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From: Valentin Schneider <valentin.schneider@arm.com>
To: Ionela Voinescu <ionela.voinescu@arm.com>,
	catalin.marinas@arm.com, will@kernel.org, mark.rutland@arm.com,
	maz@kernel.org, suzuki.poulose@arm.com, sudeep.holla@arm.com,
	dietmar.eggemann@arm.com
Cc: peterz@infradead.org, mingo@redhat.com, ggherdovich@suse.cz,
	vincent.guittot@linaro.org, linux-arm-kernel@lists.infradead.org,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 6/6] arm64: use activity monitors for frequency invariance
Date: Wed, 29 Jan 2020 17:13:53 +0000	[thread overview]
Message-ID: <96fdead6-9896-5695-6744-413300d424f5@arm.com> (raw)
In-Reply-To: <20191218182607.21607-7-ionela.voinescu@arm.com>

Only commenting on the bits that should be there regardless of using the
workqueues or not;

On 18/12/2019 18:26, Ionela Voinescu wrote:
> +static void cpu_amu_fie_init_workfn(struct work_struct *work)
> +{
> +	u64 core_cnt, const_cnt, ratio;
> +	struct cpu_amu_work *amu_work;
> +	int cpu = smp_processor_id();
> +
> +	if (!cpu_has_amu_feat()) {
> +		pr_debug("CPU%d: counters are not supported.\n", cpu);
> +		return;
> +	}
> +
> +	core_cnt = read_sysreg_s(SYS_AMEVCNTR0_CORE_EL0);
> +	const_cnt = read_sysreg_s(SYS_AMEVCNTR0_CONST_EL0);
> +
> +	if (unlikely(!core_cnt || !const_cnt)) {
> +		pr_err("CPU%d: cycle counters are not enabled.\n", cpu);
> +		return;
> +	}
> +
> +	amu_work = container_of(work, struct cpu_amu_work, cpu_work);
> +	if (unlikely(!(amu_work->cpuinfo_max_freq))) {
> +		pr_err("CPU%d: invalid maximum frequency.\n", cpu);
> +		return;
> +	}
> +
> +	/*
> +	 * Pre-compute the fixed ratio between the frequency of the
> +	 * constant counter and the maximum frequency of the CPU (hz).

I can't resist: s/hz/Hz/

> +	 */
> +	ratio = (u64)arch_timer_get_rate() << (2 * SCHED_CAPACITY_SHIFT);
> +	ratio = div64_u64(ratio, amu_work->cpuinfo_max_freq * 1000);

Nit: we're missing a comment somewhere that the unit of this is in kHz
(which explains the * 1000).

> +	this_cpu_write(arch_max_freq_scale, (unsigned long)ratio);
> +

Okay so what we get in the tick is:

  /\ core
  --------
  /\ const

And we want that to be SCHED_CAPACITY_SCALE when running at max freq. IOW we
want to turn

  max_freq
  ----------
  const_freq

into SCHED_CAPACITY_SCALE, so we can just multiply that by:

  const_freq
  ---------- * SCHED_CAPACITY_SCALE
  max_freq

But what the ratio you are storing here is 

                          const_freq
  arch_max_freq_scale =   ---------- * SCHED_CAPACITY_SCALE²
                           max_freq

(because x << 2 * SCHED_CAPACITY_SHIFT == x << 20)


In topology_freq_scale_tick() you end up doing

  /\ core   arch_max_freq_scale
  ------- * --------------------
  /\ const  SCHED_CAPACITY_SCALE

which gives us what we want (SCHED_CAPACITY_SCALE at max freq).


Now, the reason why we multiply our ratio by the square of
SCHED_CAPACITY_SCALE was not obvious to me, but you pointed me out that the
frequency of the arch timer can be *really* low compared to the max CPU freq.

For instance on h960:

  [    0.000000] arch_timer: cp15 timer(s) running at 1.92MHz (phys)

  $ root@valsch-h960:~# cat /sys/devices/system/cpu/cpufreq/policy4/cpuinfo_max_freq 
  2362000

So our ratio would be

  1'920'000 * 1024
  ----------------
    2'362'000'000

Which is ~0.83, so that becomes simply 0...


I had a brief look at the Arm ARM, for the arch timer it says it is
"typically in the range 1-50MHz", but then also gives an example with 20KHz
in a low-power mode.

If we take say 5GHz max CPU frequency, our lower bound for the arch timer
(with that SCHED_CAPACITY_SCALE² trick) is about ~4.768KHz. It's not *too*
far from that 20KHz, but I'm not sure we would actually be executing stuff
in that low-power mode.

Long story short, we're probably fine, but it would nice to shove some of
the above into comments (especially the SCHED_CAPACITY_SCALE² trick)

  parent reply	other threads:[~2020-01-29 17:14 UTC|newest]

Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-18 18:26 [PATCH v2 0/6] arm64: ARMv8.4 Activity Monitors support Ionela Voinescu
2019-12-18 18:26 ` [PATCH v2 1/6] arm64: add support for the AMU extension v1 Ionela Voinescu
2020-01-23 17:04   ` Valentin Schneider
2020-01-23 18:32     ` Ionela Voinescu
2020-01-24 12:00       ` Valentin Schneider
2020-01-28 11:00         ` Ionela Voinescu
2020-01-28 16:34   ` Suzuki Kuruppassery Poulose
2020-01-29 16:42     ` Ionela Voinescu
2019-12-18 18:26 ` [PATCH v2 2/6] arm64: trap to EL1 accesses to AMU counters from EL0 Ionela Voinescu
2020-01-23 17:04   ` Valentin Schneider
2020-01-23 17:34     ` Ionela Voinescu
2019-12-18 18:26 ` [PATCH v2 3/6] arm64/kvm: disable access to AMU registers from kvm guests Ionela Voinescu
2020-01-27 15:33   ` Valentin Schneider
2020-01-28 15:48     ` Ionela Voinescu
2020-01-28 17:26     ` Suzuki Kuruppassery Poulose
2020-01-28 17:37       ` Valentin Schneider
2020-01-28 17:52         ` Ionela Voinescu
2019-12-18 18:26 ` [PATCH v2 4/6] Documentation: arm64: document support for the AMU extension Ionela Voinescu
2020-01-27 16:47   ` Valentin Schneider
2020-01-28 16:53     ` Ionela Voinescu
2020-01-28 18:36       ` Valentin Schneider
2020-01-30 15:04   ` Suzuki Kuruppassery Poulose
2020-01-30 16:45     ` Ionela Voinescu
2020-01-30 18:26       ` Suzuki K Poulose
2020-01-31  9:54         ` Ionela Voinescu
2019-12-18 18:26 ` [PATCH v2 5/6] TEMP: sched: add interface for counter-based frequency invariance Ionela Voinescu
2020-01-29 19:37   ` Peter Zijlstra
2020-01-30 15:33     ` Ionela Voinescu
2019-12-18 18:26 ` [PATCH v2 6/6] arm64: use activity monitors for " Ionela Voinescu
2020-01-23 11:49   ` Lukasz Luba
2020-01-23 17:07     ` Ionela Voinescu
2020-01-24  1:19       ` Lukasz Luba
2020-01-24 13:12         ` Ionela Voinescu
2020-01-24 15:17           ` Lukasz Luba
2020-01-28 17:36             ` Ionela Voinescu
2020-01-29 17:13   ` Valentin Schneider [this message]
2020-01-29 17:52     ` Ionela Voinescu
2020-01-29 23:39     ` Valentin Schneider
2020-01-30 15:49       ` Ionela Voinescu
2020-01-30 16:11         ` Valentin Schneider

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