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Subject[tip: locking/core] timekeeping: Split jiffies seqlock
The following commit has been merged into the locking/core branch of tip:

Commit-ID: e5d4d1756b07d9490a0269a9e68c1e05ee1feb9b
Gitweb: https://git.kernel.org/tip/e5d4d1756b07d9490a0269a9e68c1e05ee1feb9b
Author: Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Sat, 21 Mar 2020 12:25:58 +01:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Sat, 21 Mar 2020 16:00:23 +01:00

timekeeping: Split jiffies seqlock

seqlock consists of a sequence counter and a spinlock_t which is used to
serialize the writers. spinlock_t is substituted by a "sleeping" spinlock
on PREEMPT_RT enabled kernels which breaks the usage in the timekeeping
code as the writers are executed in hard interrupt and therefore
non-preemptible context even on PREEMPT_RT.

The spinlock in seqlock cannot be unconditionally replaced by a
raw_spinlock_t as many seqlock users have nesting spinlock sections or
other code which is not suitable to run in truly atomic context on RT.

Instead of providing a raw_seqlock API for a single use case, open code the
seqlock for the jiffies use case and implement it with a raw_spinlock_t and
a sequence counter.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200321113242.120587764@linutronix.de
---
kernel/time/jiffies.c | 7 ++++---
kernel/time/tick-common.c | 10 ++++++----
kernel/time/tick-sched.c | 19 ++++++++++++-------
kernel/time/timekeeping.c | 6 ++++--
kernel/time/timekeeping.h | 3 ++-
5 files changed, 28 insertions(+), 17 deletions(-)

diff --git a/kernel/time/jiffies.c b/kernel/time/jiffies.c
index d23b434..eddcf49 100644
--- a/kernel/time/jiffies.c
+++ b/kernel/time/jiffies.c
@@ -58,7 +58,8 @@ static struct clocksource clocksource_jiffies = {
.max_cycles = 10,
};

-__cacheline_aligned_in_smp DEFINE_SEQLOCK(jiffies_lock);
+__cacheline_aligned_in_smp DEFINE_RAW_SPINLOCK(jiffies_lock);
+__cacheline_aligned_in_smp seqcount_t jiffies_seq;

#if (BITS_PER_LONG < 64)
u64 get_jiffies_64(void)
@@ -67,9 +68,9 @@ u64 get_jiffies_64(void)
u64 ret;

do {
- seq = read_seqbegin(&jiffies_lock);
+ seq = read_seqcount_begin(&jiffies_seq);
ret = jiffies_64;
- } while (read_seqretry(&jiffies_lock, seq));
+ } while (read_seqcount_retry(&jiffies_seq, seq));
return ret;
}
EXPORT_SYMBOL(get_jiffies_64);
diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
index 7e5d352..6c9c342 100644
--- a/kernel/time/tick-common.c
+++ b/kernel/time/tick-common.c
@@ -84,13 +84,15 @@ int tick_is_oneshot_available(void)
static void tick_periodic(int cpu)
{
if (tick_do_timer_cpu == cpu) {
- write_seqlock(&jiffies_lock);
+ raw_spin_lock(&jiffies_lock);
+ write_seqcount_begin(&jiffies_seq);

/* Keep track of the next tick event */
tick_next_period = ktime_add(tick_next_period, tick_period);

do_timer(1);
- write_sequnlock(&jiffies_lock);
+ write_seqcount_end(&jiffies_seq);
+ raw_spin_unlock(&jiffies_lock);
update_wall_time();
}

@@ -162,9 +164,9 @@ void tick_setup_periodic(struct clock_event_device *dev, int broadcast)
ktime_t next;

do {
- seq = read_seqbegin(&jiffies_lock);
+ seq = read_seqcount_begin(&jiffies_seq);
next = tick_next_period;
- } while (read_seqretry(&jiffies_lock, seq));
+ } while (read_seqcount_retry(&jiffies_seq, seq));

clockevents_switch_state(dev, CLOCK_EVT_STATE_ONESHOT);

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index a792d21..4be756b 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -65,7 +65,8 @@ static void tick_do_update_jiffies64(ktime_t now)
return;

/* Reevaluate with jiffies_lock held */
- write_seqlock(&jiffies_lock);
+ raw_spin_lock(&jiffies_lock);
+ write_seqcount_begin(&jiffies_seq);

delta = ktime_sub(now, last_jiffies_update);
if (delta >= tick_period) {
@@ -91,10 +92,12 @@ static void tick_do_update_jiffies64(ktime_t now)
/* Keep the tick_next_period variable up to date */
tick_next_period = ktime_add(last_jiffies_update, tick_period);
} else {
- write_sequnlock(&jiffies_lock);
+ write_seqcount_end(&jiffies_seq);
+ raw_spin_unlock(&jiffies_lock);
return;
}
- write_sequnlock(&jiffies_lock);
+ write_seqcount_end(&jiffies_seq);
+ raw_spin_unlock(&jiffies_lock);
update_wall_time();
}

@@ -105,12 +108,14 @@ static ktime_t tick_init_jiffy_update(void)
{
ktime_t period;

- write_seqlock(&jiffies_lock);
+ raw_spin_lock(&jiffies_lock);
+ write_seqcount_begin(&jiffies_seq);
/* Did we start the jiffies update yet ? */
if (last_jiffies_update == 0)
last_jiffies_update = tick_next_period;
period = last_jiffies_update;
- write_sequnlock(&jiffies_lock);
+ write_seqcount_end(&jiffies_seq);
+ raw_spin_unlock(&jiffies_lock);
return period;
}

@@ -676,10 +681,10 @@ static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)

/* Read jiffies and the time when jiffies were updated last */
do {
- seq = read_seqbegin(&jiffies_lock);
+ seq = read_seqcount_begin(&jiffies_seq);
basemono = last_jiffies_update;
basejiff = jiffies;
- } while (read_seqretry(&jiffies_lock, seq));
+ } while (read_seqcount_retry(&jiffies_seq, seq));
ts->last_jiffies = basejiff;
ts->timer_expires_base = basemono;

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index ca69290..856280d 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -2397,8 +2397,10 @@ EXPORT_SYMBOL(hardpps);
*/
void xtime_update(unsigned long ticks)
{
- write_seqlock(&jiffies_lock);
+ raw_spin_lock(&jiffies_lock);
+ write_seqcount_begin(&jiffies_seq);
do_timer(ticks);
- write_sequnlock(&jiffies_lock);
+ write_seqcount_end(&jiffies_seq);
+ raw_spin_unlock(&jiffies_lock);
update_wall_time();
}
diff --git a/kernel/time/timekeeping.h b/kernel/time/timekeeping.h
index 141ab3a..099737f 100644
--- a/kernel/time/timekeeping.h
+++ b/kernel/time/timekeeping.h
@@ -25,7 +25,8 @@ static inline void sched_clock_resume(void) { }
extern void do_timer(unsigned long ticks);
extern void update_wall_time(void);

-extern seqlock_t jiffies_lock;
+extern raw_spinlock_t jiffies_lock;
+extern seqcount_t jiffies_seq;

#define CS_NAME_LEN 32

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