Date:	Wed, 23 Apr 2014 02:04:45 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Christoph Lameter <cl@...ux.com>,
	Kevin Hilman <khilman@...aro.org>,
	Mike Galbraith <bitbucket@...ine.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Tejun Heo <tj@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH] workqueue: allow changing attributions of ordered
 workqueue

Hi Lai,

So actually I'll need to use apply_workqueue_attr() on the next patchset. So
I'm considering this patch.

Some comments below:

On Tue, Apr 15, 2014 at 05:58:08PM +0800, Lai Jiangshan wrote:
> From 534f1df8a5a03427b0fc382150fbd34e05648a28 Mon Sep 17 00:00:00 2001
> From: Lai Jiangshan <laijs@...fujitsu.com>
> Date: Tue, 15 Apr 2014 17:52:19 +0800
> Subject: [PATCH] workqueue: allow changing attributions of ordered workqueue
> 
> Allow changing ordered workqueue's cpumask
> Allow changing ordered workqueue's nice value
> Allow registering ordered workqueue to SYSFS
> 
> Still disallow changing ordered workqueue's max_active which breaks ordered guarantee
> Still disallow changing ordered workqueue's no_numa which breaks ordered guarantee
> 
> Changing attributions will introduce new pwqs in a given workqueue, and
> old implement doesn't have any solution to handle multi-pwqs on ordered workqueues,
> so changing attributions for ordered workqueues is disallowed.
> 
> After I investigated several solutions which try to handle multi-pwqs on ordered workqueues,
> I found the solution which reuse max_active are the simplest.
> In this solution, the new introduced pwq's max_active is kept as *ZERO* until it become
> the oldest pwq.

I don't see where this zero value is enforced. Do you mean 1? That's the initial value of
ordered max_active pools.

> Thus only one (the oldest) pwq is active, and ordered is guarantee.
> 
> This solution forces ordered on higher level while the non-reentrancy is also
> forced. so we don't need to queue any work to its previous pool. And we shouldn't
> do it. we must disallow any work repeatedly requeues itself back-to-back
> which keeps the oldest pwq stay and make newest(if have) pwq starvation(non-active for ever).
> 
> Build test only.
> This patch depends on previous patch:
> workqueue: add __WQ_FREEZING and remove POOL_FREEZING
> 
> Frederic:
> 	You can pick this patch to your updated patchset before
> 	TJ apply it.
> 
> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
> ---
>  kernel/workqueue.c |   66 ++++++++++++++++++++++++++++++---------------------
>  1 files changed, 39 insertions(+), 27 deletions(-)
> 
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 92c9ada..fadcc4a 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -1350,8 +1350,14 @@ retry:
>  	 * If @work was previously on a different pool, it might still be
>  	 * running there, in which case the work needs to be queued on that
>  	 * pool to guarantee non-reentrancy.
> +	 *
> +	 * pwqs are guaranteed active orderly for ordered workqueue, and
> +	 * it guarantees non-reentrancy for works. So any work doesn't need
> +	 * to be queued on previous pool. And the works shouldn't be queued
> +	 * on previous pool, since we need to guarantee the prevous pwq
> +	 * releasable to avoid work-stavation on the newest pool.

BTW, who needs this reentrancy guarantee? Is this an implicit guarantee for
all workqueues that is there for backward compatibility? I've seen some
patches dealing with that lately but I don't recall the details.

>  	 */
> -	last_pool = get_work_pool(work);
> +	last_pool = wq->flags & __WQ_ORDERED ? NULL : get_work_pool(work);

Does it hurt performance to do this old worker recovery? It seems to
when I look at get_work_pool() and find_worker_executing_pool().

Perhaps we can generalize this check to all wqs which have only one
worker?

Anyway that's just optimizations. Nothing that needs to be done in this
patch.

>  	if (last_pool && last_pool != pwq->pool) {
>  		struct worker *worker;
>  
> @@ -3179,6 +3185,10 @@ static ssize_t wq_numa_store(struct device *dev, struct device_attribute *attr,
>  	struct workqueue_attrs *attrs;
>  	int v, ret;
>  
> +	/* Creating per-node pwqs breaks ordering guarantee. Keep no_numa = 1 */
> +	if (WARN_ON(wq->flags & __WQ_ORDERED))
> +		return -EINVAL;
> +
>  	attrs = wq_sysfs_prep_attrs(wq);
>  	if (!attrs)
>  		return -ENOMEM;
> @@ -3239,14 +3249,6 @@ int workqueue_sysfs_register(struct workqueue_struct *wq)
>  	struct wq_device *wq_dev;
>  	int ret;
>  
> -	/*
> -	 * Adjusting max_active or creating new pwqs by applyting
> -	 * attributes breaks ordering guarantee.  Disallow exposing ordered
> -	 * workqueues.
> -	 */
> -	if (WARN_ON(wq->flags & __WQ_ORDERED))
> -		return -EINVAL;
> -
>  	wq->wq_dev = wq_dev = kzalloc(sizeof(*wq_dev), GFP_KERNEL);
>  	if (!wq_dev)
>  		return -ENOMEM;
> @@ -3568,6 +3570,13 @@ static void rcu_free_pwq(struct rcu_head *rcu)
>  			container_of(rcu, struct pool_workqueue, rcu));
>  }
>  
> +static struct pool_workqueue *oldest_pwq(struct workqueue_struct *wq)
> +{
> +	return list_last_entry(&wq->pwqs, struct pool_workqueue, pwqs_node);
> +}
> +
> +static void pwq_adjust_max_active(struct pool_workqueue *pwq);
> +
>  /*
>   * Scheduled on system_wq by put_pwq() when an unbound pwq hits zero refcnt
>   * and needs to be destroyed.
> @@ -3583,14 +3592,12 @@ static void pwq_unbound_release_workfn(struct work_struct *work)
>  	if (WARN_ON_ONCE(!(wq->flags & WQ_UNBOUND)))
>  		return;
>  
> -	/*
> -	 * Unlink @pwq.  Synchronization against wq->mutex isn't strictly
> -	 * necessary on release but do it anyway.  It's easier to verify
> -	 * and consistent with the linking path.
> -	 */
>  	mutex_lock(&wq->mutex);
>  	list_del_rcu(&pwq->pwqs_node);
>  	is_last = list_empty(&wq->pwqs);
> +	/* try to active the oldest pwq when needed */
> +	if (!is_last && (wq->flags & __WQ_ORDERED))
> +		pwq_adjust_max_active(oldest_pwq(wq));

So this switches the new pwq's max active from 0 to 1?
I still can't find where it is set to 0.

>  	mutex_unlock(&wq->mutex);
>  
>  	mutex_lock(&wq_pool_mutex);
> @@ -3609,6 +3616,16 @@ static void pwq_unbound_release_workfn(struct work_struct *work)
>  	}
>  }
>  
> +static bool pwq_active(struct pool_workqueue *pwq)
> +{
> +	/* Only the oldest pwq is active in the ordered wq */
> +	if (pwq->wq->flags & __WQ_ORDERED)
> +		return pwq == oldest_pwq(pwq->wq);
> +
> +	/* All pwqs in the non-ordered wq are active */
> +	return true;
> +}
> +
>  /**
>   * pwq_adjust_max_active - update a pwq's max_active to the current setting
>   * @pwq: target pool_workqueue
> @@ -3626,7 +3643,7 @@ static void pwq_adjust_max_active(struct pool_workqueue *pwq)
>  
>  	spin_lock_irq(&pwq->pool->lock);
>  
> -	if (!(wq->flags & __WQ_FREEZING)) {
> +	if (!(wq->flags & __WQ_FREEZING) && pwq_active(pwq)) {

Ah! So that must be how we initially set the new max_active to 0.

>  		pwq->max_active = wq->saved_max_active;
>  
>  		while (!list_empty(&pwq->delayed_works) &&
> @@ -3680,11 +3697,11 @@ static void link_pwq(struct pool_workqueue *pwq)
>  	 */
>  	pwq->work_color = wq->work_color;
>  
> +	/* link in @pwq on the head of &wq->pwqs */
> +	list_add_rcu(&pwq->pwqs_node, &wq->pwqs);
> +
>  	/* sync max_active to the current setting */
>  	pwq_adjust_max_active(pwq);
> -
> -	/* link in @pwq */
> -	list_add_rcu(&pwq->pwqs_node, &wq->pwqs);
>  }
>  
>  /* obtain a pool matching @attr and create a pwq associating the pool and @wq */
> @@ -3810,8 +3827,8 @@ int apply_workqueue_attrs(struct workqueue_struct *wq,
>  	if (WARN_ON(!(wq->flags & WQ_UNBOUND)))
>  		return -EINVAL;
>  
> -	/* creating multiple pwqs breaks ordering guarantee */
> -	if (WARN_ON((wq->flags & __WQ_ORDERED) && !list_empty(&wq->pwqs)))
> +	/* creating multiple per-node pwqs breaks ordering guarantee */
> +	if (WARN_ON((wq->flags & __WQ_ORDERED) && !attrs->no_numa))
>  		return -EINVAL;
>  
>  	pwq_tbl = kzalloc(wq_numa_tbl_len * sizeof(pwq_tbl[0]), GFP_KERNEL);
> @@ -4004,7 +4021,7 @@ out_unlock:
>  static int alloc_and_link_pwqs(struct workqueue_struct *wq)
>  {
>  	bool highpri = wq->flags & WQ_HIGHPRI;
> -	int cpu, ret;
> +	int cpu;
>  
>  	if (!(wq->flags & WQ_UNBOUND)) {
>  		wq->cpu_pwqs = alloc_percpu(struct pool_workqueue);
> @@ -4025,12 +4042,7 @@ static int alloc_and_link_pwqs(struct workqueue_struct *wq)
>  		}
>  		return 0;
>  	} else if (wq->flags & __WQ_ORDERED) {
> -		ret = apply_workqueue_attrs(wq, ordered_wq_attrs[highpri]);
> -		/* there should only be single pwq for ordering guarantee */
> -		WARN(!ret && (wq->pwqs.next != &wq->dfl_pwq->pwqs_node ||
> -			      wq->pwqs.prev != &wq->dfl_pwq->pwqs_node),
> -		     "ordering guarantee broken for workqueue %s\n", wq->name);
> -		return ret;
> +		return apply_workqueue_attrs(wq, ordered_wq_attrs[highpri]);
>  	} else {
>  		return apply_workqueue_attrs(wq, unbound_std_wq_attrs[highpri]);
>  	}

So I have mixed feelings with this patch given the complication. But it's probably
better to take that direction.

I just wish we had some way to automatically detect situations where a work
mistakenly runs through re-entrancy. Because if it ever happens randomly,
it's going to be a hell to debug.

Thanks.

> -- 
> 1.7.4.4
> 
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