diff options
| -rw-r--r-- | arch/mips/kernel/cevt-r4k.c | 82 |
1 files changed, 80 insertions, 2 deletions
diff --git a/arch/mips/kernel/cevt-r4k.c b/arch/mips/kernel/cevt-r4k.c index 8dfe6a6..e4c21bb 100644 --- a/arch/mips/kernel/cevt-r4k.c +++ b/arch/mips/kernel/cevt-r4k.c @@ -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; |
