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Subject[PATCH v3 2/4] sched_clock: Optimize cache line usage
Date
Currently sched_clock(), a very hot code path, is not optimized to
minimise its cache profile. In particular:

1. cd is not ____cacheline_aligned,

2. struct clock_data does not distinguish between hotpath and
coldpath data, reducing locality of reference in the hotpath,

3. Some hotpath data is missing from struct clock_data and is marked
__read_mostly (which more or less guarantees it will not share a
cache line with cd).

This patch corrects these problems by extracting all hotpath data
into a separate structure and using ____cacheline_aligned to ensure
the hotpath uses a single (64 byte) cache line.

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 | 98 ++++++++++++++++++++++++++++++-----------------
1 file changed, 62 insertions(+), 36 deletions(-)

diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index 3d21a8719444..cb69a47dfee4 100644
--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c
@@ -18,28 +18,44 @@
#include <linux/seqlock.h>
#include <linux/bitops.h>

-struct clock_data {
- ktime_t wrap_kt;
+/**
+ * struct clock_read_data - data required to read from sched_clock
+ *
+ * Care must be taken when updating this structure; it is read by
+ * some very hot code paths. It occupies <=48 bytes and, when combined
+ * with the seqcount used to synchronize access, comfortably fits into
+ * a 64 byte cache line.
+ */
+struct clock_read_data {
u64 epoch_ns;
u64 epoch_cyc;
- seqcount_t seq;
- unsigned long rate;
+ u64 sched_clock_mask;
+ u64 (*read_sched_clock)(void);
u32 mult;
u32 shift;
bool suspended;
};

+/**
+ * struct clock_data - all data needed for sched_clock (including
+ * registration of a new clock source)
+ *
+ * The ordering of this structure has been chosen to optimize cache
+ * performance. In particular seq and read_data (combined) should fit
+ * into a single 64 byte cache line.
+ */
+struct clock_data {
+ seqcount_t seq;
+ struct clock_read_data read_data;
+ ktime_t wrap_kt;
+ unsigned long rate;
+};
+
static struct hrtimer sched_clock_timer;
static int irqtime = -1;

core_param(irqtime, irqtime, int, 0400);

-static struct clock_data cd = {
- .mult = NSEC_PER_SEC / HZ,
-};
-
-static u64 __read_mostly sched_clock_mask;
-
static u64 notrace jiffy_sched_clock_read(void)
{
/*
@@ -49,7 +65,10 @@ static u64 notrace jiffy_sched_clock_read(void)
return (u64)(jiffies - INITIAL_JIFFIES);
}

-static u64 __read_mostly (*read_sched_clock)(void) = jiffy_sched_clock_read;
+static struct clock_data cd ____cacheline_aligned = {
+ .read_data = { .mult = NSEC_PER_SEC / HZ,
+ .read_sched_clock = jiffy_sched_clock_read, },
+};

static inline u64 notrace cyc_to_ns(u64 cyc, u32 mult, u32 shift)
{
@@ -60,15 +79,16 @@ unsigned long long notrace sched_clock(void)
{
u64 cyc, res;
unsigned long seq;
+ struct clock_read_data *rd = &cd.read_data;

do {
seq = raw_read_seqcount_begin(&cd.seq);

- res = cd.epoch_ns;
- if (!cd.suspended) {
- cyc = read_sched_clock();
- cyc = (cyc - cd.epoch_cyc) & sched_clock_mask;
- res += cyc_to_ns(cyc, cd.mult, cd.shift);
+ 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);
}
} while (read_seqcount_retry(&cd.seq, seq));

@@ -83,16 +103,17 @@ static void notrace update_sched_clock(void)
unsigned long flags;
u64 cyc;
u64 ns;
+ struct clock_read_data *rd = &cd.read_data;

- cyc = read_sched_clock();
- ns = cd.epoch_ns +
- cyc_to_ns((cyc - cd.epoch_cyc) & sched_clock_mask,
- cd.mult, cd.shift);
+ cyc = rd->read_sched_clock();
+ ns = rd->epoch_ns +
+ cyc_to_ns((cyc - rd->epoch_cyc) & rd->sched_clock_mask,
+ rd->mult, rd->shift);

raw_local_irq_save(flags);
raw_write_seqcount_begin(&cd.seq);
- cd.epoch_ns = ns;
- cd.epoch_cyc = cyc;
+ rd->epoch_ns = ns;
+ rd->epoch_cyc = cyc;
raw_write_seqcount_end(&cd.seq);
raw_local_irq_restore(flags);
}
@@ -109,9 +130,9 @@ void __init sched_clock_register(u64 (*read)(void), int bits,
{
u64 res, wrap, new_mask, new_epoch, cyc, ns;
u32 new_mult, new_shift;
- ktime_t new_wrap_kt;
unsigned long r;
char r_unit;
+ struct clock_read_data *rd = &cd.read_data;

if (cd.rate > rate)
return;
@@ -130,17 +151,18 @@ 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 = read_sched_clock();
- ns = cd.epoch_ns + cyc_to_ns((cyc - cd.epoch_cyc) & sched_clock_mask,
- cd.mult, cd.shift);
+ cyc = rd->read_sched_clock();
+ ns = rd->epoch_ns +
+ cyc_to_ns((cyc - rd->epoch_cyc) & rd->sched_clock_mask,
+ rd->mult, rd->shift);

raw_write_seqcount_begin(&cd.seq);
- read_sched_clock = read;
- sched_clock_mask = new_mask;
- cd.mult = new_mult;
- cd.shift = new_shift;
- cd.epoch_cyc = new_epoch;
- cd.epoch_ns = ns;
+ rd->read_sched_clock = read;
+ rd->sched_clock_mask = new_mask;
+ rd->mult = new_mult;
+ rd->shift = new_shift;
+ rd->epoch_cyc = new_epoch;
+ rd->epoch_ns = ns;
raw_write_seqcount_end(&cd.seq);

r = rate;
@@ -172,7 +194,7 @@ void __init sched_clock_postinit(void)
* If no sched_clock function has been provided at that point,
* make it the final one one.
*/
- if (read_sched_clock == jiffy_sched_clock_read)
+ if (cd.read_data.read_sched_clock == jiffy_sched_clock_read)
sched_clock_register(jiffy_sched_clock_read, BITS_PER_LONG, HZ);

update_sched_clock();
@@ -188,17 +210,21 @@ void __init sched_clock_postinit(void)

static int sched_clock_suspend(void)
{
+ struct clock_read_data *rd = &cd.read_data;
+
update_sched_clock();
hrtimer_cancel(&sched_clock_timer);
- cd.suspended = true;
+ rd->suspended = true;
return 0;
}

static void sched_clock_resume(void)
{
- cd.epoch_cyc = read_sched_clock();
+ struct clock_read_data *rd = &cd.read_data;
+
+ rd->epoch_cyc = rd->read_sched_clock();
hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL);
- cd.suspended = false;
+ rd->suspended = false;
}

static struct syscore_ops sched_clock_ops = {
--
1.9.3


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