linux-kernel - Re: [PATCH v3 10/11] perf/x86/intel: Perform rotation on Intel CQM RMIDs

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Message-ID: <20141110205846.GE10501@worktop.programming.kicks-ass.net>
Date:	Mon, 10 Nov 2014 21:58:46 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Matt Fleming <matt@...sole-pimps.org>
Cc:	Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Kanaka Juvva <kanaka.d.juvva@...el.com>,
	Matt Fleming <matt.fleming@...el.com>
Subject: Re: [PATCH v3 10/11] perf/x86/intel: Perform rotation on Intel CQM
 RMIDs

On Mon, Nov 10, 2014 at 08:43:53PM +0000, Matt Fleming wrote:
> On Fri, 07 Nov, at 01:06:12PM, Peter Zijlstra wrote:
> > On Thu, Nov 06, 2014 at 12:23:21PM +0000, Matt Fleming wrote:
> > > +/*
> > > + * Exchange the RMID of a group of events.
> > > + */
> > > +static unsigned int
> > > +intel_cqm_xchg_rmid(struct perf_event *group, unsigned int rmid)
> > > +{
> > > +	struct perf_event *event;
> > > +	unsigned int old_rmid = group->hw.cqm_rmid;
> > > +	struct list_head *head = &group->hw.cqm_group_entry;
> > > +
> > > +	lockdep_assert_held(&cache_mutex);
> > > +
> > > +	/*
> > > +	 * If our RMID is being deallocated, perform a read now.
> > > +	 */
> > > +	if (__rmid_valid(old_rmid) && !__rmid_valid(rmid)) {
> > > +		struct intel_cqm_count_info info;
> > > +
> > > +		local64_set(&group->count, 0);
> > > +		info.event = group;
> > > +
> > > +		preempt_disable();
> > > +		smp_call_function_many(&cqm_cpumask, __intel_cqm_event_count,
> > > +				       &info, 1);
> > > +		preempt_enable();
> > > +	}
> > 
> > This suffers the same issue as before, why not call that one function
> > and not reimplement it?
> > 
> > Also, I don't think we'd ever swap an rmid for another valid one, right?
> > So we could do this read/update unconditionally.
> 
> No, we never swap a valid RMID for another valid one, but we do make a
> invalid -> valid transition, so doing the read wouldn't make sense
> in that situation.

Ah indeed.
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