[PATCH wq/for-6.9 v4 0/4] workqueue: Enable unbound cpumask update on ordered workqueues

[PATCH wq/for-6.9 v4 0/4] workqueue: Enable unbound cpumask update on ordered workqueues

[Waiman Long]

 v4:
  - [v3] https://lore.kernel.org/lkml/20240205194602.871505-1-longman@redhat.com/
  - Rebase on the latest for-6.9 branch again & discard the use of
    __WQ_ORDERED_EXPLICIT and resetting of __WQ_ORDERED.
  - Add a new patch 1 to change the ordering of pwq's in wq->pwqs from
    oldest to newest.
  - Change the terminalogy from freeze/thaw to plug/unplug.
  - Allow more than 2 pwq's in wq->pwqs of ordered workqueue but only the
    oldest one is unplugged. This eliminates the need to wait for
    the draining of extra pwq in workqueue_apply_unbound_cpumask().

 v3:
  - [v2] https://lore.kernel.org/lkml/20240203154334.791910-1-longman@redhat.com/
  - Drop patch 1 as it has been merged into the for-6.9 branch.
  - Use rcu_access_pointer() to access wq->dfl_pwq.
  - Use RCU protection instead of acquiring wq->mutex in
    apply_wqattrs_cleanup().

 v2:
  - [v1] https://lore.kernel.org/all/20240130183336.511948-1-longman@redhat.com/
  - Rebased on top of wq's for-v6.9 branch.
  - Use the new pwq_tryinc_nr_active() mechanism to freeze the new
    pwq of an ordered workqueue until the old pwq has been properly
    drained to maintain ordering.
  - Make rescuer follow changes in workqueue unbound cpumask as well
    as its sysfs cpumask, if available.

Ordered workqueues does not currently follow changes made to the
global unbound cpumask because per-pool workqueue changes may break
the ordering guarantee. IOW, a work function in an ordered workqueue
may run on a cpuset isolated CPU.

This series enables ordered workqueues to follow changes made to the
global unbound cpumask by temporaily suspending (plugging) the execution
of work items in the newly allocated pool_workqueue until the old pwq
has been properly drained.

The cpumask of the rescuer task of each workqueue is also made to follow
changes in workqueue unbound cpumask as well as its sysfs cpumask,
if available.

Juri Lelli (1):
  kernel/workqueue: Let rescuers follow unbound wq cpumask changes

Waiman Long (3):
  workqueue: Link pwq's into wq->pwqs from oldest to newest
  workqueue: Enable unbound cpumask update on ordered workqueues
  workqueue: Bind unbound workqueue rescuer to wq_unbound_cpumask

 kernel/workqueue.c | 111 ++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 99 insertions(+), 12 deletions(-)

-- 
2.39.3

[PATCH wq/for-6.9 v4 1/4] workqueue: Link pwq's into wq->pwqs from oldest to newest

[Waiman Long]

Add a new pwq into the tail of wq->pwqs so that pwq iteration will
start from the oldest pwq to the newest. This ordering will facilitate
the inclusion of ordered workqueues in a wq_unbound_cpumask update.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/workqueue.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index cf514ba0dfc3..fa7bd3b34f52 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -4804,7 +4804,7 @@ static void link_pwq(struct pool_workqueue *pwq)
 	pwq->work_color = wq->work_color;
 
 	/* link in @pwq */
-	list_add_rcu(&pwq->pwqs_node, &wq->pwqs);
+	list_add_tail_rcu(&pwq->pwqs_node, &wq->pwqs);
 }
 
 /* obtain a pool matching @attr and create a pwq associating the pool and @wq */
-- 
2.39.3

[PATCH wq/for-6.9 v4 2/4] workqueue: Enable unbound cpumask update on ordered workqueues

[Waiman Long]

Ordered workqueues does not currently follow changes made to the
global unbound cpumask because per-pool workqueue changes may break
the ordering guarantee. IOW, a work function in an ordered workqueue
may run on an isolated CPU.

This patch enables ordered workqueues to follow changes made to the
global unbound cpumask by temporaily plug or suspend the newly allocated
pool_workqueue from executing newly queued work items until the old
pwq has been properly drained. For ordered workqueues, there should
only be one pwq that is unplugged, the rests should be plugged.

This enables ordered workqueues to follow the unbound cpumask changes
like other unbound workqueues at the expense of some delay in execution
of work functions during the transition period.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/workqueue.c | 102 ++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 92 insertions(+), 10 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index fa7bd3b34f52..49fe082fe328 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -255,6 +255,7 @@ struct pool_workqueue {
 	int			refcnt;		/* L: reference count */
 	int			nr_in_flight[WORK_NR_COLORS];
 						/* L: nr of in_flight works */
+	bool			plugged;	/* L: execution suspended */
 
 	/*
 	 * nr_active management and WORK_STRUCT_INACTIVE:
@@ -1708,6 +1709,9 @@ static bool pwq_tryinc_nr_active(struct pool_workqueue *pwq, bool fill)
 		goto out;
 	}
 
+	if (unlikely(pwq->plugged))
+		return false;
+
 	/*
 	 * Unbound workqueue uses per-node shared nr_active $nna. If @pwq is
 	 * already waiting on $nna, pwq_dec_nr_active() will maintain the
@@ -1782,6 +1786,58 @@ static bool pwq_activate_first_inactive(struct pool_workqueue *pwq, bool fill)
 	}
 }
 
+/**
+ * unplug_oldest_pwq - restart an oldest plugged pool_workqueue
+ * @wq: workqueue_struct to be restarted
+ * @pwq: pwq to be excluded
+ *
+ * pwq's are linked into wq->pwqs with the oldest first. For ordered
+ * workqueues, only the oldest pwq is unplugged, the others are plugged to
+ * suspend execution until the oldest one is drained. When this happens, the
+ * next oldest one (first plugged pwq in iteration) will be unplugged to
+ * restart work item execution to ensure proper work item ordering.
+ *
+ *    dfl_pwq --------------+     [P] - plugged
+ *                          |
+ *                          v
+ *    pwqs -> A -> B [P] -> C [P] (newest)
+ *            |    |        |
+ *            1    3        5
+ *            |    |        |
+ *            2    4        6
+ *
+ * The unplugging is done either in apply_wqattrs_cleanup() [fast path] when
+ * the workqueue was idle or in pwq_release_workfn() [slow path] when the
+ * workqueue was busy.
+ */
+static void unplug_oldest_pwq(struct workqueue_struct *wq,
+			      struct pool_workqueue *exlude_pwq)
+{
+	struct pool_workqueue *pwq;
+	unsigned long flags;
+	bool found = false;
+
+	for_each_pwq(pwq, wq) {
+		if (pwq == exlude_pwq)
+			continue;
+		if (!pwq->plugged)
+			return;	/* No unplug needed */
+		found = true;
+		break;
+	}
+	if (WARN_ON_ONCE(!found))
+		return;
+
+	raw_spin_lock_irqsave(&pwq->pool->lock, flags);
+	if (!pwq->plugged)
+		goto out_unlock;
+	pwq->plugged = false;
+	if (pwq_activate_first_inactive(pwq, true))
+		kick_pool(pwq->pool);
+out_unlock:
+	raw_spin_unlock_irqrestore(&pwq->pool->lock, flags);
+}
+
 /**
  * node_activate_pending_pwq - Activate a pending pwq on a wq_node_nr_active
  * @nna: wq_node_nr_active to activate a pending pwq for
@@ -4740,6 +4796,13 @@ static void pwq_release_workfn(struct kthread_work *work)
 		mutex_lock(&wq->mutex);
 		list_del_rcu(&pwq->pwqs_node);
 		is_last = list_empty(&wq->pwqs);
+
+		/*
+		 * For ordered workqueue with a plugged dfl_pwq, restart it now.
+		 */
+		if (!is_last && (wq->flags & __WQ_ORDERED))
+			unplug_oldest_pwq(wq, NULL);
+
 		mutex_unlock(&wq->mutex);
 	}
 
@@ -4906,8 +4969,26 @@ static void apply_wqattrs_cleanup(struct apply_wqattrs_ctx *ctx)
 
 		for_each_possible_cpu(cpu)
 			put_pwq_unlocked(ctx->pwq_tbl[cpu]);
+
+		/*
+		 * Acquire rcu_read_lock() before refcnt can become 0 to
+		 * ensure that ctx->dfl_pwq won't be freed & we can
+		 * iterate wq->pwqs.
+		 */
+		rcu_read_lock();
 		put_pwq_unlocked(ctx->dfl_pwq);
 
+		/*
+		 * It is possible that ctx->dfl_pwq (previous wq->dfl_pwq)
+		 * may not be the oldest one with the plugged flag still set.
+		 * unplug_oldest_pwq() will still do the right thing to allow
+		 * only one unplugged pwq in the workqueue.
+		 */
+		if ((ctx->wq->flags & __WQ_ORDERED) &&
+		     ctx->dfl_pwq && !ctx->dfl_pwq->refcnt)
+			unplug_oldest_pwq(ctx->wq, ctx->dfl_pwq);
+		rcu_read_unlock();
+
 		free_workqueue_attrs(ctx->attrs);
 
 		kfree(ctx);
@@ -4966,6 +5047,15 @@ apply_wqattrs_prepare(struct workqueue_struct *wq,
 	cpumask_copy(new_attrs->__pod_cpumask, new_attrs->cpumask);
 	ctx->attrs = new_attrs;
 
+	/*
+	 * For initialized ordered workqueues, there is only one pwq (dfl_pwq).
+	 * Set the plugged flag of ctx->dfl_pwq to suspend execution of newly
+	 * queued work items until execution of older work items in the old
+	 * pwq's have completed.
+	 */
+	if (!list_empty(&wq->pwqs) && (wq->flags & __WQ_ORDERED))
+		ctx->dfl_pwq->plugged = true;
+
 	ctx->wq = wq;
 	return ctx;
 
@@ -5006,10 +5096,6 @@ static int apply_workqueue_attrs_locked(struct workqueue_struct *wq,
 	if (WARN_ON(!(wq->flags & WQ_UNBOUND)))
 		return -EINVAL;
 
-	/* creating multiple pwqs breaks ordering guarantee */
-	if (!list_empty(&wq->pwqs) && WARN_ON(wq->flags & __WQ_ORDERED))
-		return -EINVAL;
-
 	ctx = apply_wqattrs_prepare(wq, attrs, wq_unbound_cpumask);
 	if (IS_ERR(ctx))
 		return PTR_ERR(ctx);
@@ -6489,9 +6575,6 @@ static int workqueue_apply_unbound_cpumask(const cpumask_var_t unbound_cpumask)
 	list_for_each_entry(wq, &workqueues, list) {
 		if (!(wq->flags & WQ_UNBOUND) || (wq->flags & __WQ_DESTROYING))
 			continue;
-		/* creating multiple pwqs breaks ordering guarantee */
-		if (wq->flags & __WQ_ORDERED)
-			continue;
 
 		ctx = apply_wqattrs_prepare(wq, wq->unbound_attrs, unbound_cpumask);
 		if (IS_ERR(ctx)) {
@@ -7006,9 +7089,8 @@ int workqueue_sysfs_register(struct workqueue_struct *wq)
 	int ret;
 
 	/*
-	 * Adjusting max_active or creating new pwqs by applying
-	 * attributes breaks ordering guarantee.  Disallow exposing ordered
-	 * workqueues.
+	 * Adjusting max_active breaks ordering guarantee.  Disallow exposing
+	 * ordered workqueues.
 	 */
 	if (WARN_ON(wq->flags & __WQ_ORDERED))
 		return -EINVAL;
-- 
2.39.3

Re: [PATCH wq/for-6.9 v4 2/4] workqueue: Enable unbound cpumask update on ordered workqueues

[Tejun Heo]

Hello, Waiman.

On Tue, Feb 06, 2024 at 08:19:09PM -0500, Waiman Long wrote:
...
> + * The unplugging is done either in apply_wqattrs_cleanup() [fast path] when
> + * the workqueue was idle or in pwq_release_workfn() [slow path] when the
> + * workqueue was busy.

I'm not sure the distinction between fast and slow paths is all that useful
here. Both are really cold paths.

> +static void unplug_oldest_pwq(struct workqueue_struct *wq,
> +			      struct pool_workqueue *exlude_pwq)
> +{
> +	struct pool_workqueue *pwq;
> +	unsigned long flags;
> +	bool found = false;
> +
> +	for_each_pwq(pwq, wq) {
> +		if (pwq == exlude_pwq)
> +			continue;
> +		if (!pwq->plugged)
> +			return;	/* No unplug needed */
> +		found = true;
> +		break;
> +	}
> +	if (WARN_ON_ONCE(!found))
> +		return;
> +
> +	raw_spin_lock_irqsave(&pwq->pool->lock, flags);
> +	if (!pwq->plugged)
> +		goto out_unlock;
> +	pwq->plugged = false;
> +	if (pwq_activate_first_inactive(pwq, true))
> +		kick_pool(pwq->pool);
> +out_unlock:
> +	raw_spin_unlock_irqrestore(&pwq->pool->lock, flags);
> +}

I don't quite understand why this needs iteration and @exclude_pwq.
Shouldn't something like the following be enough?

static void unplug_oldest_pwq(struct workqueue_struct *wq)
{
	struct pool_workqueue *pwq;

	raw_spin_lock_irq(&pwq->pool->lock);
	pwq = list_first_entry_or_null(&pwq->pwqs, ...);
	if (pwq)
		pwq->plugged = false;
	raw_spin_unlock_irq(&pwq->pool->lock);
}

> @@ -4740,6 +4796,13 @@ static void pwq_release_workfn(struct kthread_work *work)
>  		mutex_lock(&wq->mutex);
>  		list_del_rcu(&pwq->pwqs_node);
>  		is_last = list_empty(&wq->pwqs);
> +
> +		/*
> +		 * For ordered workqueue with a plugged dfl_pwq, restart it now.
> +		 */
> +		if (!is_last && (wq->flags & __WQ_ORDERED))
> +			unplug_oldest_pwq(wq, NULL);

This makes sense.

> @@ -4906,8 +4969,26 @@ static void apply_wqattrs_cleanup(struct apply_wqattrs_ctx *ctx)
...
> +		/*
> +		 * It is possible that ctx->dfl_pwq (previous wq->dfl_pwq)
> +		 * may not be the oldest one with the plugged flag still set.
> +		 * unplug_oldest_pwq() will still do the right thing to allow
> +		 * only one unplugged pwq in the workqueue.
> +		 */
> +		if ((ctx->wq->flags & __WQ_ORDERED) &&
> +		     ctx->dfl_pwq && !ctx->dfl_pwq->refcnt)
> +			unplug_oldest_pwq(ctx->wq, ctx->dfl_pwq);
> +		rcu_read_unlock();

But why do we need this? Isn't all that needed to call unplug_oldest during
workqueue initialization and chaining unplugging from pwq release from there
on?

Thanks.

-- 
tejun

Re: [PATCH wq/for-6.9 v4 2/4] workqueue: Enable unbound cpumask update on ordered workqueues

[Waiman Long]

On 2/7/24 12:25, Tejun Heo wrote:
> Hello, Waiman.
>
> On Tue, Feb 06, 2024 at 08:19:09PM -0500, Waiman Long wrote:
> ...
>> + * The unplugging is done either in apply_wqattrs_cleanup() [fast path] when
>> + * the workqueue was idle or in pwq_release_workfn() [slow path] when the
>> + * workqueue was busy.
> I'm not sure the distinction between fast and slow paths is all that useful
> here. Both are really cold paths.
Yes, both are cold paths. Maybe a more accurate description is with 
respect to the latency that a new work item may experience since 
apply_wqattrs_cleanup() should be executed earlier than 
pwq_release_workfn().
>
>> +static void unplug_oldest_pwq(struct workqueue_struct *wq,
>> +			      struct pool_workqueue *exlude_pwq)
>> +{
>> +	struct pool_workqueue *pwq;
>> +	unsigned long flags;
>> +	bool found = false;
>> +
>> +	for_each_pwq(pwq, wq) {
>> +		if (pwq == exlude_pwq)
>> +			continue;
>> +		if (!pwq->plugged)
>> +			return;	/* No unplug needed */
>> +		found = true;
>> +		break;
>> +	}
>> +	if (WARN_ON_ONCE(!found))
>> +		return;
>> +
>> +	raw_spin_lock_irqsave(&pwq->pool->lock, flags);
>> +	if (!pwq->plugged)
>> +		goto out_unlock;
>> +	pwq->plugged = false;
>> +	if (pwq_activate_first_inactive(pwq, true))
>> +		kick_pool(pwq->pool);
>> +out_unlock:
>> +	raw_spin_unlock_irqrestore(&pwq->pool->lock, flags);
>> +}
> I don't quite understand why this needs iteration and @exclude_pwq.
> Shouldn't something like the following be enough?
>
> static void unplug_oldest_pwq(struct workqueue_struct *wq)
> {
> 	struct pool_workqueue *pwq;
>
> 	raw_spin_lock_irq(&pwq->pool->lock);
> 	pwq = list_first_entry_or_null(&pwq->pwqs, ...);
> 	if (pwq)
> 		pwq->plugged = false;
> 	raw_spin_unlock_irq(&pwq->pool->lock);
> }
>
It is because this function can be called from apply_wqattrs_cleanup() 
where I need to exclude ctx->dfl_pwq from being considered.
>> @@ -4740,6 +4796,13 @@ static void pwq_release_workfn(struct kthread_work *work)
>>   		mutex_lock(&wq->mutex);
>>   		list_del_rcu(&pwq->pwqs_node);
>>   		is_last = list_empty(&wq->pwqs);
>> +
>> +		/*
>> +		 * For ordered workqueue with a plugged dfl_pwq, restart it now.
>> +		 */
>> +		if (!is_last && (wq->flags & __WQ_ORDERED))
>> +			unplug_oldest_pwq(wq, NULL);
> This makes sense.
>
>> @@ -4906,8 +4969,26 @@ static void apply_wqattrs_cleanup(struct apply_wqattrs_ctx *ctx)
> ...
>> +		/*
>> +		 * It is possible that ctx->dfl_pwq (previous wq->dfl_pwq)
>> +		 * may not be the oldest one with the plugged flag still set.
>> +		 * unplug_oldest_pwq() will still do the right thing to allow
>> +		 * only one unplugged pwq in the workqueue.
>> +		 */
>> +		if ((ctx->wq->flags & __WQ_ORDERED) &&
>> +		     ctx->dfl_pwq && !ctx->dfl_pwq->refcnt)
>> +			unplug_oldest_pwq(ctx->wq, ctx->dfl_pwq);
>> +		rcu_read_unlock();
> But why do we need this? Isn't all that needed to call unplug_oldest during
> workqueue initialization and chaining unplugging from pwq release from there
> on?

Yes, it is possible to just do unplug_oldest_pwq() in 
pwq_release_workfn() and don't do it in apply_wqattrs_cleanup(). As said 
above, I just want to reduce the latency when the old pwq to be retired 
is idle. I can certainly update the patch to just do it in 
pwq_release_workfn() if you don't that it is necessary to do that too in 
apply_wqattrs_cleanup(). That will eliminate the need for the extra 
arugment and simplify unplug_oldest_pwq().

Cheers,
Longman

Re: [PATCH wq/for-6.9 v4 2/4] workqueue: Enable unbound cpumask update on ordered workqueues

[Tejun Heo]

Hello, Waiman.

On Wed, Feb 07, 2024 at 03:59:06PM -0500, Waiman Long wrote:
> > But why do we need this? Isn't all that needed to call unplug_oldest during
> > workqueue initialization and chaining unplugging from pwq release from there
> > on?
> 
> Yes, it is possible to just do unplug_oldest_pwq() in pwq_release_workfn()
> and don't do it in apply_wqattrs_cleanup(). As said above, I just want to
> reduce the latency when the old pwq to be retired is idle. I can certainly

They should retire as soon as all the work items are done.

> update the patch to just do it in pwq_release_workfn() if you don't that it
> is necessary to do that too in apply_wqattrs_cleanup(). That will eliminate
> the need for the extra arugment and simplify unplug_oldest_pwq().

So, yeah, let's please keep it simple for now. We can add optimizations
later if this becomes a problem, right?

Thanks.

-- 
tejun

[PATCH wq/for-6.9 v4 3/4] kernel/workqueue: Let rescuers follow unbound wq cpumask changes

[Waiman Long]

From: Juri Lelli <juri.lelli@redhat.com>

When workqueue cpumask changes are committed the associated rescuer (if
one exists) affinity is not touched and this might be a problem down the
line for isolated setups.

Make sure rescuers affinity is updated every time a workqueue cpumask
changes, so that rescuers can't break isolation.

 [longman: set_cpus_allowed_ptr() will block until the designated task
  is enqueued on an allowed CPU, no wake_up_process() needed. Also use
  the unbound_effective_cpumask() helper as suggested by Tejun.]

Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/workqueue.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 49fe082fe328..3044ad6f9496 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -5084,6 +5084,11 @@ static void apply_wqattrs_commit(struct apply_wqattrs_ctx *ctx)
 	/* update node_nr_active->max */
 	wq_update_node_max_active(ctx->wq, -1);
 
+	/* rescuer needs to respect wq cpumask changes */
+	if (ctx->wq->rescuer)
+		set_cpus_allowed_ptr(ctx->wq->rescuer->task,
+				     unbound_effective_cpumask(ctx->wq));
+
 	mutex_unlock(&ctx->wq->mutex);
 }
 
-- 
2.39.3

[PATCH wq/for-6.9 v4 4/4] workqueue: Bind unbound workqueue rescuer to wq_unbound_cpumask

[Waiman Long]

Commit 85f0ab43f9de ("kernel/workqueue: Bind rescuer to unbound
cpumask for WQ_UNBOUND") modified init_rescuer() to bind rescuer of
an unbound workqueue to the cpumask in wq->unbound_attrs. However
unbound_attrs->cpumask's of all workqueues are initialized to
cpu_possible_mask and will only be changed if it has the WQ_SYSFS flag
to expose a cpumask sysfs file to be written by users. So this patch
doesn't achieve what it is intended to do.

If an unbound workqueue is created after wq_unbound_cpumask is modified
and there is no more unbound cpumask update after that, the unbound
rescuer will be bound to all CPUs unless the workqueue is created
with the WQ_SYSFS flag and a user explicitly modified its cpumask
sysfs file.  Fix this problem by binding directly to wq_unbound_cpumask
in init_rescuer().

Fixes: 85f0ab43f9de ("kernel/workqueue: Bind rescuer to unbound cpumask for WQ_UNBOUND")
Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/workqueue.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 3044ad6f9496..81800cc2ee28 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -5332,7 +5332,7 @@ static int init_rescuer(struct workqueue_struct *wq)
 
 	wq->rescuer = rescuer;
 	if (wq->flags & WQ_UNBOUND)
-		kthread_bind_mask(rescuer->task, wq->unbound_attrs->cpumask);
+		kthread_bind_mask(rescuer->task, wq_unbound_cpumask);
 	else
 		kthread_bind_mask(rescuer->task, cpu_possible_mask);
 	wake_up_process(rescuer->task);
-- 
2.39.3