Currently cd.read_data.suspended is read by the hotpath function sched_clock(). This variable need not be accessed on the hotpath. In fact, once it is removed, we can remove the conditional branches from sched_clock() and install a dummy read_sched_clock function to suspend the clock.
The new master copy of the function pointer (actual_read_sched_clock) is introduced and is used this for all reads of the clock hardware except those within sched_clock itself.
Suggested-by: Thomas Gleixner tglx@linutronix.de Signed-off-by: Daniel Thompson daniel.thompson@linaro.org Cc: Russell King linux@arm.linux.org.uk Cc: Will Deacon will.deacon@arm.com Cc: Catalin Marinas catalin.marinas@arm.com --- kernel/time/sched_clock.c | 40 +++++++++++++++++++++++++--------------- 1 file changed, 25 insertions(+), 15 deletions(-)
diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c index 695b2ac2e8b4..5d6407951fb8 100644 --- a/kernel/time/sched_clock.c +++ b/kernel/time/sched_clock.c @@ -28,10 +28,9 @@ * @read_sched_clock: Current clock source (or dummy source when suspended) * @mult: Multipler for scaled math conversion * @shift: Shift value for scaled math conversion - * @suspended: Flag to indicate if the clock is suspended (stopped) * * Care must be taken when updating this structure; it is read by - * some very hot code paths. It occupies <=48 bytes and, when combined + * some very hot code paths. It occupies <=40 bytes and, when combined * with the seqcount used to synchronize access, comfortably fits into * a 64 byte cache line. */ @@ -42,7 +41,6 @@ struct clock_read_data { u64 (*read_sched_clock)(void); u32 mult; u32 shift; - bool suspended; };
/** @@ -64,6 +62,7 @@ struct clock_data { struct clock_read_data read_data; ktime_t wrap_kt; unsigned long rate; + u64 (*actual_read_sched_clock)(void); };
static struct hrtimer sched_clock_timer; @@ -83,6 +82,8 @@ static u64 notrace jiffy_sched_clock_read(void) static struct clock_data cd ____cacheline_aligned = { .read_data = { .mult = NSEC_PER_SEC / HZ, .read_sched_clock = jiffy_sched_clock_read, }, + .actual_read_sched_clock = jiffy_sched_clock_read, + };
static inline u64 notrace cyc_to_ns(u64 cyc, u32 mult, u32 shift) @@ -99,12 +100,9 @@ unsigned long long notrace sched_clock(void) do { seq = raw_read_seqcount_begin(&cd.seq);
- res = rd->epoch_ns; - if (!rd->suspended) { - cyc = rd->read_sched_clock(); - cyc = (cyc - rd->epoch_cyc) & rd->sched_clock_mask; - res += cyc_to_ns(cyc, rd->mult, rd->shift); - } + cyc = (rd->read_sched_clock() - rd->epoch_cyc) & + rd->sched_clock_mask; + res = rd->epoch_ns + cyc_to_ns(cyc, rd->mult, rd->shift); } while (read_seqcount_retry(&cd.seq, seq));
return res; @@ -120,7 +118,7 @@ static void notrace update_sched_clock(void) u64 ns; struct clock_read_data *rd = &cd.read_data;
- cyc = rd->read_sched_clock(); + cyc = cd.actual_read_sched_clock(); ns = rd->epoch_ns + cyc_to_ns((cyc - rd->epoch_cyc) & rd->sched_clock_mask, rd->mult, rd->shift); @@ -166,10 +164,11 @@ void __init sched_clock_register(u64 (*read)(void), int bits,
/* update epoch for new counter and update epoch_ns from old counter*/ new_epoch = read(); - cyc = rd->read_sched_clock(); + cyc = cd.actual_read_sched_clock(); ns = rd->epoch_ns + cyc_to_ns((cyc - rd->epoch_cyc) & rd->sched_clock_mask, rd->mult, rd->shift); + cd.actual_read_sched_clock = read;
raw_write_seqcount_begin(&cd.seq); rd->read_sched_clock = read; @@ -209,7 +208,7 @@ void __init sched_clock_postinit(void) * If no sched_clock function has been provided at that point, * make it the final one one. */ - if (cd.read_data.read_sched_clock == jiffy_sched_clock_read) + if (cd.actual_read_sched_clock == jiffy_sched_clock_read) sched_clock_register(jiffy_sched_clock_read, BITS_PER_LONG, HZ);
update_sched_clock(); @@ -223,13 +222,24 @@ void __init sched_clock_postinit(void) hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL); }
+/* + * Clock read function for use when the clock is suspended. + * + * This function makes it appear to sched_clock() as if the clock + * stopped counting at its last update. + */ +static u64 notrace suspended_sched_clock_read(void) +{ + return cd.read_data.epoch_cyc; +} + static int sched_clock_suspend(void) { struct clock_read_data *rd = &cd.read_data;
update_sched_clock(); hrtimer_cancel(&sched_clock_timer); - rd->suspended = true; + rd->read_sched_clock = suspended_sched_clock_read; return 0; }
@@ -237,9 +247,9 @@ static void sched_clock_resume(void) { struct clock_read_data *rd = &cd.read_data;
- rd->epoch_cyc = rd->read_sched_clock(); + rd->epoch_cyc = cd.actual_read_sched_clock(); hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL); - rd->suspended = false; + rd->read_sched_clock = cd.actual_read_sched_clock; }
static struct syscore_ops sched_clock_ops = {