From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nick Piggin Date: Wed, 07 Jan 2009 07:40:13 +0000 Subject: Re: [PATCH] configure HAVE_UNSTABLE_SCHED_CLOCK for SGI_SN systems Message-Id: <20090107074013.GF21629@wotan.suse.de> List-Id: References: <20090106162741.GA7991@sgi.com> <57C9024A16AD2D4C97DC78E552063EA35CB95575@orsmsx505.amr.corp.intel.com> <20090106201950.GA3850@sgi.com> <57C9024A16AD2D4C97DC78E552063EA35CB955B4@orsmsx505.amr.corp.intel.com> <1231275441.11687.110.camel@twins> <20090106225054.GB3850@sgi.com> <1231283763.11687.135.camel@twins> <20090107030030.GH3390@wotan.suse.de> <1231313289.11687.172.camel@twins> In-Reply-To: <1231313289.11687.172.camel@twins> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Peter Zijlstra Cc: Robin Holt , "Luck, Tony" , Dimitri Sivanich , "linux-ia64@vger.kernel.org" , Greg KH , "linux-kernel@vger.kernel.org" , Gregory Haskins , Tony Luck On Wed, Jan 07, 2009 at 08:28:09AM +0100, Peter Zijlstra wrote: > On Wed, 2009-01-07 at 04:00 +0100, Nick Piggin wrote: > > On Wed, Jan 07, 2009 at 12:16:03AM +0100, Peter Zijlstra wrote: > > > > > > > > But doesn't scheduler tick advance the rq->clock? Why do the others > > > > need to fiddle with a remote runqueue's clock? When that cpu starts > > > > taking ticks again, it will update it's rq->clock field and start the > > > > processes. I guess I am a lot underinformed about the new scheduler > > > > design. > > > > > > We try to do better than tick based time accounting these days. > > > > But if you contain the drift to within one tick, it shouldn't be much > > problem to just truncate negative deltas I would have thought? The > > time between events on different CPUs is pretty fuzzy at the ns level > > anyway, I think ;) > > That's basically what the HAVE_UNSTABLE_SCHED_CLOCK code does. It takes > a tick timestamp and tries to improve on that by using strict per cpu > sched_clock() deltas. > > What we do to obtain remote time, is basically calculate local time and > pull remote time fwd if that was behind. > > While doing that, it filters out any backward motion and large fwd leaps > so as to stay no worse than a jiffie clock. OK, that's good. I guess the optimisations to remove that code should have been called HAVE_STABLE_SCHED_CLOCK and have archs turn it on on a case by case basis.