From 112f38a4a31668eb6a7d91d128296a26afdf7c4b Mon Sep 17 00:00:00 2001 From: Russell King Date: Wed, 15 Dec 2010 19:23:07 +0000 Subject: ARM: sched_clock: provide common infrastructure for sched_clock() Provide common sched_clock() infrastructure for platforms to use to create a 64-bit ns based sched_clock() implementation from a counter running at a non-variable clock rate. This implementation is based upon maintaining an epoch for the counter and an epoch for the nanosecond time. When we desire a sched_clock() time, we calculate the number of counter ticks since the last epoch update, convert this to nanoseconds and add to the epoch nanoseconds. We regularly refresh these epochs within the counter wrap interval. We perform a similar calculation as above, and store the new epochs. We read and write the epochs in such a way that sched_clock() can easily (and locklessly) detect when an update is in progress, and repeat the loading of these constants when they're known not to be stable. The one caveat is that sched_clock() is not called in the middle of an update. We achieve that by disabling IRQs. Finally, if the clock rate is known at compile time, the counter to ns conversion factors can be specified, allowing sched_clock() to be tightly optimized. We ensure that these factors are correct by providing an initialization function which performs a run-time check. Acked-by: Peter Zijlstra Tested-by: Santosh Shilimkar Tested-by: Will Deacon Tested-by: Mikael Pettersson Tested-by: Eric Miao Tested-by: Olof Johansson Tested-by: Jamie Iles Reviewed-by: Nicolas Pitre Signed-off-by: Russell King --- arch/arm/kernel/sched_clock.c | 69 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 69 insertions(+) create mode 100644 arch/arm/kernel/sched_clock.c (limited to 'arch/arm/kernel/sched_clock.c') diff --git a/arch/arm/kernel/sched_clock.c b/arch/arm/kernel/sched_clock.c new file mode 100644 index 000000000000..2cdcc9287c74 --- /dev/null +++ b/arch/arm/kernel/sched_clock.c @@ -0,0 +1,69 @@ +/* + * sched_clock.c: support for extending counters to full 64-bit ns counter + * + * This program is free software; you can redistribute it and/or modify + * it under the terms of the GNU General Public License version 2 as + * published by the Free Software Foundation. + */ +#include +#include +#include +#include +#include +#include + +#include + +static void sched_clock_poll(unsigned long wrap_ticks); +static DEFINE_TIMER(sched_clock_timer, sched_clock_poll, 0, 0); +static void (*sched_clock_update_fn)(void); + +static void sched_clock_poll(unsigned long wrap_ticks) +{ + mod_timer(&sched_clock_timer, round_jiffies(jiffies + wrap_ticks)); + sched_clock_update_fn(); +} + +void __init init_sched_clock(struct clock_data *cd, void (*update)(void), + unsigned int clock_bits, unsigned long rate) +{ + unsigned long r, w; + u64 res, wrap; + char r_unit; + + sched_clock_update_fn = update; + + /* calculate the mult/shift to convert counter ticks to ns. */ + clocks_calc_mult_shift(&cd->mult, &cd->shift, rate, NSEC_PER_SEC, 60); + + r = rate; + if (r >= 4000000) { + r /= 1000000; + r_unit = 'M'; + } else { + r /= 1000; + r_unit = 'k'; + } + + /* calculate how many ns until we wrap */ + wrap = cyc_to_ns((1ULL << clock_bits) - 1, cd->mult, cd->shift); + do_div(wrap, NSEC_PER_MSEC); + w = wrap; + + /* calculate the ns resolution of this counter */ + res = cyc_to_ns(1ULL, cd->mult, cd->shift); + pr_info("sched_clock: %u bits at %lu%cHz, resolution %lluns, wraps every %lums\n", + clock_bits, r, r_unit, res, w); + + /* + * Start the timer to keep sched_clock() properly updated and + * sets the initial epoch. + */ + sched_clock_timer.data = msecs_to_jiffies(w - (w / 10)); + sched_clock_poll(sched_clock_timer.data); + + /* + * Ensure that sched_clock() starts off at 0ns + */ + cd->epoch_ns = 0; +} -- cgit v1.2.2 From 211baa7016894c02fc18693e21ca479cd08ac0c0 Mon Sep 17 00:00:00 2001 From: Russell King Date: Tue, 11 Jan 2011 16:23:04 +0000 Subject: ARM: sched_clock: allow init_sched_clock() to be called early sched_clock is supposed to be initialized early - in the recently added init_early platform hook. However, in doing so we end up calling mod_timer() before the timer lists are initialized, resulting in an oops. Split the initialization in two - the part which the platform calls early which starts things off. The addition of the timer can be delayed until after we have more of the kernel initialized - when the normal time sources are initialized. Signed-off-by: Russell King --- arch/arm/kernel/sched_clock.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) (limited to 'arch/arm/kernel/sched_clock.c') diff --git a/arch/arm/kernel/sched_clock.c b/arch/arm/kernel/sched_clock.c index 2cdcc9287c74..784464a21c1c 100644 --- a/arch/arm/kernel/sched_clock.c +++ b/arch/arm/kernel/sched_clock.c @@ -60,10 +60,15 @@ void __init init_sched_clock(struct clock_data *cd, void (*update)(void), * sets the initial epoch. */ sched_clock_timer.data = msecs_to_jiffies(w - (w / 10)); - sched_clock_poll(sched_clock_timer.data); + update(); /* * Ensure that sched_clock() starts off at 0ns */ cd->epoch_ns = 0; } + +void __init sched_clock_postinit(void) +{ + sched_clock_poll(sched_clock_timer.data); +} -- cgit v1.2.2 From edc4d272551594729f63ca4cde1612608494091f Mon Sep 17 00:00:00 2001 From: Russell King Date: Tue, 11 Jan 2011 16:44:02 +0000 Subject: ARM: sched_clock: make minsec argument to clocks_calc_mult_shift() zero The purpose of the minsec argument is to prevent 64-bit math overflow when the number of cycles is multiplied up. However, the multipler is 32-bit, and in the sched_clock() case, the cycle counter is up to 32-bit as well. So the math can never overflow. With a value of 60, and clock rates greater than 71MHz, the calculated multiplier is unnecessarily reduced in value, which reduces accuracy by maybe 70ppt. It's almost not worth bothering with as the oscillator driving the counter won't be any more than 1ppm - unless you're using a rubidium lamp or caesium fountain frequency standard. So, set the minsec argument to zero. Signed-off-by: Russell King --- arch/arm/kernel/sched_clock.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'arch/arm/kernel/sched_clock.c') diff --git a/arch/arm/kernel/sched_clock.c b/arch/arm/kernel/sched_clock.c index 784464a21c1c..9a46370fe9da 100644 --- a/arch/arm/kernel/sched_clock.c +++ b/arch/arm/kernel/sched_clock.c @@ -34,7 +34,7 @@ void __init init_sched_clock(struct clock_data *cd, void (*update)(void), sched_clock_update_fn = update; /* calculate the mult/shift to convert counter ticks to ns. */ - clocks_calc_mult_shift(&cd->mult, &cd->shift, rate, NSEC_PER_SEC, 60); + clocks_calc_mult_shift(&cd->mult, &cd->shift, rate, NSEC_PER_SEC, 0); r = rate; if (r >= 4000000) { -- cgit v1.2.2