From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932821AbaHGR2I (ORCPT ); Thu, 7 Aug 2014 13:28:08 -0400 Received: from casper.infradead.org ([85.118.1.10]:48116 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932128AbaHGR2H (ORCPT ); Thu, 7 Aug 2014 13:28:07 -0400 Date: Thu, 7 Aug 2014 19:27:53 +0200 From: Peter Zijlstra To: "Paul E. McKenney" Cc: Steven Rostedt , Oleg Nesterov , linux-kernel@vger.kernel.org, mingo@kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com, bobby.prani@gmail.com Subject: Re: [PATCH v3 tip/core/rcu 3/9] rcu: Add synchronous grace-period waiting for RCU-tasks Message-ID: <20140807172753.GG3588@twins.programming.kicks-ass.net> References: <20140802225857.GC8101@linux.vnet.ibm.com> <20140805205711.7a52076c@gandalf.local.home> <20140806012139.GY8101@linux.vnet.ibm.com> <20140806084708.GR9918@twins.programming.kicks-ass.net> <20140806120958.GZ8101@linux.vnet.ibm.com> <20140806163035.GG19379@twins.programming.kicks-ass.net> <20140806224518.GA8101@linux.vnet.ibm.com> <20140807084544.GJ19379@twins.programming.kicks-ass.net> <20140807150031.GB5821@linux.vnet.ibm.com> <20140807152600.GW9918@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="KzTpGdFWKkiGyjCs" Content-Disposition: inline In-Reply-To: <20140807152600.GW9918@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --KzTpGdFWKkiGyjCs Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Thu, Aug 07, 2014 at 05:26:00PM +0200, Peter Zijlstra wrote: > > > So do we really need the call_rcu_task() thing and why isn't something > > > like synchronize_tasks() good enough? > >=20 > > Sounds like a question for Steven. > >=20 > > > So the thing is, the one proposed user is very rare (*) and for that > > > you're adding overhead outside of that user (a separate kthread) and > > > your adding overhead when its not used. > >=20 > > If that really was the case, that would be bad. However, in the latest > > versions, that is no longer the case. > >=20 > > > * I'm assuming that, since tracing is 'rare' and this is some tracing > > > thing. > >=20 > > Another good point for Steven. >=20 > Yes.. and he's back now, so please :-) Right, Steve (and Paul) please explain _why_ this is an 'RCU' at all? _Why_ do we have call_rcu_task(), and why is it entwined in the 'normal' RCU stuff? We've got SRCU -- which btw started out simple, without call_srcu() -- and that lives entirely independent. And SRCU is far more an actual RCU than this thing is, its got read side primitives and everything. Also, I cannot think of any other use besides trampolines for this thing, but that might be my limited imagination. --KzTpGdFWKkiGyjCs Content-Type: application/pgp-signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (GNU/Linux) iQIcBAEBAgAGBQJT47cZAAoJEHZH4aRLwOS6xpcP/03fjku6RxtwJRSuP1ylt0xW aYHfRuA2Y9ZaHa4UBVK9cfe6Arh4ASw2RhUqYjfF6guPNlzjY+SDczxz7f9XYdWa KKb+jlBadndNMGI8Ynuhk9hsEj1MxGW/Yvpm0SLmRr6E9ZyGphzH35r7hJcXmotU gfsaJZDry7HfWgfqtv11HXnqIgf96s5T9jQ8o6z+dBvvZtKCTyCPedKqPiichaKU so0MVqM8XvQpXMd0z7NoJGPxaamzOabUq95rr5+8HXAfLWcqnc7bD4ygr3d0xMDW RzOQo+GrRpVBEisxrRh/y3twP00fu0EJlnervRhPcYcYLek2flXkgQZLb1skblUS JLtX0KOpNO/fs5iW2Y1G24ZuEDi/XMWoB+FXC8IT6Q/ID2cfCDl55BaI/oRG0VVO X2cDmHRJN2e0hfclDkFLuhN7ewkd/GqI+xAHpxxad0Jjx8l5clgzzURULgJiT8La Qede2KoaAH6jX+qHFcQHpDrxCyEINtbmiN5mHOtySlM0qyfunmfyIYycnexm1D+D n1hj4uWcPw9+YHi12rRYzZIgCzFqVp7UX92QCOLC4iFEwBoFqrwYZD/dA/WB+wMQ wlgRmXsCXjPcFDsvbuiDNbVEmrJo6eshlXjuGdHezH+HzskGlTVAPkRUXCHJ2sKr JvU7hCI5ZEL78+LbAigB =tsHC -----END PGP SIGNATURE----- --KzTpGdFWKkiGyjCs--