linux-kernel - Re: [PATCH 01/10] workqueue: Move pwq->max_active to wq->max

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Date: Tue, 26 Dec 2023 17:13:40 +0800
From: Lai Jiangshan <jiangshanlai@...il.com>
To: Tejun Heo <tj@...nel.org>
Cc: linux-kernel@...r.kernel.org, Naohiro.Aota@....com, kernel-team@...a.com
Subject: Re: [PATCH 01/10] workqueue: Move pwq->max_active to wq->max_active

On Wed, Dec 20, 2023 at 3:25 PM Tejun Heo <tj@...nel.org> wrote:

> +static void wq_adjust_max_active(struct workqueue_struct *wq)
> +{
> +       struct pool_workqueue *pwq;
> +
> +       lockdep_assert_held(&wq->mutex);
> +
> +       if ((wq->flags & WQ_FREEZABLE) && workqueue_freezing) {
> +               wq->max_active = 0;
> +               return;
> +       }
> +
> +       if (wq->max_active == wq->saved_max_active)
> +               return;
> +
> +       wq->max_active = wq->saved_max_active;
> +

If a work item gets queued now, it will get scheduled earlier than a
previous queued one which is still in the inactive list.

To solve it, I recommend adding wq->queue_max_active which will be
updated after the following code and used only when queue_work().
But it requires round-robin through PWQs the second time after
wq->queue_max_active is updated to catch the new inactivated items.

Or just keep pwq->max_active and will be
updated after activating inactivated items and used only when queue_work().

> +       for_each_pwq(pwq, wq) {
> +               unsigned long flags;
> +
> +               /* this function can be called during early boot w/ irq disabled */
> +               raw_spin_lock_irqsave(&pwq->pool->lock, flags);
> +
> +               while (!list_empty(&pwq->inactive_works) &&
> +                      pwq->nr_active < wq->max_active)
> +                       pwq_activate_first_inactive(pwq);
> +
> +               kick_pool(pwq->pool);
> +
> +               raw_spin_unlock_irqrestore(&pwq->pool->lock, flags);
> +       }
> +}
> +