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

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Message-ID: <20170321171605.GB29116@leverpostej>
Date:   Tue, 21 Mar 2017 17:16:05 +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 10, 2017 at 01:28:45AM -0500, Anurup M wrote:
> Add hrtimer support which use poll method to avoid counter overflow
> when overflow IRQ is not supported in hardware.
> The L3 cache PMU use N-N SPI interrupt which has no support in kernel
> mainline. So use hrtimer to poll and update event counter to avoid
> overflow condition for L3 cache PMU.
> An interval of 10 seconds is used for the hrtimer.

This should be folded with the previous patch, given that it is
necessary for counters to work correctly.

[...]

> +/*
> + * Default timer frequency to poll and avoid counter overflow.
> + * CPU speed = 2.4Ghz, Therefore Access time = 0.4ns
> + * L1 cache - 2 way set associative
> + * L2  - 16 way set associative
> + * L3  - 16 way set associative. L3 cache has 4 banks.
> + *
> + * Overflow time = 2^31 * (access time L1 + access time L2 + access time L3)
> + * = 2^31 * ((2 * 0.4ns) + (16 * 0.4ns) + (4 * 16 * 0.4ns)) = 70 seconds
> + *
> + * L3 cache is also used by devices like PCIe, SAS etc. at
> + * the same time. So the overflow time could be even smaller.
> + * So on a safe side we use a timer interval of 10sec
> + */
> +#define L3C_HRTIMER_INTERVAL (10LL * MSEC_PER_SEC)

This sounds fine.

[...]

> +/*
> + * sysfs hrtimer_interval attributes
> + */
> +ssize_t hisi_hrtimer_interval_sysfs_show(struct device *dev,
> +					 struct device_attribute *attr,
> +					 char *buf)
> +{
> +	struct pmu *pmu = dev_get_drvdata(dev);
> +	struct hisi_pmu *hisi_pmu = to_hisi_pmu(pmu);
> +
> +	if (hisi_pmu->hrt_duration)
> +		return sprintf(buf, "%llu\n", hisi_pmu->hrt_duration);
> +	return 0;
> +}

I don't think that we need a sysfs property for this.

[...]

> +/* 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?

Thanks,
Mark.