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Date: Tue, 7 Nov 2023 08:30:08 +0100
From: Daniel Bristot de Oliveira <bristot@...nel.org>
To: Joel Fernandes <joel@...lfernandes.org>
Cc: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Valentin Schneider <vschneid@...hat.com>,
linux-kernel@...r.kernel.org,
Luca Abeni <luca.abeni@...tannapisa.it>,
Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>,
Thomas Gleixner <tglx@...utronix.de>,
Vineeth Pillai <vineeth@...byteword.org>,
Shuah Khan <skhan@...uxfoundation.org>,
Phil Auld <pauld@...hat.com>
Subject: Re: [PATCH v5 6/7] sched/deadline: Deferrable dl server
On 11/6/23 20:32, Joel Fernandes wrote:
> Hi Daniel,
>
> On Sat, Nov 4, 2023 at 6:59 AM Daniel Bristot de Oliveira
> <bristot@...nel.org> wrote:
>>
>> Among the motivations for the DL servers is the real-time throttling
>> mechanism. This mechanism works by throttling the rt_rq after
>> running for a long period without leaving space for fair tasks.
>>
>> The base dl server avoids this problem by boosting fair tasks instead
>> of throttling the rt_rq. The point is that it boosts without waiting
>> for potential starvation, causing some non-intuitive cases.
>>
>> For example, an IRQ dispatches two tasks on an idle system, a fair
>> and an RT. The DL server will be activated, running the fair task
>> before the RT one. This problem can be avoided by deferring the
>> dl server activation.
>>
>> By setting the zerolax option, the dl_server will dispatch an
>> SCHED_DEADLINE reservation with replenished runtime, but throttled.
>>
>> The dl_timer will be set for (period - runtime) ns from start time.
>> Thus boosting the fair rq on its 0-laxity time with respect to
>> rt_rq.
>>
>> If the fair scheduler has the opportunity to run while waiting
>> for zerolax time, the dl server runtime will be consumed. If
>> the runtime is completely consumed before the zerolax time, the
>> server will be replenished while still in a throttled state. Then,
>> the dl_timer will be reset to the new zerolax time
>>
>> If the fair server reaches the zerolax time without consuming
>> its runtime, the server will be boosted, following CBS rules
>> (thus without breaking SCHED_DEADLINE).
>>
>> Signed-off-by: Daniel Bristot de Oliveira <bristot@...nel.org>
>> ---
>> include/linux/sched.h | 2 +
>> kernel/sched/deadline.c | 100 +++++++++++++++++++++++++++++++++++++++-
>> kernel/sched/fair.c | 3 ++
>> 3 files changed, 103 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>> index 5ac1f252e136..56e53e6fd5a0 100644
>> --- a/include/linux/sched.h
>> +++ b/include/linux/sched.h
>> @@ -660,6 +660,8 @@ struct sched_dl_entity {
>> unsigned int dl_non_contending : 1;
>> unsigned int dl_overrun : 1;
>> unsigned int dl_server : 1;
>> + unsigned int dl_zerolax : 1;
>> + unsigned int dl_zerolax_armed : 1;
>>
>> /*
>> * Bandwidth enforcement timer. Each -deadline task has its
>> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
>> index 1d7b96ca9011..69ee1fbd60e4 100644
>> --- a/kernel/sched/deadline.c
>> +++ b/kernel/sched/deadline.c
>> @@ -772,6 +772,14 @@ static inline void replenish_dl_new_period(struct sched_dl_entity *dl_se,
>> /* for non-boosted task, pi_of(dl_se) == dl_se */
>> dl_se->deadline = rq_clock(rq) + pi_of(dl_se)->dl_deadline;
>> dl_se->runtime = pi_of(dl_se)->dl_runtime;
>> +
>> + /*
>> + * If it is a zerolax reservation, throttle it.
>> + */
>> + if (dl_se->dl_zerolax) {
>> + dl_se->dl_throttled = 1;
>> + dl_se->dl_zerolax_armed = 1;
>> + }
>> }
>>
>> /*
>> @@ -828,6 +836,7 @@ static inline void setup_new_dl_entity(struct sched_dl_entity *dl_se)
>> * could happen are, typically, a entity voluntarily trying to overcome its
>> * runtime, or it just underestimated it during sched_setattr().
>> */
>> +static int start_dl_timer(struct sched_dl_entity *dl_se);
>> static void replenish_dl_entity(struct sched_dl_entity *dl_se)
>> {
>> struct dl_rq *dl_rq = dl_rq_of_se(dl_se);
>> @@ -874,6 +883,28 @@ static void replenish_dl_entity(struct sched_dl_entity *dl_se)
>> dl_se->dl_yielded = 0;
>> if (dl_se->dl_throttled)
>> dl_se->dl_throttled = 0;
>> +
>> + /*
>> + * If this is the replenishment of a zerolax reservation,
>> + * clear the flag and return.
>> + */
>> + if (dl_se->dl_zerolax_armed) {
>> + dl_se->dl_zerolax_armed = 0;
>> + return;
>> + }
>> +
>> + /*
>> + * A this point, if the zerolax server is not armed, and the deadline
>> + * is in the future, throttle the server and arm the zerolax timer.
>> + */
>> + if (dl_se->dl_zerolax &&
>> + dl_time_before(dl_se->deadline - dl_se->runtime, rq_clock(rq))) {
>> + if (!is_dl_boosted(dl_se)) {
>> + dl_se->dl_zerolax_armed = 1;
>> + dl_se->dl_throttled = 1;
>> + start_dl_timer(dl_se);
>> + }
>> + }
>> }
>>
>> /*
>> @@ -1024,6 +1055,13 @@ static void update_dl_entity(struct sched_dl_entity *dl_se)
>> }
>>
>> replenish_dl_new_period(dl_se, rq);
>> + } else if (dl_server(dl_se) && dl_se->dl_zerolax) {
>> + /*
>> + * The server can still use its previous deadline, so throttle
>> + * and arm the zero-laxity timer.
>> + */
>> + dl_se->dl_zerolax_armed = 1;
>> + dl_se->dl_throttled = 1;
>> }
>> }
>>
>> @@ -1056,8 +1094,20 @@ static int start_dl_timer(struct sched_dl_entity *dl_se)
>> * We want the timer to fire at the deadline, but considering
>> * that it is actually coming from rq->clock and not from
>> * hrtimer's time base reading.
>> + *
>> + * The zerolax reservation will have its timer set to the
>> + * deadline - runtime. At that point, the CBS rule will decide
>> + * if the current deadline can be used, or if a replenishment
>> + * is required to avoid add too much pressure on the system
>> + * (current u > U).
>> */
>> - act = ns_to_ktime(dl_next_period(dl_se));
>> + if (dl_se->dl_zerolax_armed) {
>> + WARN_ON_ONCE(!dl_se->dl_throttled);
>> + act = ns_to_ktime(dl_se->deadline - dl_se->runtime);
>
> Just a question, here if dl_se->deadline - dl_se->runtime is large,
> then does that mean that server activation will be much more into the
> future? So say I want to give CFS 30%, then it will take 70% of the
> period before CFS preempts RT thus "starving" CFS for this duration. I
> think that's Ok for smaller periods and runtimes, though.
I think you are answering yourself here :-)
If the default values are not good, change them o/
The current interface allows you to have more responsive/small chuck of CPU
or less responsive/large chucks of CPU... you can even place RT bellow CFS
for a "bounded amount of time" by disabling the defer option... per CPU.
All at once with different periods patterns on CPUs to increase the
changes of having a cfs rq ready on another CPU... like...
[3/10 - 2/6 - 1.5/5 - 1/3 no defer] in a 4 cpus system :-).
The default setup is based on the throttling to avoid changing
the historical behavior for those that... are happy with them.
-- Daniel
> - Joel
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