linux-kernel - Re: [PATCH 1/4] tile: support arch_irq_work

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Date:	Tue, 24 Mar 2015 18:06:26 -0400
From:	Chris Metcalf <cmetcalf@...hip.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	<linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 1/4] tile: support arch_irq_work_raise

On 3/24/2015 5:14 PM, Peter Zijlstra wrote:
> On Tue, Mar 24, 2015 at 03:21:32PM -0400, cmetcalf@...hip.com wrote:
>> From: Chris Metcalf <cmetcalf@...hip.com>
>>
>> Tile includes a hypervisor hook to deliver messages to arbitrary
>> tiles, so we can use that to raise an interrupt as soon as
>> possible on our own core.  Unfortunately the Tilera hypervisor
>> disabled that support on principle in previous releases, but
>> it will be available in MDE 4.3.4 and later.
> Can you program a timer in the (recent) past which will instantly
> trigger an interrupt? This is what PPC64 does to implement the self-ipi.

I looked in git history a bit and see commit 105988c015943 from 2009,
which looks like it was the basis for the powerpc support.  I'm a little leery
of just randomly changing the in-flight timer decrementer value, though;
does the timer event_handler properly handle being called early, and
then properly handle resetting the decrementer to the right value?
I guess if both of those things are true, it seems plausible to adopt the
approach you suggested.

On the other hand, the approach I coded up avoids making any of those
slightly scary assumptions about the timer subsystem, and I don't really
have a problem with saying you need a recent Tilera hypervisor binary if
you want to use NOHZ_FULL...

-- 
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com

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