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Message-ID: <CAGtn9rkwd1aw=Nept2kLTsMR+mGAbCKvzM1ELieNpT4sPN4zyw@mail.gmail.com>
Date: Mon, 25 Mar 2024 15:01:51 -0400
From: Ewan Milne <emilne@...hat.com>
To: Ming Lei <ming.lei@...hat.com>
Cc: Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
linux-kernel@...r.kernel.org, Tim Chen <tim.c.chen@...ux.intel.com>,
Juri Lelli <juri.lelli@...hat.com>, Andrew Theurer <atheurer@...hat.com>,
Joe Mario <jmario@...hat.com>, Sebastian Jug <sejug@...hat.com>,
Frederic Weisbecker <frederic@...nel.org>, Bart Van Assche <bvanassche@....org>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH V5] blk-mq: don't schedule block kworker on isolated CPUs
On Thu, Mar 21, 2024 at 10:13 PM Ming Lei <ming.lei@...hat.com> wrote:
>
> Kernel parameter of `isolcpus=` or 'nohz_full=' are used to isolate CPUs
> for specific task, and it isn't expected to let block IO disturb these CPUs.
> blk-mq kworker shouldn't be scheduled on isolated CPUs. Also if isolated
> CPUs is run for blk-mq kworker, long block IO latency can be caused.
>
> Kernel workqueue only respects CPU isolation for WQ_UNBOUND, for bound
> WQ, the responsibility is on user because CPU is specified as WQ API
> parameter, such as mod_delayed_work_on(cpu), queue_delayed_work_on(cpu)
> and queue_work_on(cpu).
>
> So not run blk-mq kworker on isolated CPUs by removing isolated CPUs
> from hctx->cpumask. Meantime use queue map to check if all CPUs in this
> hw queue are offline instead of hctx->cpumask, this way can avoid any
> cost in fast IO code path, and is safe since hctx->cpumask are only
> used in the two cases.
>
> Cc: Tim Chen <tim.c.chen@...ux.intel.com>
> Cc: Juri Lelli <juri.lelli@...hat.com>
> Cc: Andrew Theurer <atheurer@...hat.com>
> Cc: Joe Mario <jmario@...hat.com>
> Cc: Sebastian Jug <sejug@...hat.com>
> Cc: Frederic Weisbecker <frederic@...nel.org>
> Cc: Bart Van Assche <bvanassche@....org>
> Cc: Tejun Heo <tj@...nel.org>
> Tesed-by: Joe Mario <jmario@...hat.com>
> Signed-off-by: Ming Lei <ming.lei@...hat.com>
> ---
> V5:
> - fix comment
> - add one helper for document benefit
> - all are suggested by Jens
> V4:
> - improve comment & commit log as suggested by Tim
> 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 | 57 +++++++++++++++++++++++++++++++++++++++++---------
> 1 file changed, 47 insertions(+), 10 deletions(-)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 555ada922cf0..06cd9c8068fc 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -28,6 +28,7 @@
> #include <linux/prefetch.h>
> #include <linux/blk-crypto.h>
> #include <linux/part_stat.h>
> +#include <linux/sched/isolation.h>
>
> #include <trace/events/block.h>
>
> @@ -2168,6 +2169,15 @@ static inline int blk_mq_first_mapped_cpu(struct blk_mq_hw_ctx *hctx)
> return cpu;
> }
>
> +/*
> + * ->next_cpu is always calculated from hctx->cpumask, so simply use
> + * it for speeding up the check
> + */
> +static bool blk_mq_hctx_empty_cpumask(struct blk_mq_hw_ctx *hctx)
> +{
> + return hctx->next_cpu >= nr_cpu_ids;
> +}
> +
> /*
> * It'd be great if the workqueue API had a way to pass
> * in a mask and had some smarts for more clever placement.
> @@ -2179,7 +2189,8 @@ 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)
> + /* Switch to unbound if no allowable CPUs in this hctx */
> + if (hctx->queue->nr_hw_queues == 1 || blk_mq_hctx_empty_cpumask(hctx))
> return WORK_CPU_UNBOUND;
>
> if (--hctx->next_cpu_batch <= 0) {
> @@ -3488,14 +3499,30 @@ 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 to rule out isolated CPUs, but userspace still
> + * might submit IOs on these isolated CPUs, so use the 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 hctx has at least one online CPU */
> + if (this_cpu != cpu)
> + return true;
> + }
> +
> + return false;
> }
>
> static int blk_mq_hctx_notify_offline(unsigned int cpu, struct hlist_node *node)
> @@ -3503,8 +3530,7 @@ 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;
>
> /*
> @@ -3912,6 +3938,8 @@ 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.
> @@ -3938,6 +3966,15 @@ 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 to avoid
> + * running block kworker on isolated CPUs
> + */
> + for_each_cpu(cpu, hctx->cpumask) {
> + if (cpu_is_isolated(cpu))
> + cpumask_clear_cpu(cpu, hctx->cpumask);
> + }
> +
> /*
> * Initialize batch roundrobin counts
> */
> --
> 2.41.0
>
>
Reviewed-by: Ewan D. Milne <emilne@...hat.com>
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