Re: [PATCH v3 15/15] blk-mq: use hk cpus only when isolcpus=io_queue is enabled

[Ming Lei]

On Tue, Aug 06, 2024 at 02:06:47PM +0200, Daniel Wagner wrote:
> When isolcpus=io_queue is enabled all hardware queues should run on the
> housekeeping CPUs only. Thus ignore the affinity mask provided by the
> driver. Also we can't use blk_mq_map_queues because it will map all CPUs
> to first hctx unless, the CPU is the same as the hctx has the affinity
> set to, e.g. 8 CPUs with isolcpus=io_queue,2-3,6-7 config
> 
>   queue mapping for /dev/nvme0n1
>         hctx0: default 2 3 4 6 7
>         hctx1: default 5
>         hctx2: default 0
>         hctx3: default 1
> 
>   PCI name is 00:05.0: nvme0n1
>         irq 57 affinity 0-1 effective 1 is_managed:0 nvme0q0
>         irq 58 affinity 4 effective 4 is_managed:1 nvme0q1
>         irq 59 affinity 5 effective 5 is_managed:1 nvme0q2
>         irq 60 affinity 0 effective 0 is_managed:1 nvme0q3
>         irq 61 affinity 1 effective 1 is_managed:1 nvme0q4
> 
> where as with blk_mq_hk_map_queues we get
> 
>   queue mapping for /dev/nvme0n1
>         hctx0: default 2 4
>         hctx1: default 3 5
>         hctx2: default 0 6
>         hctx3: default 1 7
> 
>   PCI name is 00:05.0: nvme0n1
>         irq 56 affinity 0-1 effective 1 is_managed:0 nvme0q0
>         irq 61 affinity 4 effective 4 is_managed:1 nvme0q1
>         irq 62 affinity 5 effective 5 is_managed:1 nvme0q2
>         irq 63 affinity 0 effective 0 is_managed:1 nvme0q3
>         irq 64 affinity 1 effective 1 is_managed:1 nvme0q4
> 
> Signed-off-by: Daniel Wagner <dwagner@xxxxxxx>
> ---
>  block/blk-mq-cpumap.c | 56 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 56 insertions(+)
> 
> diff --git a/block/blk-mq-cpumap.c b/block/blk-mq-cpumap.c
> index c1277763aeeb..7e026c2ffa02 100644
> --- a/block/blk-mq-cpumap.c
> +++ b/block/blk-mq-cpumap.c
> @@ -60,11 +60,64 @@ unsigned int blk_mq_num_online_queues(unsigned int max_queues)
>  }
>  EXPORT_SYMBOL_GPL(blk_mq_num_online_queues);
>  
> +static bool blk_mq_hk_map_queues(struct blk_mq_queue_map *qmap)
> +{
> +	struct cpumask *hk_masks;
> +	cpumask_var_t isol_mask;
> +
> +	unsigned int queue, cpu;
> +
> +	if (!housekeeping_enabled(HK_TYPE_IO_QUEUE))
> +		return false;
> +
> +	/* map housekeeping cpus to matching hardware context */
> +	hk_masks = group_cpus_evenly(qmap->nr_queues);
> +	if (!hk_masks)
> +		goto fallback;
> +
> +	for (queue = 0; queue < qmap->nr_queues; queue++) {
> +		for_each_cpu(cpu, &hk_masks[queue])
> +			qmap->mq_map[cpu] = qmap->queue_offset + queue;
> +	}
> +
> +	kfree(hk_masks);
> +
> +	/* map isolcpus to hardware context */
> +	if (!alloc_cpumask_var(&isol_mask, GFP_KERNEL))
> +		goto fallback;
> +
> +	queue = 0;
> +	cpumask_andnot(isol_mask,
> +		       cpu_possible_mask,
> +		       housekeeping_cpumask(HK_TYPE_IO_QUEUE));
> +
> +	for_each_cpu(cpu, isol_mask) {
> +		qmap->mq_map[cpu] = qmap->queue_offset + queue;
> +		queue = (queue + 1) % qmap->nr_queues;
> +	}
> +

With patch 14 and the above change, managed irq's affinity becomes not
matched with blk-mq mapping any more.

If the last CPU in managed irq's affinity becomes offline, blk-mq
mapping may have other isolated CPUs, so IOs in this hctx won't be
drained from blk_mq_hctx_notify_offline() in case of CPU offline,
but genirq still shutdowns this manage irq.

So IO hang risk is introduced here, it should be the reason of your
hang observation.


Thanks, 
Ming