From: Anna-Maria Behnsen <anna-maria@linutronix.de>
To: Frederic Weisbecker <frederic@kernel.org>
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 v10a] timers: Move marking timer bases idle into tick_nohz_stop_tick()
Date: Tue, 20 Feb 2024 16:23:26 +0100 [thread overview]
Message-ID: <87o7cb40sx.fsf@somnus> (raw)
In-Reply-To: <ZdTA8N7TkGG66Ay6@lothringen>
Frederic Weisbecker <frederic@kernel.org> writes:
> On Tue, Feb 20, 2024 at 03:00:57PM +0100, Anna-Maria Behnsen wrote:
>> Frederic Weisbecker <frederic@kernel.org> writes:
>>
>> > Le Tue, Feb 20, 2024 at 01:02:18PM +0100, Anna-Maria Behnsen a écrit :
>> >> Frederic Weisbecker <frederic@kernel.org> writes:
>> >>
>> >> > Le Tue, Feb 20, 2024 at 11:48:19AM +0100, Anna-Maria Behnsen a écrit :
>> >> >> Frederic Weisbecker <frederic@kernel.org> writes:
>> >> >> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
>> >> >> index 01fb50c1b17e..b93f0e6f273f 100644
>> >> >> --- a/kernel/time/tick-sched.c
>> >> >> +++ b/kernel/time/tick-sched.c
>> >> >> @@ -895,21 +895,6 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
>> >> >> /* Make sure we won't be trying to stop it twice in a row. */
>> >> >> ts->timer_expires_base = 0;
>> >> >>
>> >> >> - /*
>> >> >> - * If this CPU is the one which updates jiffies, then give up
>> >> >> - * the assignment and let it be taken by the CPU which runs
>> >> >> - * the tick timer next, which might be this CPU as well. If we
>> >> >> - * don't drop this here, the jiffies might be stale and
>> >> >> - * do_timer() never gets invoked. Keep track of the fact that it
>> >> >> - * was the one which had the do_timer() duty last.
>> >> >> - */
>> >> >> - if (cpu == tick_do_timer_cpu) {
>> >> >> - tick_do_timer_cpu = TICK_DO_TIMER_NONE;
>> >> >> - ts->do_timer_last = 1;
>> >> >> - } else if (tick_do_timer_cpu != TICK_DO_TIMER_NONE) {
>> >> >> - ts->do_timer_last = 0;
>> >> >> - }
>> >> >> -
>> >> >> /* Skip reprogram of event if it's not changed */
>> >> >> if (ts->tick_stopped && (expires == ts->next_tick)) {
>> >> >> /* Sanity check: make sure clockevent is actually programmed */
>> >> >
>> >> > That should work but then you lose the optimization that resets
>> >> > ts->do_timer_last even if the next timer hasn't changed.
>> >> >
>> >>
>> >> Beside of this optimization thing, I see onther problem. But I'm not
>> >> sure, if I understood it correctly: When the CPU drops the
>> >> tick_do_timer_cpu assignment and stops the tick, it is possible, that
>> >> this CPU nevertheless executes tick_sched_do_timer() and then reassigns
>> >> to tick_do_timer_cpu?
>> >
>> > Yes but in this case a timer interrupt has executed and ts->next_tick
>> > is cleared, so the above skip reprogramm branch is not taken.
>> >
>>
>> Yes... So I need to change it without dropping the
>> optimization. Otherwise someone might complain about it.
>>
>> Two possible solutions:
>>
>> a) split out this if/else thing for dropping the tick_do_timer_cpu
>> assignment into a separate function and call it:
>> - before the return in the skip reprogramm branch
>> - and after the if clause which contains stopping the tick (where it
>> is executed in the current proposal)
>>
>> b) Take my current proposal and add before the return in the skip
>> reprogramm branch the following lines:
>>
>> if (tick_do_timer_cpu != TICK_DO_TIMER_NONE)
>> ts->do_timer_last = 0;
>>
>> as the first part of the tick_do_timer_cpu/last logic shouldn't be
>> required (because then also ts->next_tick is already cleared).
>>
>> What do you prefere? Or do you prefere something else?
>
> Wouldn't the following work? If timer_idle is false, then the tick isn't
> even stopped and there is nothing to do? So you can early return.
>
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index fdd57f1af1d7..1b2984acafbd 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -924,6 +924,9 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
> expires = ts->timer_expires;
> }
>
> + if (!timer_idle)
> + return;
> +
> /*
> * If this CPU is the one which updates jiffies, then give up
> * the assignment and let it be taken by the CPU which runs
Yes... And then I can drop the if (!timer_idle) thing inside
!ts->tick_stopped branch.
next prev parent reply other threads:[~2024-02-20 15:23 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 [this message]
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
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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