linux-kernel - Re: [PATCH v6 08/11] drivers: perf: hisi: use poll method to avoid L3C counter overflow

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Date:   Fri, 24 Mar 2017 11:43:25 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Anurup M <anurupvasu@...il.com>
Cc:     will.deacon@....com, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, anurup.m@...wei.com,
        zhangshaokun@...ilicon.com, tanxiaojun@...wei.com,
        xuwei5@...ilicon.com, sanil.kumar@...ilicon.com,
        john.garry@...wei.com, gabriele.paoloni@...wei.com,
        shiju.jose@...wei.com, huangdaode@...ilicon.com,
        linuxarm@...wei.com, dikshit.n@...wei.com, shyju.pv@...wei.com
Subject: Re: [PATCH v6 08/11] drivers: perf: hisi: use poll method to avoid
 L3C counter overflow

On Fri, Mar 24, 2017 at 12:13:18PM +0530, Anurup M wrote:
> On Tuesday 21 March 2017 10:46 PM, Mark Rutland wrote:
> >On Fri, Mar 10, 2017 at 01:28:45AM -0500, Anurup M wrote:

> >>+/* The counter overflow IRQ is not supported for some PMUs
> >>+ * use hrtimer to periodically poll and avoid overflow
> >>+ */
> >>+static enum hrtimer_restart hisi_hrtimer_callback(struct hrtimer *hrtimer)
> >>+{
> >>+	struct hisi_pmu *hisi_pmu = container_of(hrtimer,
> >>+						 struct hisi_pmu, hrtimer);
> >>+	struct perf_event *event;
> >>+	struct hw_perf_event *hwc;
> >>+	unsigned long flags;
> >>+
> >>+	/* Return if no active events */
> >>+	if (!hisi_pmu->num_active)
> >>+		return HRTIMER_NORESTART;
> >>+
> >>+	local_irq_save(flags);
> >>+
> >>+	/* Update event count for each active event */
> >>+	list_for_each_entry(event, &hisi_pmu->active_list, active_entry) {
> >>+		hwc = &event->hw;
> >>+		/* Read hardware counter and update the Perf event counter */
> >>+		hisi_pmu->ops->event_update(event, hwc, GET_CNTR_IDX(hwc));
> >>+	}
> >How do we ensure that we don't take the interrupt in the middle of a
> >sequence of accesses to the HW?
> 
> The L3 cache and MN PMU does not use the overflow IRQ and it does
> not occur here
> as the interrupt Mask register is by default masked in hardware.

I was referring to the timer interrupt which backs the hrtimer.

i.e. how do we guarantee that hisi_hrtimer_callback() is not called
while we are in the middle of a RMW sequence? Are interrupts disabled
for all of those seqeunces?

Thanks,
Mark.