On Wed, Jul 02, 2025 at 06:33:59PM +0200, Daniel Wagner wrote:
> When isolcpus=io_queue is enabled, and the last housekeeping CPU for a
> given hctx goes offline, there would be no CPU left to handle I/O. To
> prevent I/O stalls, prevent offlining housekeeping CPUs that are still
> serving isolated CPUs.
>
> When isolcpus=io_queue is enabled and the last housekeeping CPU
> for a given hctx goes offline, no CPU would be left to handle I/O.
> To prevent I/O stalls, disallow offlining housekeeping CPUs that are
> still serving isolated CPUs.
>
> Signed-off-by: Daniel Wagner <wagi@xxxxxxxxxx>
> ---
> block/blk-mq.c | 42 ++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 42 insertions(+)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 0c61492724d228736f975f1d8f195515603801b6..87240644f73ed0490a5459e042c68e0e168f727d 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -3681,6 +3681,43 @@ static bool blk_mq_hctx_has_requests(struct blk_mq_hw_ctx *hctx)
> return data.has_rq;
> }
>
> +static bool blk_mq_hctx_can_offline_hk_cpu(struct blk_mq_hw_ctx *hctx,
> + unsigned int this_cpu)
> +{
> + const struct cpumask *hk_mask = housekeeping_cpumask(HK_TYPE_IO_QUEUE);
> +
> + for (int i = 0; i < hctx->nr_ctx; i++) {
> + struct blk_mq_ctx *ctx = hctx->ctxs[i];
> +
> + if (ctx->cpu == this_cpu)
> + continue;
> +
> + /*
> + * Check if this context has at least one online
> + * housekeeping CPU; in this case the hardware context is
> + * usable.
> + */
> + if (cpumask_test_cpu(ctx->cpu, hk_mask) &&
> + cpu_online(ctx->cpu))
> + break;
> +
> + /*
> + * The context doesn't have any online housekeeping CPUs,
> + * but there might be an online isolated CPU mapped to
> + * it.
> + */
> + if (cpu_is_offline(ctx->cpu))
> + continue;
> +
> + pr_warn("%s: trying to offline hctx%d but there is still an online isolcpu CPU %d mapped to it\n",
> + hctx->queue->disk->disk_name,
> + hctx->queue_num, ctx->cpu);
> + return false;
> + }
> +
> + return true;
> +}
> +
> static bool blk_mq_hctx_has_online_cpu(struct blk_mq_hw_ctx *hctx,
> unsigned int this_cpu)
> {
> @@ -3712,6 +3749,11 @@ static int blk_mq_hctx_notify_offline(unsigned int cpu, struct hlist_node *node)
> struct blk_mq_hw_ctx *hctx = hlist_entry_safe(node,
> struct blk_mq_hw_ctx, cpuhp_online);
>
> + if (housekeeping_enabled(HK_TYPE_IO_QUEUE)) {
> + if (!blk_mq_hctx_can_offline_hk_cpu(hctx, cpu))
> + return -EINVAL;
> + }
> +
> if (blk_mq_hctx_has_online_cpu(hctx, cpu))
> return 0;
>
>
> --
> 2.50.0
>
Thanks Daniel and great work!
Reviewed-by: Aaron Tomlin <atomlin@xxxxxxxxxxx>
--
Aaron Tomlin
