linux-kernel - Re: [patch 3/6] x86, perf, cqm: Remove pointless spinlock from state cache

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Date:	Tue, 19 May 2015 12:51:21 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Matt Fleming <matt@...eblueprint.co.uk>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
	x86@...nel.org, Matt Fleming <matt.fleming@...el.com>,
	Will Auld <will.auld@...el.com>,
	Kanaka Juvva <kanaka.d.juvva@...el.com>
Subject: Re: [patch 3/6] x86, perf, cqm: Remove pointless spinlock from state
 cache

On Tue, May 19, 2015 at 10:13:18AM +0100, Matt Fleming wrote:
> On Tue, 19 May, at 12:00:53AM, Thomas Gleixner wrote:
> > struct intel_cqm_state is a strict per cpu cache of the rmid and the
> > usage counter. It can never be modified from a remote cpu.
> > 
> > The 3 functions which modify the content: start, stop and del (del
> > maps to stop) are called from the perf core with interrupts disabled
> > which is enough protection for the per cpu state values.
> > 
> > Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> > ---
> >  arch/x86/kernel/cpu/perf_event_intel_cqm.c |   17 ++++++-----------
> >  1 file changed, 6 insertions(+), 11 deletions(-)
> 
> The state locking code was taken from Peter's original patch last year,
> so it would be good for him to chime in that this is safe. It's probably
> just that it was necessary in Peter's patches but after I refactored
> bits I forgot to rip it out.
> 
> But yeah, from reading the code again the lock does look entirely
> superfluous.

I think that all stems from a point in time when it wasn't at all clear
to me what the hardware looked like, but what do I know, I can't even
remember last week.

All the patches looked good to me, so I already queued them.

I'll add your Ack on them.
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