MIPS: cevt-r4k: Dynamically calculate min_delta_ns (1fa40555) · Commits · 戴 / test

arch/mips/kernel/cevt-r4k.c

+80 −2

Original line number	Diff line number	Diff line
		@@ -28,6 +28,83 @@ static int mips_next_event(unsigned long delta,
		return res;
		}

		/**
		* calculate_min_delta() - Calculate a good minimum delta for mips_next_event().
		*
		* Running under virtualisation can introduce overhead into mips_next_event() in
		* the form of hypervisor emulation of CP0_Count/CP0_Compare registers,
		* potentially with an unnatural frequency, which makes a fixed min_delta_ns
		* value inappropriate as it may be too small.
		*
		* It can also introduce occasional latency from the guest being descheduled.
		*
		* This function calculates a good minimum delta based roughly on the 75th
		* percentile of the time taken to do the mips_next_event() sequence, in order
		* to handle potentially higher overhead while also eliminating outliers due to
		* unpredictable hypervisor latency (which can be handled by retries).
		*
		* Return: An appropriate minimum delta for the clock event device.
		*/
		static unsigned int calculate_min_delta(void)
		{
		unsigned int cnt, i, j, k, l;
		unsigned int buf1[4], buf2[3];
		unsigned int min_delta;

		/*
		* Calculate the median of 5 75th percentiles of 5 samples of how long
		* it takes to set CP0_Compare = CP0_Count + delta.
		*/
		for (i = 0; i < 5; ++i) {
		for (j = 0; j < 5; ++j) {
		/*
		* This is like the code in mips_next_event(), and
		* directly measures the borderline "safe" delta.
		*/
		cnt = read_c0_count();
		write_c0_compare(cnt);
		cnt = read_c0_count() - cnt;

		/* Sorted insert into buf1 */
		for (k = 0; k < j; ++k) {
		if (cnt < buf1[k]) {
		l = min_t(unsigned int,
		j, ARRAY_SIZE(buf1) - 1);
		for (; l > k; --l)
		buf1[l] = buf1[l - 1];
		break;
		}
		}
		if (k < ARRAY_SIZE(buf1))
		buf1[k] = cnt;
		}

		/* Sorted insert of 75th percentile into buf2 */
		for (k = 0; k < i; ++k) {
		if (buf1[ARRAY_SIZE(buf1) - 1] < buf2[k]) {
		l = min_t(unsigned int,
		i, ARRAY_SIZE(buf2) - 1);
		for (; l > k; --l)
		buf2[l] = buf2[l - 1];
		break;
		}
		}
		if (k < ARRAY_SIZE(buf2))
		buf2[k] = buf1[ARRAY_SIZE(buf1) - 1];
		}

		/* Use 2 * median of 75th percentiles */
		min_delta = buf2[ARRAY_SIZE(buf2) - 1] * 2;

		/* Don't go too low */
		if (min_delta < 0x300)
		min_delta = 0x300;

		pr_debug("%s: median 75th percentile=%#x, min_delta=%#x\n",
		__func__, buf2[ARRAY_SIZE(buf2) - 1], min_delta);
		return min_delta;
		}

		DEFINE_PER_CPU(struct clock_event_device, mips_clockevent_device);
		int cp0_timer_irq_installed;

		@@ -177,7 +254,7 @@ int r4k_clockevent_init(void)
		{
		unsigned int cpu = smp_processor_id();
		struct clock_event_device *cd;
		unsigned int irq;
		unsigned int irq, min_delta;

		if (!cpu_has_counter \|\| !mips_hpt_frequency)
		return -ENXIO;
		@@ -203,7 +280,8 @@ int r4k_clockevent_init(void)

		/* Calculate the min / max delta */
		cd->max_delta_ns = clockevent_delta2ns(0x7fffffff, cd);
		cd->min_delta_ns = clockevent_delta2ns(0x300, cd);
		min_delta = calculate_min_delta();
		cd->min_delta_ns = clockevent_delta2ns(min_delta, cd);

		cd->rating = 300;
		cd->irq = irq;

Admin message