Tested-by: Joe Mario <jmario@xxxxxxxxxx>
Running fio on an nvme, I verified that block kworker threads no longer ran on isolated cpus.
I used:
# timeout -s INT 60 bpftrace -e 'kprobe:nvme_queue_rq{ @["CPU=",cpu] = count(); }'
The kernel with this V3 patch showed no hits on isolated cpus.
An unpatched kernel showed 40587 hits on isolated cpus.
Joe
On 10/24/23 10:57 PM, Ming Lei wrote:
> Kernel parameter of `isolcpus=` or 'nohz_full=' are used for isolating CPUs
> for specific task, and user often won't want block IO to disturb these CPUs,
> also long IO latency may be caused if blk-mq kworker is scheduled on these
> isolated CPUs.
>
> Kernel workqueue only respects this limit for WQ_UNBOUND, for bound wq,
> the responsibility should be on wq user.
>
> So don't not run block kworker on isolated CPUs by ruling out isolated CPUs
> from hctx->cpumask. Meantime in cpuhp handler, use queue map to check if
> all CPUs in this hw queue are offline, this way can avoid any cost in fast
> IO code path.
>
> Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
> Cc: Andrew Theurer <atheurer@xxxxxxxxxx>
> Cc: Joe Mario <jmario@xxxxxxxxxx>
> Cc: Sebastian Jug <sejug@xxxxxxxxxx>
> Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Cc: Bart Van Assche <bvanassche@xxxxxxx>
> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> ---
>
> V3:
> - avoid to check invalid cpu as reported by Bart
> - take current cpu(to be offline, not done yet) into account
> - simplify blk_mq_hctx_has_online_cpu()
>
> V2:
> - remove module parameter, meantime use queue map to check if
> all cpus in one hctx are offline
>
> block/blk-mq.c | 51 ++++++++++++++++++++++++++++++++++++++++----------
> 1 file changed, 41 insertions(+), 10 deletions(-)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index e2d11183f62e..4556978ce71b 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -29,6 +29,7 @@
> #include <linux/prefetch.h>
> #include <linux/blk-crypto.h>
> #include <linux/part_stat.h>
> +#include <linux/sched/isolation.h>
>
> #include <trace/events/block.h>
>
> @@ -2158,7 +2159,11 @@ static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
> bool tried = false;
> int next_cpu = hctx->next_cpu;
>
> - if (hctx->queue->nr_hw_queues == 1)
> + /*
> + * In case of single queue or no allowed CPU for scheduling
> + * worker, don't bound our worker with any CPU
> + */
> + if (hctx->queue->nr_hw_queues == 1 || next_cpu >= nr_cpu_ids)
> return WORK_CPU_UNBOUND;
>
> if (--hctx->next_cpu_batch <= 0) {
> @@ -3459,14 +3464,30 @@ static bool blk_mq_hctx_has_requests(struct blk_mq_hw_ctx *hctx)
> return data.has_rq;
> }
>
> -static inline bool blk_mq_last_cpu_in_hctx(unsigned int cpu,
> - struct blk_mq_hw_ctx *hctx)
> +static bool blk_mq_hctx_has_online_cpu(struct blk_mq_hw_ctx *hctx,
> + unsigned int this_cpu)
> {
> - if (cpumask_first_and(hctx->cpumask, cpu_online_mask) != cpu)
> - return false;
> - if (cpumask_next_and(cpu, hctx->cpumask, cpu_online_mask) < nr_cpu_ids)
> - return false;
> - return true;
> + enum hctx_type type = hctx->type;
> + int cpu;
> +
> + /*
> + * hctx->cpumask has rule out isolated CPUs, but userspace still
> + * might submit IOs on these isolated CPUs, so use queue map to
> + * check if all CPUs mapped to this hctx are offline
> + */
> + for_each_online_cpu(cpu) {
> + struct blk_mq_hw_ctx *h = blk_mq_map_queue_type(hctx->queue,
> + type, cpu);
> +
> + if (h != hctx)
> + continue;
> +
> + /* this current CPU isn't put offline yet */
> + if (this_cpu != cpu)
> + return true;
> + }
> +
> + return false;
> }
>
> static int blk_mq_hctx_notify_offline(unsigned int cpu, struct hlist_node *node)
> @@ -3474,8 +3495,7 @@ 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 (!cpumask_test_cpu(cpu, hctx->cpumask) ||
> - !blk_mq_last_cpu_in_hctx(cpu, hctx))
> + if (blk_mq_hctx_has_online_cpu(hctx, cpu))
> return 0;
>
> /*
> @@ -3883,6 +3903,8 @@ static void blk_mq_map_swqueue(struct request_queue *q)
> }
>
> queue_for_each_hw_ctx(q, hctx, i) {
> + int cpu;
> +
> /*
> * If no software queues are mapped to this hardware queue,
> * disable it and free the request entries.
> @@ -3909,6 +3931,15 @@ static void blk_mq_map_swqueue(struct request_queue *q)
> */
> sbitmap_resize(&hctx->ctx_map, hctx->nr_ctx);
>
> + /*
> + * rule out isolated CPUs from hctx->cpumask for avoiding to
> + * run wq worker on isolated CPU
> + */
> + for_each_cpu(cpu, hctx->cpumask) {
> + if (cpu_is_isolated(cpu))
> + cpumask_clear_cpu(cpu, hctx->cpumask);
> + }
> +
> /*
> * Initialize batch roundrobin counts
> */
