Date: Wed, 29 Apr 2009 13:19:34 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Robert Richter <robert.richter@amd.com>, Paul Mackerras <paulus@samba.org>,
       LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 04/29] x86/perfcounters: rework
	pmc_amd_save_disable_all() and pmc_amd_restore_all()
Message-ID: <20090429111934.GA8298@elte.hu>
References: <1241002046-8832-1-git-send-email-robert.richter@amd.com> <1241002046-8832-5-git-send-email-robert.richter@amd.com> <1241003260.8021.236.camel@laptop> <20090429111159.GJ2373@elte.hu> <1241003785.8021.248.camel@laptop>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1241003785.8021.248.camel@laptop>
User-Agent: Mutt/1.5.18 (2008-05-17)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1700
Lines: 46


* Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

> On Wed, 2009-04-29 at 13:11 +0200, Ingo Molnar wrote:
> > * Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> > 
> > > On Wed, 2009-04-29 at 12:47 +0200, Robert Richter wrote:
> > > > MSR reads and writes are expensive. This patch adds checks to avoid
> > > > its usage where possible.
> > > 
> > > save_disable_all()
> > > enable(1)
> > > restore_all()
> > > 
> > > would not correctly enable 1 with the below modification as we do 
> > > not write the configuration into the msr, on which restore relies, 
> > > as it only toggles the _ENABLE bit.
> > > 
> > > That said, I'm not sure if that's really an issue, but its why the 
> > > does does as it does.
> > > 
> > > A better abstraction could perhaps avoid this issue all-together.
> > 
> > Could we remove the disable-all facility altogether and make the 
> > core code NMI-safe? The current approach wont scale on CPUs that 
> > dont have global-disable features.
> > 
> > disable-all was arguably a hack i introduced and which spread too 
> > far. Can you see a conceptual need for it?
> 
> power suffers the same issue and simply iterates the things like 
> amd does now.
> 
> The thing is, with a global disable you get slightly better 
> coupling, so in that respect it might be nice to have.

ok. With system-wide profiling there's no global disable/enable in 
the fastpath. Do we have any of them in the per task counter 
fastpath?

	Ingo
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/