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Message-ID: <b4c57171-be10-ffe3-514b-2464806004c9@arm.com>
Date: Thu, 26 May 2022 09:38:01 +0100
From: Lukasz Luba <lukasz.luba@....com>
To: linux-kernel@...r.kernel.org
Cc: corbet@....net, mingo@...hat.com, peterz@...radead.org,
juri.lelli@...hat.com, vincent.guittot@...aro.org,
dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
mgorman@...e.de, bristot@...hat.com, xuewen.yan@...soc.com,
linux-doc@...r.kernel.org
Subject: Re: [PATCH] sched: thermal_load_avg: Change the raising/decaying
period mechanism
Hi all,
Gentle ping.
Are there any objections to speed-up this thermal clock?
Regards,
Lukasz
On 4/29/22 10:12, Lukasz Luba wrote:
> The thermal pressure mechanism consists of two parts:
> 1) PELT-like signal with its own clock (rq_clock_thermal()) which is
> responsible for the raising/decaying characteristics
> 2) instantaneous information provided in 'thermal_pressure' variable,
> which is set by thermal framework or drivers to notify about
> the throttling.
>
> Add a new mechanism which allows to change the raising/decaying
> characteristics of the PELT-like thermal signal. To make this happen
> modify how the rq_clock_thermal() counts. Instead of only slowing down the
> clock, which results in longer raising/decaying periods, make it faster.
> Thanks to that the information about throttling can faster arrive at the
> right place in the scheduler. This faster propagation of information
> is useful for the latency sensitive stuff, such as RT tasks. In
> a situation of CPU capacity inversion, such task might suffer when
> staying on the lower capacity CPU.
>
> Change the boot parameter 'sched_thermal_decay_shift' allowed values
> and use the negatives to speed up the thermal clock.
>
> Signed-off-by: Lukasz Luba <lukasz.luba@....com>
> ---
> Hi all,
>
> This patch addresses an issue of missing configuration for the
> thermal pressure raising/decaying characteristic to be more instantaneous.
> This more sharp signal might be better if the raw thermal signal
> already has 'some logic' or doesn't change that often, like in IPA
> (every 100ms).
>
> I've prepared a notebook with experiments with different shifter
> values [0, -2, -3, -4, 2, 4] for IPA thermal update periods:
> 50ms, 100ms (and also jumping between cooling states 0, 1 or 0, 3).
> It presents two signals: instantaneous thermal update and thermal_load_avg().
> It would be useful for discussion. I can provide more details if needed.
>
> Regards,
> Lukasz Luba
>
> [1] https://nbviewer.org/github/lukaszluba-arm/lisa/blob/public_tests/thermal_pressure_delays-all-ipa.ipynb
>
>
>
> Documentation/admin-guide/kernel-parameters.txt | 15 +++++++++++----
> kernel/sched/fair.c | 2 +-
> kernel/sched/sched.h | 17 +++++++++++++++--
> 3 files changed, 27 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index c2d1f8b5e8f3..ba32540f1fbf 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -5117,15 +5117,22 @@
> pressure signal. Thermal pressure signal follows the
> default decay period of other scheduler pelt
> signals(usually 32 ms but configurable). Setting
> - sched_thermal_decay_shift will left shift the decay
> - period for the thermal pressure signal by the shift
> - value.
> + a positive value for sched_thermal_decay_shift will
> + left shift the decay period for the thermal pressure
> + signal by the shift value. This would make
> + raising/decaying characteristic longer. Setting
> + a negative value will right shift the decay period
> + by the shift value and make the raising/decaying
> + characteristic more sharp.
> i.e. with the default pelt decay period of 32 ms
> sched_thermal_decay_shift thermal pressure decay pr
> + -2 8 ms
> + -1 16 ms
> + 0 32 ms
> 1 64 ms
> 2 128 ms
> and so on.
> - Format: integer between 0 and 10
> + Format: integer between -5 and 10
> Default is 0.
>
> scftorture.holdoff= [KNL]
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 5146163bfabb..93cb7db5939c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -100,7 +100,7 @@ static int __init setup_sched_thermal_decay_shift(char *str)
> if (kstrtoint(str, 0, &_shift))
> pr_warn("Unable to set scheduler thermal pressure decay shift parameter\n");
>
> - sched_thermal_decay_shift = clamp(_shift, 0, 10);
> + sched_thermal_decay_shift = clamp(_shift, -5, 10);
> return 1;
> }
> __setup("sched_thermal_decay_shift=", setup_sched_thermal_decay_shift);
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index de53be905739..cb453c0f3572 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1478,8 +1478,14 @@ static inline u64 rq_clock_task(struct rq *rq)
> /**
> * By default the decay is the default pelt decay period.
> * The decay shift can change the decay period in
> - * multiples of 32.
> + * multiples of 32 to make it longer or to make it shorter using
> + * negative values as on the example below.
> * Decay shift Decay period(ms)
> + * -5 1
> + * -4 2
> + * -3 4
> + * -2 8
> + * -1 16
> * 0 32
> * 1 64
> * 2 128
> @@ -1490,7 +1496,14 @@ extern int sched_thermal_decay_shift;
>
> static inline u64 rq_clock_thermal(struct rq *rq)
> {
> - return rq_clock_task(rq) >> sched_thermal_decay_shift;
> + u64 thermal_clock = rq_clock_task(rq);
> +
> + if (sched_thermal_decay_shift < 0)
> + thermal_clock <<= -sched_thermal_decay_shift;
> + else
> + thermal_clock >>= sched_thermal_decay_shift;
> +
> + return thermal_clock;
> }
>
> static inline void rq_clock_skip_update(struct rq *rq)
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