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Date: Tue, 5 May 2015 12:51:02 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Rik van Riel <riel@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>, Ingo Molnar <mingo@kernel.org>,
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        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: question about RCU dynticks_nesting
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On Tue, May 05, 2015 at 12:48:34PM +0200, Peter Zijlstra wrote:
> 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.

Hmm, I suppose you could do the rcu_read_lock_nesting thing in a per-cpu
counter too and transfer that into the task_struct on context switch.

If you manage to put both sides of that in the same cache things should
not add significant overhead.

You'd have to move the rcu_read_lock_nesting into the thread_info, which
would be painful as you'd have to go touch all archs etc..