linux-kernel - Re: [PATCH 2/6] irq_work: Implement remote queueing

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Message-ID: <53A9E117.4010704@wwwdotorg.org>
Date:	Tue, 24 Jun 2014 14:35:35 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Kevin Hilman <khilman@...aro.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	"linux-next@...r.kernel.org" <linux-next@...r.kernel.org>
Subject: Re: [PATCH 2/6] irq_work: Implement remote queueing

On 06/24/2014 02:33 PM, Stephen Warren wrote:
> On 06/10/2014 09:15 AM, Frederic Weisbecker wrote:
>> irq work currently only supports local callbacks. However its code
>> is mostly ready to run remote callbacks and we have some potential user.
>>
>> The full nohz subsystem currently open codes its own remote irq work
>> on top of the scheduler ipi when it wants a CPU to reevaluate its next
>> tick. However this ad hoc solution bloats the scheduler IPI.
>>
>> Lets just extend the irq work subsystem to support remote queuing on top
>> of the generic SMP IPI to handle this kind of user. This shouldn't add
>> noticeable overhead.
> 
> I'm running next-20140624 on an ARM system, and this patch causes CPU
> hot(un)plug to Oops for me; the following fires:
> 
> void irq_work_run(void)
> {
> 	BUG_ON(!in_irq());
> 
> I found that Linus's master (8b8f5d971584 "Merge tag 'compress-3.16-rc3'
> of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/driver-core")
> works fine. I found that this commit inside the tip(?) tree works fine
> (478850160636 "irq_work: Implement remote queueing"). However, if I
> merge the two together, I hit that BUG_ON.

I forgot to mention that the conflicting commit from Linus' tree is
8d056c48e486 "CPU hotplug, smp: flush any pending IPI callbacks before
CPU offline".

> I think the issue is:
> 
> This commit adds a call from
> generic_smp_call_function_single_interrupt() to irq_work_run().
> 
> Srivatsa's patch adds a call from hotplug_cfd() to
> flush_smp_call_function_queue() to, which I imagine happens in
> non-interrupt context. Note that this patch moves most of the body of
> generic_smp_call_function_single_interrupt() into
> flush_smp_call_function_queue().
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