![Joonsoo Kim](/assets/img/avatar_default.png)
If we want load epoch_cyc and epoch_ns atomically, we should update epoch_cyc_copy first of all. This notify reader that updating is in progress. If we update epoch_cyc first like as current implementation, there is subtle error case. Look at the below example. <Initial Condition> cyc = 9 ns = 900 cyc_copy = 9 == CASE 1 == <CPU A = reader> <CPU B = updater> write cyc = 10 read cyc = 10 read ns = 900 write ns = 1000 write cyc_copy = 10 read cyc_copy = 10 output = (10, 900) == CASE 2 == <CPU A = reader> <CPU B = updater> read cyc = 9 write cyc = 10 write ns = 1000 read ns = 1000 read cyc_copy = 9 write cyc_copy = 10 output = (9, 1000) If atomic read is ensured, output should be (9, 900) or (10, 1000). But, output in example case are not. So, change updating sequence in order to correct this problem. Cc: <stable@vger.kernel.org> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com> Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
207 lines
4.8 KiB
C
207 lines
4.8 KiB
C
/*
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* sched_clock.c: support for extending counters to full 64-bit ns counter
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License version 2 as
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* published by the Free Software Foundation.
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*/
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#include <linux/clocksource.h>
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#include <linux/init.h>
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#include <linux/jiffies.h>
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#include <linux/kernel.h>
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#include <linux/moduleparam.h>
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#include <linux/sched.h>
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#include <linux/syscore_ops.h>
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#include <linux/timer.h>
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#include <asm/sched_clock.h>
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struct clock_data {
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u64 epoch_ns;
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u32 epoch_cyc;
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u32 epoch_cyc_copy;
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u32 mult;
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u32 shift;
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bool suspended;
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bool needs_suspend;
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};
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static void sched_clock_poll(unsigned long wrap_ticks);
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static DEFINE_TIMER(sched_clock_timer, sched_clock_poll, 0, 0);
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static int irqtime = -1;
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core_param(irqtime, irqtime, int, 0400);
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static struct clock_data cd = {
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.mult = NSEC_PER_SEC / HZ,
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};
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static u32 __read_mostly sched_clock_mask = 0xffffffff;
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static u32 notrace jiffy_sched_clock_read(void)
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{
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return (u32)(jiffies - INITIAL_JIFFIES);
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}
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static u32 __read_mostly (*read_sched_clock)(void) = jiffy_sched_clock_read;
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static inline u64 cyc_to_ns(u64 cyc, u32 mult, u32 shift)
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{
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return (cyc * mult) >> shift;
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}
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static unsigned long long cyc_to_sched_clock(u32 cyc, u32 mask)
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{
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u64 epoch_ns;
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u32 epoch_cyc;
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if (cd.suspended)
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return cd.epoch_ns;
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/*
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* Load the epoch_cyc and epoch_ns atomically. We do this by
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* ensuring that we always write epoch_cyc, epoch_ns and
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* epoch_cyc_copy in strict order, and read them in strict order.
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* If epoch_cyc and epoch_cyc_copy are not equal, then we're in
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* the middle of an update, and we should repeat the load.
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*/
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do {
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epoch_cyc = cd.epoch_cyc;
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smp_rmb();
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epoch_ns = cd.epoch_ns;
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smp_rmb();
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} while (epoch_cyc != cd.epoch_cyc_copy);
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return epoch_ns + cyc_to_ns((cyc - epoch_cyc) & mask, cd.mult, cd.shift);
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}
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/*
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* Atomically update the sched_clock epoch.
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*/
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static void notrace update_sched_clock(void)
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{
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unsigned long flags;
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u32 cyc;
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u64 ns;
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cyc = read_sched_clock();
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ns = cd.epoch_ns +
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cyc_to_ns((cyc - cd.epoch_cyc) & sched_clock_mask,
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cd.mult, cd.shift);
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/*
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* Write epoch_cyc and epoch_ns in a way that the update is
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* detectable in cyc_to_fixed_sched_clock().
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*/
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raw_local_irq_save(flags);
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cd.epoch_cyc_copy = cyc;
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smp_wmb();
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cd.epoch_ns = ns;
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smp_wmb();
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cd.epoch_cyc = cyc;
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raw_local_irq_restore(flags);
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}
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static void sched_clock_poll(unsigned long wrap_ticks)
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{
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mod_timer(&sched_clock_timer, round_jiffies(jiffies + wrap_ticks));
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update_sched_clock();
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}
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void __init setup_sched_clock(u32 (*read)(void), int bits, unsigned long rate)
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{
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unsigned long r, w;
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u64 res, wrap;
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char r_unit;
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BUG_ON(bits > 32);
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WARN_ON(!irqs_disabled());
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WARN_ON(read_sched_clock != jiffy_sched_clock_read);
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read_sched_clock = read;
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sched_clock_mask = (1 << bits) - 1;
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/* calculate the mult/shift to convert counter ticks to ns. */
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clocks_calc_mult_shift(&cd.mult, &cd.shift, rate, NSEC_PER_SEC, 0);
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r = rate;
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if (r >= 4000000) {
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r /= 1000000;
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r_unit = 'M';
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} else if (r >= 1000) {
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r /= 1000;
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r_unit = 'k';
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} else
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r_unit = ' ';
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/* calculate how many ns until we wrap */
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wrap = cyc_to_ns((1ULL << bits) - 1, cd.mult, cd.shift);
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do_div(wrap, NSEC_PER_MSEC);
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w = wrap;
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/* calculate the ns resolution of this counter */
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res = cyc_to_ns(1ULL, cd.mult, cd.shift);
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pr_info("sched_clock: %u bits at %lu%cHz, resolution %lluns, wraps every %lums\n",
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bits, r, r_unit, res, w);
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/*
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* Start the timer to keep sched_clock() properly updated and
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* sets the initial epoch.
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*/
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sched_clock_timer.data = msecs_to_jiffies(w - (w / 10));
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update_sched_clock();
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/*
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* Ensure that sched_clock() starts off at 0ns
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*/
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cd.epoch_ns = 0;
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/* Enable IRQ time accounting if we have a fast enough sched_clock */
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if (irqtime > 0 || (irqtime == -1 && rate >= 1000000))
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enable_sched_clock_irqtime();
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pr_debug("Registered %pF as sched_clock source\n", read);
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}
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unsigned long long notrace sched_clock(void)
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{
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u32 cyc = read_sched_clock();
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return cyc_to_sched_clock(cyc, sched_clock_mask);
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}
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void __init sched_clock_postinit(void)
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{
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/*
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* If no sched_clock function has been provided at that point,
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* make it the final one one.
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*/
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if (read_sched_clock == jiffy_sched_clock_read)
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setup_sched_clock(jiffy_sched_clock_read, 32, HZ);
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sched_clock_poll(sched_clock_timer.data);
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}
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static int sched_clock_suspend(void)
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{
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sched_clock_poll(sched_clock_timer.data);
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cd.suspended = true;
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return 0;
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}
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static void sched_clock_resume(void)
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{
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cd.epoch_cyc = read_sched_clock();
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cd.epoch_cyc_copy = cd.epoch_cyc;
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cd.suspended = false;
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}
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static struct syscore_ops sched_clock_ops = {
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.suspend = sched_clock_suspend,
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.resume = sched_clock_resume,
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};
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static int __init sched_clock_syscore_init(void)
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{
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register_syscore_ops(&sched_clock_ops);
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return 0;
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}
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device_initcall(sched_clock_syscore_init);
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