linux-kernel - Re: IRQ affinity notifiers vs RT

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Message-ID: <1380584663.2562.27.camel@bwh-desktop.uk.level5networks.com>
Date:	Tue, 1 Oct 2013 00:44:23 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	"Alexandra N. Kossovsky" <Alexandra.Kossovsky@...etlabs.ru>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: IRQ affinity notifiers vs RT

On Mon, 2013-09-23 at 14:58 +0200, Sebastian Andrzej Siewior wrote:
> On 08/30/2013 11:29 PM, Ben Hutchings wrote:
> > Sebastian, I saw you came up with a fix for this but apparently without
> > seeing my earlier message:
> 
> Yes Ben, I haven't seen it. If I was on Cc I very sorry for overlooking
> it.

You weren't, as I didn't realise you were maintaining the RT patch set
then.

> > On Thu, 2013-07-25 at 00:31 +0100, Ben Hutchings wrote:
> 
> >> Workqueue code uses spin_lock_irq() on the workqueue lock, which with
> >> PREEMPT_RT enabled doesn't actually block IRQs.
> >>
> >> In 3.6, the irq_cpu_rmap functions relies on a workqueue flush to finish
> >> any outstanding notifications before freeing the cpu_rmap that they use.
> >> This won't be reliable if the notification is scheduled after releasing
> >> the irq_desc lock.
> >>
> >> However, following commit 896f97ea95c1 ('lib: cpu_rmap: avoid flushing
> >> all workqueues') in 3.8, I think that it is sufficient to do only
> >> kref_get(&desc->affinity_notify->kref) in __irq_set_affinity_locked()
> >> and then call schedule_work() in irq_set_affinity() after releasing the
> >> lock.  Something like this (untested):
> > 
> > Does the following make sense to you?
> 
> This was suggested by the original submitter on rt-users@....o (Joe
> Korty) where I've been made aware of this for the first time. This okay
> except for the part where the workqueue is not scheduled if calling by
> the __ function (i.e. the mips case). If I read the code correctly, the
> CPU goes offline and affinity change should be updated / users notified
> and this is not the case with this patch.
> 
> It is a valid question why only one mips SoC needs the function. If you
> look at commit 0c3263870f ("MIPS: Octeon: Rewrite interrupt handling
> code.") you can see that tglx himself made this adjustment so it might
> be valid :) Therefore I assume that we may get more callers of this
> function and the workqueue should be executed and I made something
> simple that works on RT.

Right, but it looks quite strange to have this thread just for a
(probably) quite rare event.  Maybe some day someone will work out how
to make Octeon's IRQ management less unusual and then you can use the
simpler approach for RT...

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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