Re: [PATCH 2/2] blk-mq: simplify queue mapping & schedule with each possisble CPU

Ming Lei
2018-01-16T15:33:18Z

On Tue, Jan 16, 2018 at 10:31:42PM +0800, jianchao.wang wrote:
> Hi minglei
>
> On 01/16/2018 08:10 PM, Ming Lei wrote:
> >>> - next_cpu = cpumask_next(hctx->next_cpu, hctx->cpumask);
> >>> + next_cpu = cpumask_next_and(hctx->next_cpu, hctx->cpumask,
> >>> + cpu_online_mask);
> >>> if (next_cpu >= nr_cpu_ids)
> >>> - next_cpu = cpumask_first(hctx->cpumask);
> >>> + next_cpu = cpumask_first_and(hctx->cpumask,cpu_online_mask);
> >> the next_cpu here could be >= nr_cpu_ids when the none of on hctx->cpumask is online.
> > That supposes not happen because storage device(blk-mq hw queue) is
> > generally C/S model, that means the queue becomes only active when
> > there is online CPU mapped to it.
> >
> > But it won't be true for non-block-IO queue, such as HPSA's queues[1], and
> > network controller RX queues.
> >
> > [1] https://urldefense.proofpoint.com/v2/url?u=https-3A__marc.info_-3Fl-3Dlinux-2Dkernel-26m-3D151601867018444-26w-3D2&d=DwIBAg&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=7WdAxUBeiTUTCy8v-7zXyr4qk7sx26ATvfo6QSTvZyQ&m=tCZdQH6JUW1dkNCN92ycoUoKfDU_qWj-7EsUoYpOeJ0&s=vgHC9sbjYQb7mtY9MUJzbVXyVEyjoNJPWEx4_rfrHxU&e=
> >
> > One thing I am still not sure(but generic irq affinity supposes to deal with
> > well) is that the CPU may become offline after the IO is just submitted,
> > then where the IRQ controller delivers the interrupt of this hw queue
> > to?
> >
> >> This could be reproduced on NVMe with a patch that could hold some rqs on ctx->rq_list,
> >> meanwhile a script online and offline the cpus. Then a panic occurred in __queue_work().
> > That shouldn't happen, when CPU offline happens the rqs in ctx->rq_list
> > are dispatched directly, please see blk_mq_hctx_notify_dead().
>
> Yes, I know. The blk_mq_hctx_notify_dead will be invoked after the cpu has been set offlined.
> Please refer to the following diagram.
>
> CPU A CPU T
> kick
> _cpu_down() -> cpuhp_thread_fun (cpuhpT kthread)
> AP_ACTIVE (clear cpu_active_mask)
> |
> v
> AP_WORKQUEUE_ONLINE (unbind workers)
> |
> v
> TEARDOWN_CPU (stop_machine)
> , | execute
> \_ _ _ _ _ _ v
> preempt V take_cpu_down ( migration kthread)
> set_cpu_online(smp_processor_id(), false) (__cpu_disable) ------> Here !!!
> TEARDOWN_CPU
> |
> cpuhpT kthead is | v
> migrated away , AP_SCHED_STARTING (migrate_tasks)
> _ _ _ _ _ _ _ _/ |
> V v
> CPU X AP_OFFLINE
>
> |
> ,
> _ _ _ _ _ /
> V
> do_idle (idle task)
> <_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ cpuhp_report_idle_dead
> complete st->done_down
> __cpu_die (cpuhpT kthread, teardown_cpu)
>
> AP_OFFLINE
> |
> v
> BRINGUP_CPU
> |
> v
> BLK_MQ_DEAD -------> Here !!!
> |
> v
> OFFLINE
>
> The cpu has been cleared in cpu_online_mask when blk_mq_hctx_notify_dead is invoked.
> If the device is NVMe which only has one cpu mapped on the hctx,
> cpumask_first_and(hctx->cpumask,cpu_online_mask) will return a bad value.

Hi Jianchao,

OK, I got it, and it should have been the only corner case in which
all CPUs mapped to this hctx become offline, and I believe the following
patch should address this case, could you give a test?

---
diff --git a/block/blk-mq.c b/block/blk-mq.c
index c376d1b6309a..23f0f3ddffcf 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1416,21 +1416,44 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
*/
static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
{
+ bool tried = false;
+
if (hctx->queue->nr_hw_queues == 1)
return WORK_CPU_UNBOUND;

if (--hctx->next_cpu_batch <= 0) {
int next_cpu;
+select_cpu:

next_cpu = cpumask_next_and(hctx->next_cpu, hctx->cpumask,
cpu_online_mask);
if (next_cpu >= nr_cpu_ids)
next_cpu = cpumask_first_and(hctx->cpumask,cpu_online_mask);

- hctx->next_cpu = next_cpu;
+ /*
+ * No online CPU can be found here when running from
+ * blk_mq_hctx_notify_dead(), so make sure hctx->next_cpu
+ * is set correctly.
+ */
+ if (next_cpu >= nr_cpu_ids)
+ hctx->next_cpu = cpumask_first_and(hctx->cpumask,
+ cpu_possible_mask);
+ else
+ hctx->next_cpu = next_cpu;
hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;
}

+ /*
+ * Do unbound schedule if we can't find a online CPU for this hctx,
+ * and it should happen only if hctx->next_cpu is becoming DEAD.
+ */
+ if (!cpu_online(hctx->next_cpu)) {
+ if (!tried) {
+ tried = true;
+ goto select_cpu;
+ }
+ return WORK_CPU_UNBOUND;
+ }
return hctx->next_cpu;
}

Thanks,
Ming