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 <[email protected]>
Cc: Juri Lelli <[email protected]>
Cc: Andrew Theurer <[email protected]>
Cc: Joe Mario <[email protected]>
Cc: Sebastian Jug <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Bart Van Assche <[email protected]>
Cc: Tejun Heo <[email protected]>
Tested-by: Joe Mario <[email protected]>
Signed-off-by: Ming Lei <[email protected]>
---
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 | 51 ++++++++++++++++++++++++++++++++++++++++----------
1 file changed, 41 insertions(+), 10 deletions(-)
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 555ada922cf0..187fbfacb397 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>
@@ -2179,7 +2180,11 @@ 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)
+ /*
+ * Switch to unbound work if all CPUs in this hw queue fall
+ * into isolated CPUs
+ */
+ if (hctx->queue->nr_hw_queues == 1 || next_cpu >= nr_cpu_ids)
return WORK_CPU_UNBOUND;
if (--hctx->next_cpu_batch <= 0) {
@@ -3488,14 +3493,30 @@ 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 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 +3524,7 @@ 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 +3932,8 @@ 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 +3960,15 @@ 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 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
*/
--
2.41.0
On Wed, Mar 20, 2024 at 10:34:46AM +0800, Ming Lei 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 <[email protected]>
> Cc: Juri Lelli <[email protected]>
> Cc: Andrew Theurer <[email protected]>
> Cc: Joe Mario <[email protected]>
> Cc: Sebastian Jug <[email protected]>
> Cc: Frederic Weisbecker <[email protected]>
> Cc: Bart Van Assche <[email protected]>
> Cc: Tejun Heo <[email protected]>
> Tested-by: Joe Mario <[email protected]>
> Signed-off-by: Ming Lei <[email protected]>
> ---
> V4:
> - improve comment & commit log as suggested by Tim
Hello Jens, Tejun and Guys,
This patch fixes one issue in OpenShift low latency environment, I appreciate
you may take a look at the patch and merge it if you are fine.
Thanks,
Ming
On 3/19/24 8:34 PM, Ming Lei 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.
In general, I think the fix is fine. Only thing that's a bit odd is:
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 555ada922cf0..187fbfacb397 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>
>
> @@ -2179,7 +2180,11 @@ 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)
> + /*
> + * Switch to unbound work if all CPUs in this hw queue fall
> + * into isolated CPUs
> + */
> + if (hctx->queue->nr_hw_queues == 1 || next_cpu >= nr_cpu_ids)
> return WORK_CPU_UNBOUND;
This relies on find_next_foo() returning >= nr_cpu_ids if the set is
empty, which is a lower level implementation detail that someone reading
this code may not know.
> if (--hctx->next_cpu_batch <= 0) {
> @@ -3488,14 +3493,30 @@ 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
^^
has to
> + * might submit IOs on these isolated CPUs, so use queue map to
^^
use the queue map
> + * check if all CPUs mapped to this hctx are offline
> + */
--
Jens Axboe
On Thu, Mar 21, 2024 at 11:07:52AM -0600, Jens Axboe wrote:
> On 3/19/24 8:34 PM, Ming Lei 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.
>
> In general, I think the fix is fine. Only thing that's a bit odd is:
Thanks for the review!
>
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index 555ada922cf0..187fbfacb397 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>
> >
> > @@ -2179,7 +2180,11 @@ 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)
> > + /*
> > + * Switch to unbound work if all CPUs in this hw queue fall
> > + * into isolated CPUs
> > + */
> > + if (hctx->queue->nr_hw_queues == 1 || next_cpu >= nr_cpu_ids)
> > return WORK_CPU_UNBOUND;
>
> This relies on find_next_foo() returning >= nr_cpu_ids if the set is
> empty, which is a lower level implementation detail that someone reading
> this code may not know.
Indeed, looks it is more readable to add one helper:
static bool blk_mq_hctx_empty_cpumask(struct blk_mq_hw_ctx *hctx)
{
return hctx->next_cpu >= nr_cpu_ids;
}
>
> > if (--hctx->next_cpu_batch <= 0) {
> > @@ -3488,14 +3493,30 @@ 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
> ^^
>
> has to
>
> > + * might submit IOs on these isolated CPUs, so use queue map to
> ^^
>
> use the queue map
OK, will fix them in V5.
thanks,
Ming