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Date: Wed, 21 Feb 2024 10:05:48 +0100
From: Anna-Maria Behnsen <anna-maria@...utronix.de>
To: linux-kernel@...r.kernel.org
Cc: Peter Zijlstra <peterz@...radead.org>,
John Stultz <jstultz@...gle.com>,
Thomas Gleixner <tglx@...utronix.de>,
Eric Dumazet <edumazet@...gle.com>,
"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
Arjan van de Ven <arjan@...radead.org>,
"Paul E . McKenney" <paulmck@...nel.org>,
Frederic Weisbecker <frederic@...nel.org>,
Rik van Riel <riel@...riel.com>,
Steven Rostedt <rostedt@...dmis.org>,
Sebastian Siewior <bigeasy@...utronix.de>,
Giovanni Gherdovich <ggherdovich@...e.cz>,
Lukasz Luba <lukasz.luba@....com>,
"Gautham R . Shenoy" <gautham.shenoy@....com>,
Srinivas Pandruvada <srinivas.pandruvada@...el.com>,
K Prateek Nayak <kprateek.nayak@....com>,
Christian Loehle <christian.loehle@....com>,
Anna-Maria Behnsen <anna-maria@...utronix.de>,
Richard Cochran <richardcochran@...il.com>
Subject: [PATCH v11 20/20] timers: Always queue timers on the local CPU
The timer pull model is in place so we can remove the heuristics which try
to guess the best target CPU at enqueue/modification time.
All non pinned timers are queued on the local CPU in the separate storage
and eventually pulled at expiry time to a remote CPU.
Originally-by: Richard Cochran (linutronix GmbH) <richardcochran@...il.com>
Signed-off-by: Anna-Maria Behnsen <anna-maria@...utronix.de>
---
v11:
- drop check for TIMER_PINNED flag and replace it with a WARN_ON_ONCE()
v9:
- Update to the changes of the preceding patches
v6:
- Update TIMER_PINNED flag description.
v5:
- Move WARN_ONCE() in add_timer_on() into a previous patch
- Fold crystallball magic related hunks into this patch
v4: Update comment about TIMER_PINNED flag (heristic is removed)
---
include/linux/timer.h | 14 ++++----------
kernel/time/timer.c | 36 +++++++++++++++---------------------
2 files changed, 19 insertions(+), 31 deletions(-)
diff --git a/include/linux/timer.h b/include/linux/timer.h
index 2be8be6dd317..14a633ba61d6 100644
--- a/include/linux/timer.h
+++ b/include/linux/timer.h
@@ -36,16 +36,10 @@
* workqueue locking issues. It's not meant for executing random crap
* with interrupts disabled. Abuse is monitored!
*
- * @TIMER_PINNED: A pinned timer will not be affected by any timer
- * placement heuristics (like, NOHZ) and will always expire on the CPU
- * on which the timer was enqueued.
- *
- * Note: Because enqueuing of timers can migrate the timer from one
- * CPU to another, pinned timers are not guaranteed to stay on the
- * initialy selected CPU. They move to the CPU on which the enqueue
- * function is invoked via mod_timer() or add_timer(). If the timer
- * should be placed on a particular CPU, then add_timer_on() has to be
- * used.
+ * @TIMER_PINNED: A pinned timer will always expire on the CPU on which the
+ * timer was enqueued. When a particular CPU is required, add_timer_on()
+ * has to be used. Enqueue via mod_timer() and add_timer() is always done
+ * on the local CPU.
*/
#define TIMER_CPUMASK 0x0003FFFF
#define TIMER_MIGRATING 0x00040000
diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 0339273f9365..9ca921383497 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -590,11 +590,16 @@ trigger_dyntick_cpu(struct timer_base *base, struct timer_list *timer)
/*
* We might have to IPI the remote CPU if the base is idle and the
- * timer is not deferrable. If the other CPU is on the way to idle
- * then it can't set base->is_idle as we hold the base lock:
+ * timer is pinned. If it is a non pinned timer, it is only queued
+ * on the remote CPU, when timer was running during queueing. Then
+ * everything is handled by remote CPU anyway. If the other CPU is
+ * on the way to idle then it can't set base->is_idle as we hold
+ * the base lock:
*/
- if (base->is_idle)
+ if (base->is_idle) {
+ WARN_ON_ONCE(!(timer->flags & TIMER_PINNED));
wake_up_nohz_cpu(base->cpu);
+ }
}
/*
@@ -941,17 +946,6 @@ static inline struct timer_base *get_timer_base(u32 tflags)
return get_timer_cpu_base(tflags, tflags & TIMER_CPUMASK);
}
-static inline struct timer_base *
-get_target_base(struct timer_base *base, unsigned tflags)
-{
-#if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON)
- if (static_branch_likely(&timers_migration_enabled) &&
- !(tflags & TIMER_PINNED))
- return get_timer_cpu_base(tflags, get_nohz_timer_target());
-#endif
- return get_timer_this_cpu_base(tflags);
-}
-
static inline void __forward_timer_base(struct timer_base *base,
unsigned long basej)
{
@@ -1106,7 +1100,7 @@ __mod_timer(struct timer_list *timer, unsigned long expires, unsigned int option
if (!ret && (options & MOD_TIMER_PENDING_ONLY))
goto out_unlock;
- new_base = get_target_base(base, timer->flags);
+ new_base = get_timer_this_cpu_base(timer->flags);
if (base != new_base) {
/*
@@ -2252,7 +2246,7 @@ static inline u64 __get_next_timer_interrupt(unsigned long basej, u64 basem,
* granularity skew (by design).
*/
if (!base_local->is_idle && time_after(nextevt, basej + 1)) {
- base_local->is_idle = base_global->is_idle = true;
+ base_local->is_idle = true;
trace_timer_base_idle(true, base_local->cpu);
}
*idle = base_local->is_idle;
@@ -2318,13 +2312,13 @@ u64 timer_base_try_to_set_idle(unsigned long basej, u64 basem, bool *idle)
void timer_clear_idle(void)
{
/*
- * 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.
+ * We do this unlocked. The worst outcome is a remote pinned timer
+ * 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. Required for BASE_LOCAL only.
*/
__this_cpu_write(timer_bases[BASE_LOCAL].is_idle, false);
- __this_cpu_write(timer_bases[BASE_GLOBAL].is_idle, false);
trace_timer_base_idle(false, smp_processor_id());
/* Activate without holding the timer_base->lock */
--
2.39.2
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