Messages in this thread Patch in this message | | | From | Anna-Maria Behnsen <> | Subject | [PATCH v8 25/25] timer: Always queue timers on the local CPU | Date | Wed, 4 Oct 2023 14:34:54 +0200 |
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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@gmail.com> Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de> --- 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 | 42 ++++++++++++++++++++---------------------- 2 files changed, 24 insertions(+), 32 deletions(-)
diff --git a/include/linux/timer.h b/include/linux/timer.h index 6f96661480dd..c39ae2ad0035 100644 --- a/include/linux/timer.h +++ b/include/linux/timer.h @@ -50,16 +50,10 @@ struct timer_list { * 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 331c59c0789a..feb915642457 100644 --- a/kernel/time/timer.c +++ b/kernel/time/timer.c @@ -590,10 +590,13 @@ 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 && timer->flags & TIMER_PINNED) wake_up_nohz_cpu(base->cpu); } @@ -941,17 +944,6 @@ 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 +1098,7 @@ __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) { /* @@ -2256,8 +2248,14 @@ u64 timer_set_idle(unsigned long basej, u64 basem, bool *idle) */ } - /* We need to mark both bases in sync */ - base_local->is_idle = base_global->is_idle = *idle; + /* + * base->is_idle information is required to wakeup a idle CPU when + * a new timer was enqueued. Only pinned timers could be enqueued + * remotely into a idle base. Therefore do maintain only + * base_local->is_idle information and ignore base_global->is_idle + * information. + */ + base_local->is_idle = *idle; raw_spin_unlock(&base_global->lock); raw_spin_unlock(&base_local->lock); @@ -2273,13 +2271,13 @@ u64 timer_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); /* Activate without holding the timer_base->lock */ tmigr_cpu_activate(); -- 2.39.2
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