linux-kernel - Re: [RFC PATCH 0/6] Convert all tasklets to workqueues

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Date:	Thu, 28 Jun 2007 20:37:36 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Dan Williams <dan.j.williams@...el.com>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Hellwig <hch@...radead.org>,
	john stultz <johnstul@...ibm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	"David S. Miller" <davem@...emloft.net>, kuznet@....inr.ac.ru
Subject: Re: [RFC PATCH 0/6] Convert all tasklets to workqueues

On 06/28, Steven Rostedt wrote:
> 
> I also don't see any nice API to have the priority set for a workqueue
> thread from within the kernel. Looks like one needs to be added,
> otherwise, I need to have the wrapper dig into the workqueue structs to
> find the thread that handles the workqueue.

It is not so trivial to implement properly. Note that CPU_UP creates a new
cwq->thread, so somehow workqueue should "remember" its priority. This means
we should record it in workqueue_struct. The most simple way is to add yet
another parameter to __create_workqueue(), but this is nasty.

So, perhaps we should add "long nice" to "struct workqueue_struct", and then

	void set_workqueue_nice(struct workqueue_struct *wq, long nice)
	{
		const cpumask_t *cpu_map = wq_cpu_map(wq);
		struct cpu_workqueue_struct *cwq;
		int cpu;

		wq->nice = nice;

		mutex_lock(&workqueue_mutex);

		for_each_cpu_mask(cpu, *cpu_map) {
			cwq = per_cpu_ptr(wq->cpu_wq, cpu);
			if (cwq->thread)
				set_user_nice(cwq->thread, nice);
		}

		mutex_unlock(&workqueue_mutex);
	}

We could use for_each_cpu_online() instead, but then we should check
is_single_threaded().

Oleg.

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