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sched_clock: Use seqcount instead of rolling our own

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We're going to increase the cyc value to 64 bits in the near
future. Doing that is going to break the custom seqcount
implementation in the sched_clock code because 64 bit numbers
aren't guaranteed to be atomic. Replace the cyc_copy with a
seqcount to avoid this problem.

Cc: Russell King <linux@arm.linux.org.uk>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
This commit is contained in:
Stephen Boyd 2013-07-18 16:21:15 -07:00 committed by John Stultz
parent 87d8b9eb7e
commit 85c3d2dd15
1 changed files with 8 additions and 19 deletions
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27
kernel/time/sched_clock.c
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@ -14,11 +14,12 @@
#include <linux/syscore_ops.h>
#include <linux/timer.h>
#include <linux/sched_clock.h>
#include <linux/seqlock.h>
struct clock_data {
u64 epoch_ns;
u32 epoch_cyc;
u32 epoch_cyc_copy;
seqcount_t seq;
unsigned long rate;
u32 mult;
u32 shift;
@ -54,23 +55,16 @@ static unsigned long long notrace sched_clock_32(void)
u64 epoch_ns;
u32 epoch_cyc;
u32 cyc;
unsigned long seq;
if (cd.suspended)
return cd.epoch_ns;
/*
* Load the epoch_cyc and epoch_ns atomically. We do this by
* ensuring that we always write epoch_cyc, epoch_ns and
* epoch_cyc_copy in strict order, and read them in strict order.
* If epoch_cyc and epoch_cyc_copy are not equal, then we're in
* the middle of an update, and we should repeat the load.
*/
do {
seq = read_seqcount_begin(&cd.seq);
epoch_cyc = cd.epoch_cyc;
smp_rmb();
epoch_ns = cd.epoch_ns;
smp_rmb();
} while (epoch_cyc != cd.epoch_cyc_copy);
} while (read_seqcount_retry(&cd.seq, seq));
cyc = read_sched_clock();
cyc = (cyc - epoch_cyc) & sched_clock_mask;
@ -90,16 +84,12 @@ static void notrace update_sched_clock(void)
ns = cd.epoch_ns +
cyc_to_ns((cyc - cd.epoch_cyc) & sched_clock_mask,
cd.mult, cd.shift);
/*
* Write epoch_cyc and epoch_ns in a way that the update is
* detectable in cyc_to_fixed_sched_clock().
*/
raw_local_irq_save(flags);
cd.epoch_cyc_copy = cyc;
smp_wmb();
write_seqcount_begin(&cd.seq);
cd.epoch_ns = ns;
smp_wmb();
cd.epoch_cyc = cyc;
write_seqcount_end(&cd.seq);
raw_local_irq_restore(flags);
}
@ -195,7 +185,6 @@ static int sched_clock_suspend(void)
static void sched_clock_resume(void)
{
cd.epoch_cyc = read_sched_clock();
cd.epoch_cyc_copy = cd.epoch_cyc;
cd.suspended = false;
}