -212,12 +213,33 @@ int irq_do_set_affinity(struct irq_data *data, const struct cpumask *mask, { struct irq_desc *desc = irq_data_to_desc(data); struct irq_chip *chip = irq_data_get_irq_chip(data); + const struct cpumask *housekeeping_mask = + housekeeping_cpumask(HK_FLAG_MANAGED_IRQ); int ret; + cpumask_var_t tmp_mask; if (!chip || !chip->irq_set_affinity) return -EINVAL; - ret = chip->irq_set_affinity(data, mask, force); + if (!zalloc_cpumask_var(&tmp_mask, GFP_KERNEL)) + return -EINVAL; + + /* + * Userspace can't change managed irq's affinity, make sure + * that isolated CPU won't be selected as the effective CPU + * if this irq's affinity includes both isolated CPU and + * housekeeping CPU. + * + * This way guarantees that isolated CPU won't be interrupted + * by IO submitted from housekeeping CPU. + */ + if (irqd_affinity_is_managed(data) && + cpumask_intersects(mask, housekeeping_mask)) + cpumask_and(tmp_mask, mask, housekeeping_mask); + else + cpumask_copy(tmp_mask, mask); + + ret = chip->irq_set_affinity(data, tmp_mask, force); switch (ret) { case IRQ_SET_MASK_OK: case IRQ_SET_MASK_OK_DONE: @@ -229,6 +251,8 @@ int irq_do_set_affinity(struct irq_data *data, const struct cpumask *mask, ret = 0; } + free_cpumask_var(tmp_mask); + return ret; } diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c index 9fcb2a695a41..008d6ac2342b 100644 --- a/kernel/sched/isolation.c +++ b/kernel/sched/isolation.c @@ -163,6 +163,12 @@ static int __init housekeeping_isolcpus_setup(char *str) continue; } + if (!strncmp(str, "managed_irq,", 12)) { + str += 12; + flags |= HK_FLAG_MANAGED_IRQ; + continue; + } + pr_warn("isolcpus: Error, unknown flag\n"); return 0; } -- Ming
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