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Date: Tue, 5 May 2015 12:48:34 +0200 From: Peter Zijlstra <peterz@...radead.org> To: Rik van Riel <riel@...hat.com> Cc: 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>, "Paul E. McKenney" <paulmck@...ibm.com>, Linus Torvalds <torvalds@...ux-foundation.org> Subject: Re: question about RCU dynticks_nesting On Mon, May 04, 2015 at 03:00:44PM -0400, Rik van Riel wrote: In case of the non-preemptible RCU, we could easily also > increase current->rcu_read_lock_nesting at the same time > we increase the preempt counter, and use that as the > indicator to test whether the cpu is in an extended > rcu quiescent state. That way there would be no extra > overhead at syscall entry or exit at all. The trick > would be getting the preempt count and the rcu read > lock nesting count in the same cache line for each task. Can't do that. Remember, on x86 we have per-cpu preempt count, and your rcu_read_lock_nesting is per task. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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