diff mbox series

lib/group_cpus.c: avoid to acquire cpu hotplug lock in group_cpus_evenly

Message ID 20230810021640.252033-1-ming.lei@redhat.com
State New
Headers
Series lib/group_cpus.c: avoid to acquire cpu hotplug lock in group_cpus_evenly |

Commit Message

Ming Lei Aug. 10, 2023, 2:16 a.m. UTC 
  group_cpus_evenly() could be part of storage driver's error handler,
such as nvme driver, when may happen during CPU hotplug, in which
storage queue has to drain its pending IOs because all CPUs associated
with the queue are offline and the queue is becoming inactive. And
handling IO needs error handler to provide forward progress.

Then dead lock is caused:

1) inside CPU hotplug handler, CPU hotplug lock is held, and blk-mq's
handler is waiting for inflight IO

2) error handler is waiting for CPU hotplug lock

3) inflight IO can't be completed in blk-mq's CPU hotplug handler because
error handling can't provide forward progress.

Solve the deadlock by not holding CPU hotplug lock in group_cpus_evenly(),
in which two stage spreads are taken: 1) the 1st stage is over all present
CPUs; 2) the end stage is over all other CPUs.

Turns out the two stage spread just needs consistent 'cpu_present_mask', and
remove the CPU hotplug lock by storing it into one local cache. This way
doesn't change correctness, because all CPUs are still covered.

Cc: Keith Busch <kbusch@kernel.org>
Cc: linux-nvme@lists.infradead.org
Cc: linux-block@lists.infradead.org
Reported-by: Yi Zhang <yi.zhang@redhat.com>
Reported-by: Guangwu Zhang <guazhang@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 lib/group_cpus.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

Comments

Guangwu Zhang Aug. 11, 2023, 10:59 a.m. UTC | #1 
Hi Ming,
I don't find any IO block after apply the patch.

Tested-by: Guangwu Zhang <guazhang@redhat.com>

Ming Lei <ming.lei@redhat.com> 于2023年8月10日周四 10:22写道：
>
> group_cpus_evenly() could be part of storage driver's error handler,
> such as nvme driver, when may happen during CPU hotplug, in which
> storage queue has to drain its pending IOs because all CPUs associated
> with the queue are offline and the queue is becoming inactive. And
> handling IO needs error handler to provide forward progress.
>
> Then dead lock is caused:
>
> 1) inside CPU hotplug handler, CPU hotplug lock is held, and blk-mq's
> handler is waiting for inflight IO
>
> 2) error handler is waiting for CPU hotplug lock
>
> 3) inflight IO can't be completed in blk-mq's CPU hotplug handler because
> error handling can't provide forward progress.
>
> Solve the deadlock by not holding CPU hotplug lock in group_cpus_evenly(),
> in which two stage spreads are taken: 1) the 1st stage is over all present
> CPUs; 2) the end stage is over all other CPUs.
>
> Turns out the two stage spread just needs consistent 'cpu_present_mask', and
> remove the CPU hotplug lock by storing it into one local cache. This way
> doesn't change correctness, because all CPUs are still covered.
>
> Cc: Keith Busch <kbusch@kernel.org>
> Cc: linux-nvme@lists.infradead.org
> Cc: linux-block@lists.infradead.org
> Reported-by: Yi Zhang <yi.zhang@redhat.com>
> Reported-by: Guangwu Zhang <guazhang@redhat.com>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>  lib/group_cpus.c | 22 ++++++++++++++++------
>  1 file changed, 16 insertions(+), 6 deletions(-)
>
> diff --git a/lib/group_cpus.c b/lib/group_cpus.c
> index aa3f6815bb12..15006e79196f 100644
> --- a/lib/group_cpus.c
> +++ b/lib/group_cpus.c
> @@ -348,6 +348,7 @@ struct cpumask *group_cpus_evenly(unsigned int numgrps)
>  {
>         unsigned int curgrp = 0, nr_present = 0, nr_others = 0;
>         cpumask_var_t *node_to_cpumask;
> +       cpumask_var_t local_cpu_present_mask;
>         cpumask_var_t nmsk, npresmsk;
>         int ret = -ENOMEM;
>         struct cpumask *masks = NULL;
> @@ -355,6 +356,16 @@ struct cpumask *group_cpus_evenly(unsigned int numgrps)
>         if (!zalloc_cpumask_var(&nmsk, GFP_KERNEL))
>                 return NULL;
>
> +       if (!zalloc_cpumask_var(&local_cpu_present_mask, GFP_KERNEL))
> +               goto fail_local_pres_mask;
> +
> +       /*
> +        * Make a local cache of 'cpu_present_mask', so the two stages
> +        * spread can observe consistent 'cpu_present_mask' without holding
> +        * cpu hotplug lock.
> +        */
> +       cpumask_copy(local_cpu_present_mask, cpu_present_mask);
> +
>         if (!zalloc_cpumask_var(&npresmsk, GFP_KERNEL))
>                 goto fail_nmsk;
>
> @@ -366,13 +377,11 @@ struct cpumask *group_cpus_evenly(unsigned int numgrps)
>         if (!masks)
>                 goto fail_node_to_cpumask;
>
> -       /* Stabilize the cpumasks */
> -       cpus_read_lock();
>         build_node_to_cpumask(node_to_cpumask);
>
>         /* grouping present CPUs first */
>         ret = __group_cpus_evenly(curgrp, numgrps, node_to_cpumask,
> -                                 cpu_present_mask, nmsk, masks);
> +                                 local_cpu_present_mask, nmsk, masks);
>         if (ret < 0)
>                 goto fail_build_affinity;
>         nr_present = ret;
> @@ -387,15 +396,13 @@ struct cpumask *group_cpus_evenly(unsigned int numgrps)
>                 curgrp = 0;
>         else
>                 curgrp = nr_present;
> -       cpumask_andnot(npresmsk, cpu_possible_mask, cpu_present_mask);
> +       cpumask_andnot(npresmsk, cpu_possible_mask, local_cpu_present_mask);
>         ret = __group_cpus_evenly(curgrp, numgrps, node_to_cpumask,
>                                   npresmsk, nmsk, masks);
>         if (ret >= 0)
>                 nr_others = ret;
>
>   fail_build_affinity:
> -       cpus_read_unlock();
> -
>         if (ret >= 0)
>                 WARN_ON(nr_present + nr_others < numgrps);
>
> @@ -406,6 +413,9 @@ struct cpumask *group_cpus_evenly(unsigned int numgrps)
>         free_cpumask_var(npresmsk);
>
>   fail_nmsk:
> +       free_cpumask_var(local_cpu_present_mask);
> +
> + fail_local_pres_mask:
>         free_cpumask_var(nmsk);
>         if (ret < 0) {
>                 kfree(masks);
> --
> 2.40.1
>
Ming Lei Aug. 14, 2023, 2:03 a.m. UTC | #2 
Hello Thomas and Guys,

On Fri, Aug 11, 2023 at 06:59:25PM +0800, Guangwu Zhang wrote:
> Hi Ming,
> I don't find any IO block after apply the patch.
> 
> Tested-by: Guangwu Zhang <guazhang@redhat.com>

Can you take a look at this fix? And this one solves a couple
of nvme io hang report in case of cpu hotplug & reset involved.

Thanks,
Ming
diff mbox series

Patch

diff --git a/lib/group_cpus.c b/lib/group_cpus.c
index aa3f6815bb12..15006e79196f 100644
--- a/lib/group_cpus.c
+++ b/lib/group_cpus.c
@@ -348,6 +348,7 @@  struct cpumask *group_cpus_evenly(unsigned int numgrps)
 {
 	unsigned int curgrp = 0, nr_present = 0, nr_others = 0;
 	cpumask_var_t *node_to_cpumask;
+	cpumask_var_t local_cpu_present_mask;
 	cpumask_var_t nmsk, npresmsk;
 	int ret = -ENOMEM;
 	struct cpumask *masks = NULL;
@@ -355,6 +356,16 @@  struct cpumask *group_cpus_evenly(unsigned int numgrps)
 	if (!zalloc_cpumask_var(&nmsk, GFP_KERNEL))
 		return NULL;
 
+	if (!zalloc_cpumask_var(&local_cpu_present_mask, GFP_KERNEL))
+		goto fail_local_pres_mask;
+
+	/*
+	 * Make a local cache of 'cpu_present_mask', so the two stages
+	 * spread can observe consistent 'cpu_present_mask' without holding
+	 * cpu hotplug lock.
+	 */
+	cpumask_copy(local_cpu_present_mask, cpu_present_mask);
+
 	if (!zalloc_cpumask_var(&npresmsk, GFP_KERNEL))
 		goto fail_nmsk;
 
@@ -366,13 +377,11 @@  struct cpumask *group_cpus_evenly(unsigned int numgrps)
 	if (!masks)
 		goto fail_node_to_cpumask;
 
-	/* Stabilize the cpumasks */
-	cpus_read_lock();
 	build_node_to_cpumask(node_to_cpumask);
 
 	/* grouping present CPUs first */
 	ret = __group_cpus_evenly(curgrp, numgrps, node_to_cpumask,
-				  cpu_present_mask, nmsk, masks);
+				  local_cpu_present_mask, nmsk, masks);
 	if (ret < 0)
 		goto fail_build_affinity;
 	nr_present = ret;
@@ -387,15 +396,13 @@  struct cpumask *group_cpus_evenly(unsigned int numgrps)
 		curgrp = 0;
 	else
 		curgrp = nr_present;
-	cpumask_andnot(npresmsk, cpu_possible_mask, cpu_present_mask);
+	cpumask_andnot(npresmsk, cpu_possible_mask, local_cpu_present_mask);
 	ret = __group_cpus_evenly(curgrp, numgrps, node_to_cpumask,
 				  npresmsk, nmsk, masks);
 	if (ret >= 0)
 		nr_others = ret;
 
  fail_build_affinity:
-	cpus_read_unlock();
-
 	if (ret >= 0)
 		WARN_ON(nr_present + nr_others < numgrps);
 
@@ -406,6 +413,9 @@  struct cpumask *group_cpus_evenly(unsigned int numgrps)
 	free_cpumask_var(npresmsk);
 
  fail_nmsk:
+	free_cpumask_var(local_cpu_present_mask);
+
+ fail_local_pres_mask:
 	free_cpumask_var(nmsk);
 	if (ret < 0) {
 		kfree(masks);