linux-kernel - Re: [PATCH 1/5] irq_work: Architecture support for remote irq work raise

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Message-ID: <20140512075650.GJ30445@twins.programming.kicks-ass.net>
Date:	Mon, 12 May 2014 09:56:50 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	"David S. Miller" <davem@...emloft.net>,
	Ingo Molnar <mingo@...nel.org>,
	Kevin Hilman <khilman@...aro.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Mackerras <paulus@...ba.org>,
	Russell King <linux@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH 1/5] irq_work: Architecture support for remote irq work
 raise

On Mon, May 12, 2014 at 01:33:53AM +0200, Frederic Weisbecker wrote:
> We are going to extend irq work to support remote queuing.
> 
> So lets add a cpu argument to arch_irq_work_raise(). The architectures
> willing to support that must then provide the backend to raise irq work
> IPIs remotely.
> 
> Initial support is provided for x86 and ARM since they are easily
> extended. The other archs that overwrite arch_irq_work_raise() seem
> to use local clock interrupts and therefore need deeper rewrite of their
> irq work support to implement remote raising.
> 

> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>

Why not borrow the smp_call_function IPI for the remote bits? We could
limit the 'safe from NMI' to the local works. And we validate this by
putting a WARN_ON(in_nmi()) in irq_work_queue_on().

At some later stage we could look at maybe merging the smp_function_call
and irq_work into a single thing, where we have the irq_work interface
as async and the smp_function_call() as sync interface.

But for now a quick 'hack' would be to call __irq_work_run() from
generic_smp_call_function_single_interrupt().

That would leave arch_irq_work_raise() as the special NMI safe local IPI
hook.

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