From: Frederic Weisbecker <frederic@kernel.org>
To: Anna-Maria Behnsen <anna-maria@linutronix.de>
Cc: linux-kernel@vger.kernel.org,
Peter Zijlstra <peterz@infradead.org>,
John Stultz <jstultz@google.com>,
Thomas Gleixner <tglx@linutronix.de>,
Eric Dumazet <edumazet@google.com>,
"Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
Arjan van de Ven <arjan@infradead.org>,
"Paul E . McKenney" <paulmck@kernel.org>,
Rik van Riel <riel@surriel.com>,
Steven Rostedt <rostedt@goodmis.org>,
Sebastian Siewior <bigeasy@linutronix.de>,
Giovanni Gherdovich <ggherdovich@suse.cz>,
Lukasz Luba <lukasz.luba@arm.com>,
"Gautham R . Shenoy" <gautham.shenoy@amd.com>,
Srinivas Pandruvada <srinivas.pandruvada@intel.com>,
K Prateek Nayak <kprateek.nayak@amd.com>
Subject: Re: [PATCH v10 18/20] timers: Implement the hierarchical pull model
Date: Mon, 29 Jan 2024 02:04:27 +0100 [thread overview]
Message-ID: <Zbb5m0hRHgk59-8z@pavilion.home> (raw)
In-Reply-To: <20240115143743.27827-19-anna-maria@linutronix.de>
Le Mon, Jan 15, 2024 at 03:37:41PM +0100, Anna-Maria Behnsen a écrit :
> +int tmigr_requires_handle_remote(void)
> +{
> + struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
> + struct tmigr_remote_data data;
> + unsigned int ret = 0;
> + unsigned long jif;
> +
> + if (tmigr_is_not_available(tmc))
> + return ret;
> +
> + data.now = get_jiffies_update(&jif);
> + data.childmask = tmc->childmask;
> + data.firstexp = KTIME_MAX;
> + data.tmc_active = !tmc->idle;
> + data.check = false;
> +
> + /*
> + * If the CPU is active, walk the hierarchy to check whether a remote
> + * expiry is required.
> + *
> + * Check is done lockless as interrupts are disabled and @tmc->idle is
> + * set only by the local CPU.
> + */
> + if (!tmc->idle) {
> + __walk_groups(&tmigr_requires_handle_remote_up, &data, tmc);
> +
> + if (data.firstexp != KTIME_MAX)
> + ret = 1;
> +
> + return ret;
> + }
> +
> + /*
> + * If the CPU is idle, check whether the recalculation of @tmc->wakeup
> + * is required. @tmc->wakeup_recalc is set, when the last active CPU
> + * went offline. The last active CPU delegated the handling of the timer
> + * migration hierarchy to another (this) CPU by updating this flag and
> + * sending a reschedule.
> + *
> + * Racy lockless check is valid:
> + * - @tmc->wakeup_recalc is set by the remote CPU before it issues
> + * reschedule IPI.
> + * - As interrupts are disabled here this CPU will either observe
> + * @tmc->wakeup_recalc set before the reschedule IPI can be handled or
> + * it will observe it when this function is called again on return
> + * from handling the reschedule IPI.
> + */
> + if (tmc->wakeup_recalc) {
> + __walk_groups(&tmigr_requires_handle_remote_up, &data, tmc);
> +
> + if (data.firstexp != KTIME_MAX)
> + ret = 1;
> +
> + raw_spin_lock(&tmc->lock);
> + WRITE_ONCE(tmc->wakeup, data.firstexp);
> + tmc->wakeup_recalc = false;
> + raw_spin_unlock(&tmc->lock);
Suppose we have:
[GRP1:0]
migrator = GRP0:1
active = GRP0:0, GRP0:1
/ \
[GRP0:0] [GRP0:1]
migrator = CPU 1 migrator = CPU 3
active = CPU 1 active = CPU 3
/ \ / \
CPUs 0 1 2 3
idle active idle active
CPU 0 and CPU 2 have no timer.
CPU 1 has a timer in a few millisecs.
[GRP1:0]
migrator = GRP0:1
active = GRP0:1
/ \
[GRP0:0] [GRP0:1]
migrator = NONE migrator = CPU 3
active = NONE active = CPU 3
/ \ / \
CPUs 0 1 2 3
idle idle idle active
CPU 1 went idle, CPU 3 will take care of CPU 1's timer. Then come two
things happening at the same time: CPU 0 has a timer interrupt, due to
RCU callbacks handling for example, and CPU 3 goes offline:
CPU 0 CPU 3
----- -----
// On top level [GRP1:0], just set migrator = TMIGR_NONE
tmigr_inactive_up() {
cpu = cpumask_any_but(cpu_online_mask, cpu);
//cpu == 0
tmc_resched = per_cpu_ptr(&tmigr_cpu, CPU 0);
raw_spin_lock(&tmc_resched->lock);
tmc_resched->wakeup_recalc = true;
raw_spin_unlock(&tmc_resched->lock);
// timer interrupt
run_local_timers() {
tmigr_requires_handle_remote() {
data.firstexp = KTIME_MAX;
// CPU 0 sees the tmc_resched->wakeup_recalc
// latest update
if (tmc->wakeup_recalc) {
tmigr_requires_handle_remote_up() {
// CPU 0 doesn't see GRP0:0
// latest update from CPU 1,
// because it has no locking
// and does a racy check.
if (!tmigr_check_migrator(group, childmask))
return true;
}
raw_spin_lock(&tmc->lock);
WRITE_ONCE(tmc->wakeup, data.firstexp);
tmc->wakeup_recalc = false;
raw_spin_unlock(&tmc->lock)
return 0;
}
// IPI is sent only now
smp_send_reschedule(cpu);
}
There is nothing that prevents CPU 0 from not seeing the hierarchy updates from
other CPUs since it checks the migrators in a racy way. As a result the timer of
CPU 1 may be ignored by CPU 0.
You'd need to lock the tmc while calling tmigr_requires_handle_remote_up(), so
that CPU 0 "inherits" everything that CPU 3 has seen, and that includes changes
from CPU 1.
But I see that tmigr_cpu_new_timer() does it right. Wouldn't it be possible to
exlusively let tmigr_cpu_new_timer() handle the wakeup_recalc thing? This is
going to be called after the end of the IRQ (whether timer interrupt or sched
IPI) in any case.
Thanks.
next prev parent reply other threads:[~2024-01-29 1:04 UTC|newest]
Thread overview: 89+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-01-15 14:37 [PATCH v10 00/20] timers: Move from a push remote at enqueue to a pull at expiry model Anna-Maria Behnsen
2024-01-15 14:37 ` [PATCH v10 01/20] timers: Restructure get_next_timer_interrupt() Anna-Maria Behnsen
2024-01-17 15:01 ` Frederic Weisbecker
2024-01-15 14:37 ` [PATCH v10 02/20] timers: Split out get next timer interrupt Anna-Maria Behnsen
2024-01-17 15:06 ` Frederic Weisbecker
2024-01-15 14:37 ` [PATCH v10 03/20] timers: Move marking timer bases idle into tick_nohz_stop_tick() Anna-Maria Behnsen
2024-01-17 16:02 ` Frederic Weisbecker
2024-01-22 11:45 ` Anna-Maria Behnsen
2024-01-22 21:49 ` Frederic Weisbecker
2024-02-19 8:52 ` [PATCH v10a] " Anna-Maria Behnsen
2024-02-19 22:37 ` Frederic Weisbecker
2024-02-20 10:48 ` Anna-Maria Behnsen
2024-02-20 11:41 ` Frederic Weisbecker
2024-02-20 12:02 ` Anna-Maria Behnsen
2024-02-20 12:34 ` Frederic Weisbecker
2024-02-20 14:00 ` Anna-Maria Behnsen
2024-02-20 15:10 ` Frederic Weisbecker
2024-02-20 15:23 ` Anna-Maria Behnsen
2024-02-20 15:25 ` Frederic Weisbecker
2024-01-15 14:37 ` [PATCH v10 04/20] timers: Optimization for timer_base_try_to_set_idle() Anna-Maria Behnsen
2024-01-17 16:45 ` Frederic Weisbecker
2024-01-22 11:48 ` Anna-Maria Behnsen
2024-01-22 22:22 ` Frederic Weisbecker
2024-01-15 14:37 ` [PATCH v10 05/20] timers: Introduce add_timer() variants which modify timer flags Anna-Maria Behnsen
2024-01-17 17:01 ` Frederic Weisbecker
2024-01-22 11:50 ` Anna-Maria Behnsen
2024-01-15 14:37 ` [PATCH v10 06/20] workqueue: Use global variant for add_timer() Anna-Maria Behnsen
2024-01-15 14:37 ` [PATCH v10 07/20] timers: add_timer_on(): Make sure TIMER_PINNED flag is set Anna-Maria Behnsen
2024-01-15 14:37 ` [PATCH v10 08/20] timers: Ease code in run_local_timers() Anna-Maria Behnsen
2024-01-15 14:37 ` [PATCH v10 09/20] timers: Split next timer interrupt logic Anna-Maria Behnsen
2024-01-23 14:28 ` Frederic Weisbecker
2024-01-15 14:37 ` [PATCH v10 10/20] timers: Keep the pinned timers separate from the others Anna-Maria Behnsen
2024-01-15 14:37 ` [PATCH v10 11/20] timers: Retrieve next expiry of pinned/non-pinned timers separately Anna-Maria Behnsen
2024-01-15 14:37 ` [PATCH v10 12/20] timers: Split out "get next timer interrupt" functionality Anna-Maria Behnsen
2024-01-15 14:37 ` [PATCH v10 13/20] timers: Add get next timer interrupt functionality for remote CPUs Anna-Maria Behnsen
2024-02-19 16:04 ` Frederic Weisbecker
2024-02-19 16:57 ` Anna-Maria Behnsen
2024-01-15 14:37 ` [PATCH v10 14/20] timers: Restructure internal locking Anna-Maria Behnsen
2024-01-24 13:56 ` Frederic Weisbecker
2024-01-15 14:37 ` [PATCH v10 15/20] timers: Check if timers base is handled already Anna-Maria Behnsen
2024-01-24 14:22 ` Frederic Weisbecker
2024-01-15 14:37 ` [PATCH v10 16/20] tick/sched: Split out jiffies update helper function Anna-Maria Behnsen
2024-01-24 14:42 ` Frederic Weisbecker
2024-01-15 14:37 ` [PATCH v10 17/20] timers: Introduce function to check timer base is_idle flag Anna-Maria Behnsen
2024-01-24 14:52 ` Frederic Weisbecker
2024-01-15 14:37 ` [PATCH v10 18/20] timers: Implement the hierarchical pull model Anna-Maria Behnsen
2024-01-25 14:30 ` Frederic Weisbecker
2024-01-28 15:58 ` Anna-Maria Behnsen
2024-01-30 15:29 ` Frederic Weisbecker
2024-01-30 16:45 ` Anna-Maria Behnsen
2024-01-26 12:53 ` Frederic Weisbecker
2024-01-27 22:54 ` Frederic Weisbecker
2024-01-29 10:50 ` Anna-Maria Behnsen
2024-01-29 22:21 ` Frederic Weisbecker
2024-01-30 13:32 ` Anna-Maria Behnsen
2024-01-29 13:50 ` Paul E. McKenney
2024-01-29 1:04 ` Frederic Weisbecker [this message]
2024-01-30 17:56 ` Anna-Maria Behnsen
2024-01-30 21:13 ` Frederic Weisbecker
2024-01-31 11:19 ` Anna-Maria Behnsen
2024-01-30 15:37 ` Frederic Weisbecker
2024-02-01 14:59 ` Anna-Maria Behnsen
2024-02-01 15:05 ` Frederic Weisbecker
2024-02-01 16:15 ` Anna-Maria Behnsen
2024-02-01 17:43 ` Frederic Weisbecker
2024-02-01 20:52 ` Anna-Maria Behnsen
2024-02-05 13:29 ` Anna-Maria Behnsen
2024-02-05 20:30 ` Frederic Weisbecker
2024-02-06 10:06 ` Anna-Maria Behnsen
2024-02-06 10:29 ` Frederic Weisbecker
2024-02-01 16:33 ` Frederic Weisbecker
2024-02-05 15:59 ` Anna-Maria Behnsen
2024-02-05 20:28 ` Frederic Weisbecker
2024-02-04 22:02 ` Frederic Weisbecker
2024-02-06 11:03 ` Anna-Maria Behnsen
2024-02-06 11:11 ` Frederic Weisbecker
2024-02-04 22:32 ` Frederic Weisbecker
2024-02-06 11:36 ` Anna-Maria Behnsen
2024-02-06 13:21 ` Frederic Weisbecker
2024-02-06 14:13 ` Anna-Maria Behnsen
2024-02-06 14:21 ` Frederic Weisbecker
2024-01-15 14:37 ` [PATCH v10 19/20] timer_migration: Add tracepoints Anna-Maria Behnsen
2024-02-01 16:47 ` Frederic Weisbecker
2024-01-15 14:37 ` [PATCH v10 20/20] timers: Always queue timers on the local CPU Anna-Maria Behnsen
2024-02-01 17:36 ` Frederic Weisbecker
2024-02-01 20:58 ` Anna-Maria Behnsen
2024-02-02 11:57 ` Frederic Weisbecker
2024-01-30 22:07 ` [PATCH v10 00/20] timers: Move from a push remote at enqueue to a pull at expiry model Christian Loehle
2024-02-01 15:03 ` Anna-Maria Behnsen
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