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

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Message-ID: <20150325080352.GC27490@worktop.programming.kicks-ass.net>
Date:	Wed, 25 Mar 2015 09:03:52 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Chris Metcalf <cmetcalf@...hip.com>
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 Tue, Mar 24, 2015 at 06:06:26PM -0400, Chris Metcalf wrote:
> 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...

Fair enough; and your call obviously. Just wanted to share that there
are alternative ways if you really need.
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