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Date: Wed, 7 Jan 2009 04:00:30 +0100
From: Nick Piggin <npiggin@suse.de>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Robin Holt <holt@sgi.com>, "Luck, Tony" <tony.luck@intel.com>,
       Dimitri Sivanich <sivanich@sgi.com>,
       "linux-ia64@vger.kernel.org" <linux-ia64@vger.kernel.org>,
       Greg KH <greg@kroah.com>,
       "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
       Gregory Haskins <ghaskins@novell.com>, Tony Luck <tony.luck@gmail.com>
Subject: Re: [PATCH] configure HAVE_UNSTABLE_SCHED_CLOCK for SGI_SN systems
Message-ID: <20090107030030.GH3390@wotan.suse.de>
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>
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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 ;)