[PATCH v5 09/18] timer: Keep the pinned timers separate from the others

Anna-Maria Behnsen
2023-03-01T14:19:00Z

Separate the storage space for pinned timers. Deferrable timers (doesn't
matter if pinned or non pinned) are still enqueued into their own base.

This is preparatory work for changing the NOHZ timer placement from a push
at enqueue time to a pull at expiry time model.

When a timer is added via add_timer_on(), TIMER_PINNED flag is required to
ensure it expires on the specified CPU. Otherwise it will be enqueued in
the global timer base which could be expired by a remote CPU. WARN_ONCE()
is added to prevent misuse.

Beside of that no functional change because all callers of add_timer_on()
already use TIMER_PINNED flag.

Originally-by: Richard Cochran (linutronix GmbH) <richardcochran@gmail.com>
Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
---
v5:
- Add WARN_ONCE() in add_timer_on()
- Decrease patch size by splitting into three patches (this patch and the
two before)
---
kernel/time/timer.c | 91 +++++++++++++++++++++++++++++++++------------
1 file changed, 68 insertions(+), 23 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 1629ccf24dd0..7656eab1bf20 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -187,12 +187,18 @@ EXPORT_SYMBOL(jiffies_64);
#define WHEEL_SIZE (LVL_SIZE * LVL_DEPTH)

#ifdef CONFIG_NO_HZ_COMMON
-# define NR_BASES 2
-# define BASE_STD 0
-# define BASE_DEF 1
+/*
+ * If multiple bases need to be locked, use the base ordering for lock
+ * nesting, i.e. lowest number first.
+ */
+# define NR_BASES 3
+# define BASE_LOCAL 0
+# define BASE_GLOBAL 1
+# define BASE_DEF 2
#else
# define NR_BASES 1
-# define BASE_STD 0
+# define BASE_LOCAL 0
+# define BASE_GLOBAL 0
# define BASE_DEF 0
#endif

@@ -902,7 +908,10 @@ static int detach_if_pending(struct timer_list *timer, struct timer_base *base,

static inline struct timer_base *get_timer_cpu_base(u32 tflags, u32 cpu)
{
- struct timer_base *base = per_cpu_ptr(&timer_bases[BASE_STD], cpu);
+ int index = tflags & TIMER_PINNED ? BASE_LOCAL : BASE_GLOBAL;
+ struct timer_base *base;
+
+ base = per_cpu_ptr(&timer_bases[index], cpu);

/*
* If the timer is deferrable and NO_HZ_COMMON is set then we need
@@ -915,7 +924,10 @@ static inline struct timer_base *get_timer_cpu_base(u32 tflags, u32 cpu)

static inline struct timer_base *get_timer_this_cpu_base(u32 tflags)
{
- struct timer_base *base = this_cpu_ptr(&timer_bases[BASE_STD]);
+ int index = tflags & TIMER_PINNED ? BASE_LOCAL : BASE_GLOBAL;
+ struct timer_base *base;
+
+ base = this_cpu_ptr(&timer_bases[index]);

/*
* If the timer is deferrable and NO_HZ_COMMON is set then we need
@@ -1264,6 +1276,12 @@ void add_timer_on(struct timer_list *timer, int cpu)
if (WARN_ON_ONCE(timer_pending(timer)))
return;

+ WARN_ONCE(!(timer->flags & TIMER_PINNED), "TIMER_PINNED flag for "
+ "add_timer_on() is missing: timer=%p function=%ps",
+ timer, timer->function);
+ /* Make sure timer flags have TIMER_PINNED flag set */
+ timer->flags |= TIMER_PINNED;
+
new_base = get_timer_cpu_base(timer->flags, cpu);

/*
@@ -1950,9 +1968,10 @@ static void forward_base_clk(struct timer_base *base, unsigned long nextevt,
*/
u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
{
- struct timer_base *base = this_cpu_ptr(&timer_bases[BASE_STD]);
+ unsigned long nextevt, nextevt_local, nextevt_global;
+ struct timer_base *base_local, *base_global;
+ bool local_first, is_idle;
u64 expires = KTIME_MAX;
- unsigned long nextevt;

/*
* Pretend that there is no timer pending if the cpu is offline.
@@ -1961,32 +1980,57 @@ u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
if (cpu_is_offline(smp_processor_id()))
return expires;

- raw_spin_lock(&base->lock);
+ base_local = this_cpu_ptr(&timer_bases[BASE_LOCAL]);
+ base_global = this_cpu_ptr(&timer_bases[BASE_GLOBAL]);

- nextevt = next_timer_interrupt(base);
+ raw_spin_lock(&base_local->lock);
+ raw_spin_lock_nested(&base_global->lock, SINGLE_DEPTH_NESTING);
+
+ nextevt_local = next_timer_interrupt(base_local);
+ nextevt_global = next_timer_interrupt(base_global);

/*
* We have a fresh next event. Check whether we can forward the
* base.
*/
- forward_base_clk(base, nextevt, basej);
+ forward_base_clk(base_local, nextevt_local, basej);
+ forward_base_clk(base_global, nextevt_global, basej);

/*
- * Base is idle if the next event is more than a tick away. Also
+ * Check whether the local event is expiring before or at the same
+ * time as the global event.
+ *
+ * Note, that nextevt_global and nextevt_local might be based on
+ * different base->clk values. So it's not guaranteed that
+ * comparing with empty bases results in a correct local_first.
+ */
+ if (base_local->timers_pending && base_global->timers_pending)
+ local_first = time_before_eq(nextevt_local, nextevt_global);
+ else
+ local_first = base_local->timers_pending;
+
+ nextevt = local_first ? nextevt_local : nextevt_global;
+
+ /*
+ * Bases are idle if the next event is more than a tick away. Also
* the tick is stopped so any added timer must forward the base clk
* itself to keep granularity small. This idle logic is only
- * maintained for the BASE_STD base, deferrable timers may still
- * see large granularity skew (by design).
+ * maintained for the BASE_LOCAL and BASE_GLOBAL base, deferrable
+ * timers may still see large granularity skew (by design).
*/
- base->is_idle = time_after(nextevt, basej + 1);
+ is_idle = time_after(nextevt, basej + 1);
+
+ /* We need to mark both bases in sync */
+ base_local->is_idle = base_global->is_idle = is_idle;

- if (base->timers_pending) {
+ if (base_local->timers_pending || base_global->timers_pending) {
/* If we missed a tick already, force 0 delta */
if (time_before(nextevt, basej))
nextevt = basej;
expires = basem + (u64)(nextevt - basej) * TICK_NSEC;
}
- raw_spin_unlock(&base->lock);
+ raw_spin_unlock(&base_global->lock);
+ raw_spin_unlock(&base_local->lock);

return cmp_next_hrtimer_event(basem, expires);
}
@@ -1998,15 +2042,14 @@ u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
*/
void timer_clear_idle(void)
{
- struct timer_base *base = this_cpu_ptr(&timer_bases[BASE_STD]);
-
/*
* We do this unlocked. The worst outcome is a remote enqueue sending
* a pointless IPI, but taking the lock would just make the window for
* sending the IPI a few instructions smaller for the cost of taking
* the lock in the exit from idle path.
*/
- base->is_idle = false;
+ __this_cpu_write(timer_bases[BASE_LOCAL].is_idle, false);
+ __this_cpu_write(timer_bases[BASE_GLOBAL].is_idle, false);
}
#endif

@@ -2052,11 +2095,13 @@ static inline void __run_timers(struct timer_base *base)
*/
static __latent_entropy void run_timer_softirq(struct softirq_action *h)
{
- struct timer_base *base = this_cpu_ptr(&timer_bases[BASE_STD]);
+ struct timer_base *base = this_cpu_ptr(&timer_bases[BASE_LOCAL]);

__run_timers(base);
- if (IS_ENABLED(CONFIG_NO_HZ_COMMON))
+ if (IS_ENABLED(CONFIG_NO_HZ_COMMON)) {
+ __run_timers(this_cpu_ptr(&timer_bases[BASE_GLOBAL]));
__run_timers(this_cpu_ptr(&timer_bases[BASE_DEF]));
+ }
}

/*
@@ -2064,7 +2109,7 @@ static __latent_entropy void run_timer_softirq(struct softirq_action *h)
*/
static void run_local_timers(void)
{
- struct timer_base *base = this_cpu_ptr(&timer_bases[BASE_STD]);
+ struct timer_base *base = this_cpu_ptr(&timer_bases[BASE_LOCAL]);

hrtimer_run_queues();

--
2.30.2