linux-kernel - Re: [PATCH 3/6] dataplane nohz: run softirqs synchronously on user entry

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Message-ID: <20150512130824.GI6776@linux.vnet.ibm.com>
Date:	Tue, 12 May 2015 06:08:24 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Chris Metcalf <cmetcalf@...hip.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Gilad Ben Yossef <giladb@...hip.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>, Tejun Heo <tj@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Christoph Lameter <cl@...ux.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/6] dataplane nohz: run softirqs synchronously on user
 entry

On Tue, May 12, 2015 at 11:32:02AM +0200, Peter Zijlstra wrote:
> On Mon, May 11, 2015 at 04:13:16PM -0400, Chris Metcalf wrote:
> > The thing you want to avoid is having two processes both
> > runnable at once
> 
> Right, because as soon as nr_running > 1 we kill the entire nohz_full
> thing. RT or not for ksoftirqd doesn't matter.
> 
> Then again, like interrupts, you basically want to avoid softirqs in
> this mode.
> 
> So I think the right solution is to figure out why the softirqs get
> raised and cure that.

Makes sense, but it also makes sense to have something that detects
when that cure fails and clean up.  And, in a test/debug environment,
also issuing some sort of diagnostic in that case.

							Thanx, Paul

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