netdev - Re: [raw v1 2/4] [NET] Use raw

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Message-ID: <20131008102142.GT3081@twins.programming.kicks-ass.net>
Date:	Tue, 8 Oct 2013 12:21:42 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Christoph Lameter <cl@...ux.com>, Tejun Heo <tj@...nel.org>,
	akpm@...uxfoundation.org, Eric Dumazet <edumazet@...gle.com>,
	netdev@...r.kernel.org, Steven Rostedt <srostedt@...hat.com>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [raw v1 2/4] [NET] Use raw_cpu ops for SNMP stats

On Tue, Oct 08, 2013 at 09:21:14AM +0200, Ingo Molnar wrote:
> 
> * Christoph Lameter <cl@...ux.com> wrote:
> 
> > SNMP stats are not protected by preemption but by bh handling.
> 
> Most forms of bh exclusion work via the preemption count though, and 
> softirq contexts themselves are generally not preemptible [to other CPUs] 
> either.
> 
> So the warnings should, in most cases, not trigger.

Right, so softirqs run either in the irq tail at which point
preempt_count += SOFTIRQ_OFFSET and thus preemption is disabled, or it
runs in ksoftirqd which has strict cpu affinity which also disables the
warning, and it also increments preempt_count with SOFTIRQ_OFFSET to
exclude the softirq from interrupts while its running, also disabling
the warning.

So it should very much not trigger.. if it does you want to know about
it.
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