linux-kernel - Re: [PATCH v4 1/2] workqueue: add new schedule_on_cpu

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Message-ID: <20130809163029.GT20515@mtj.dyndns.org>
Date:	Fri, 9 Aug 2013 12:30:29 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Chris Metcalf <cmetcalf@...era.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Cody P Schafer <cody@...ux.vnet.ibm.com>
Subject: Re: [PATCH v4 1/2] workqueue: add new schedule_on_cpu_mask() API

Hello, Chris.

On Fri, Aug 09, 2013 at 12:12:43PM -0400, Chris Metcalf wrote:
> I could certainly make schedule_on_cpu_mask() do sanity checking,
> perhaps via a WARN_ON_ONCE() if offline cpus were specified, and
> otherwise just have it create a local struct cpumask that it and's
> with cpu_online_mask, suitably wrapping with
> get_online_cpus()/put_online_cpus().  (I'm not sure how to test if a
> cpu has ever been online, vs whether it's online right now.)  I

I think you'll have to collect it from CPU_ONLINE of
workqueue_cpu_up_callback() and I think it probably wouldn't be a bad
idea to allow scheduling on CPUs which have been up but aren't
currently as that's the current rule for other interfaces anyway.

> don't want to unnecessarily slow down the existing
> schedule_on_each_cpu(), so perhaps the implementation should have a
> static schedule_on_cpu_mask_internal() function that is the same as
> my previous schedule_on_cpu_mask(), allowing schedule_on_each_cpu()
> to call it directly to bypass the checking.

Hmmm.... it's unlikely to make noticeable difference given that it's
gonna be bouncing multiple cachelines across all online CPUs.

> That said... I wonder if it might make sense to treat this API the
> same as other APIs that already take a cpu?  schedule_work_on(),
> schedule_delayed_work_on(), and queue_delayed_work_on() all take a
> cpu parameter without API comment or validity checking;
> queue_work_on() just says "the caller must ensure [the cpu] can't go
> away".  Does it make sense to just add a similar comment to
> schedule_on_cpu_mask() rather than make this API the first to
> actually do cpu validity checking?

Yeah, we've been lazy with the sanity check and I think it's a good
opportunity to add it.  Let's worry about other paths later.

Thanks.

-- 
tejun
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