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Date: Tue, 5 May 2015 12:53:46 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: Rik van Riel <riel@...hat.com>,
Paolo Bonzini <pbonzini@...hat.com>,
Ingo Molnar <mingo@...nel.org>,
Andy Lutomirski <luto@...capital.net>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
X86 ML <x86@...nel.org>, williams@...hat.com,
Andrew Lutomirski <luto@...nel.org>, fweisbec@...hat.com,
Heiko Carstens <heiko.carstens@...ibm.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: question about RCU dynticks_nesting
On Mon, May 04, 2015 at 12:39:23PM -0700, Paul E. McKenney wrote:
> But in non-preemptible RCU, we have PREEMPT=n, so there is no preempt
> counter in production kernels. Even if there was, we have to sample this
> on other CPUs, so the overhead of preempt_disable() and preempt_enable()
> would be where kernel entry/exit is, so I expect that this would be a
> net loss in overall performance.
We unconditionally have the preempt_count, its just not used much for
PREEMPT_COUNT=n kernels.
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