linux-kernel - Re: [PATCH v2] Added perf functionality to mmdc driver

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Message-ID: <20160815194945.GB1120@svinekod>
Date:	Mon, 15 Aug 2016 20:49:45 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Zhengyu Shen <zhengyu.shen@....com>
Cc:	"shawnguo@...nel.org" <shawnguo@...nel.org>,
	"peterz@...radead.org" <peterz@...radead.org>,
	Frank Li <frank.li@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"acme@...nel.org" <acme@...nel.org>,
	"alexander.shishkin@...ux.intel.com" 
	<alexander.shishkin@...ux.intel.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"lznuaa@...il.com" <lznuaa@...il.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2] Added perf functionality to mmdc driver

On Tue, Aug 16, 2016 at 02:40:43PM +0000, Zhengyu Shen wrote:
> > > 	Added cpumask and migration handling support to driver
> > > 	Validated event during event_init
> > > 	Added code to properly stop counters
> > > 	Used perf_invalid_context instead of perf_sw_context
> > > 	Added hrtimer to poll for overflow
> > > 	Added better description
> > > 	Added support for multiple mmdcs
> > 
> > As I commented on v1 w.r.t. the above, I would appreciate being Cc'd on
> > future versions of this patch.
> 
> Sorry about that, I'll be sure to CC you in the future. 
> 
> > > +static void mmdc_event_start(struct perf_event *event, int flags) {
> > > +	struct mmdc_pmu *pmu_mmdc = to_mmdc_pmu(event->pmu);
> > > +	void __iomem *mmdc_base, *reg;
> > > +
> > > +	local64_set(&event->count, 0);
> > > +	mmdc_base = pmu_mmdc->mmdc_base;
> > > +	reg = mmdc_base + MMDC_MADPCR0;
> > > +	hrtimer_start(&pmu_mmdc->hrtimer, mmdc_timer_period(),
> > > +			HRTIMER_MODE_REL_PINNED);
> > 
> > Why is a hrtimer necessary? Is this just copy-pasted from CCN, or do you
> > have similar HW issues?
> > 
> > Is there no overflow interrupt?
> 
> When overflow occurs, a register bit is set to one. There is no overflow
> interrupt which is why the timer is needed. 

I see. Please have add comment in the driver explaining this, so that this is
obvious.

Does the counter itself wrap and continue counting, or does it saturate?

How have you tuned your polling period so as to avoid missing events in the
case of an overflow?

Thanks,
Mark.