linux-kernel - Re: question about RCU dynticks

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Date:	Tue, 5 May 2015 05:30:39 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 Tue, May 05, 2015 at 12:51:02PM +0200, Peter Zijlstra wrote:
> 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..

Last I tried doing that, things got really messy at context-switch time.
Perhaps I simply didn't do the save/restore in the right place?

							Thanx, Paul

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