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

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Date:	Sat, 09 May 2015 09:04:03 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Chris Metcalf <cmetcalf@...hip.com>
Cc:	Gilad Ben Yossef <giladb@...hip.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>, Tejun Heo <tj@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/6] dataplane nohz: run softirqs synchronously on user
 entry

On Fri, 2015-05-08 at 13:58 -0400, Chris Metcalf wrote:
> For tasks which have elected dataplane functionality, we run
> any pending softirqs for the core before returning to userspace,
> rather than ever scheduling ksoftirqd to run.  The problem we
> fix is that by allowing another task to run on the core, we
> guarantee more interrupts in the future to the dataplane task,
> which is exactly what dataplane mode is required to prevent.

If ksoftirqd were rt class, softirqs would be gone when the soloist gets
the CPU back and heads to userspace.  Being a soloist, it has no use for
a priority, so why can't it just let ksoftirqd run if it raises the
occasional softirq?  Meeting a contended lock while processing it will
wreck the soloist regardless of who does that processing.

	-Mike

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