Re: [PATCH v1 1/3] arm64: topology: Add arch_freq_get_on_cpu() support

[Beata Michalska]

Hi Vanshidhar,

On Thu, Feb 29, 2024 at 08:25:13AM -0800, Vanshidhar Konda wrote:
> AMU counters are used by the Frequency Invariance Engine (FIE) to
> estimate the CPU utilization during each tick. The delta of the AMU
> counters between two ticks can also be used to estimate the average CPU
> frequency of each core over the tick duration. Measure the AMU counters
> during tick, compute the delta and store it. When the frequency of the
> core is queried, use the stored delta to determine the frequency.
> 
> arch_freq_get_on_cpu() is used on x86 systems to estimate the frequency
> of each CPU. It can be wired up on arm64 for the same functionality.
> 
> Signed-off-by: Vanshidhar Konda <vanshikonda@xxxxxxxxxxxxxxxxxxxxxx>
> ---
>  arch/arm64/kernel/topology.c | 114 +++++++++++++++++++++++++++++------
>  1 file changed, 96 insertions(+), 18 deletions(-)
> 
> diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> index 1a2c72f3e7f8..db8d14525cf4 100644
> --- a/arch/arm64/kernel/topology.c
> +++ b/arch/arm64/kernel/topology.c
> @@ -17,6 +17,8 @@
>  #include <linux/cpufreq.h>
>  #include <linux/init.h>
>  #include <linux/percpu.h>
> +#include <linux/sched/isolation.h>
> +#include <linux/seqlock_types.h>
>  
>  #include <asm/cpu.h>
>  #include <asm/cputype.h>
> @@ -82,20 +84,54 @@ int __init parse_acpi_topology(void)
>  #undef pr_fmt
>  #define pr_fmt(fmt) "AMU: " fmt
>  
> +struct amu_counters {
> +	seqcount_t	seq;
> +	unsigned long	last_update;
> +	u64		core_cnt;
> +	u64		const_cnt;
> +	u64		delta_core_cnt;
> +	u64		delta_const_cnt;
> +};
It still might not be necessary to track both last taken sample and deltas from
previous ones, see[1].
I could send v3 of [1] and take into account the changes you have suggested here,
namely the last tick recorded. Otherwise few comments below.
> +
>  /*
>   * Ensure that amu_scale_freq_tick() will return SCHED_CAPACITY_SCALE until
>   * the CPU capacity and its associated frequency have been correctly
>   * initialized.
>   */
> -static DEFINE_PER_CPU_READ_MOSTLY(unsigned long, arch_max_freq_scale) =  1UL << (2 * SCHED_CAPACITY_SHIFT);
> -static DEFINE_PER_CPU(u64, arch_const_cycles_prev);
> -static DEFINE_PER_CPU(u64, arch_core_cycles_prev);
> +static DEFINE_PER_CPU_READ_MOSTLY(unsigned long, arch_max_freq_scale) =
> +	1UL << (2 * SCHED_CAPACITY_SHIFT);
> +static DEFINE_PER_CPU_SHARED_ALIGNED(struct amu_counters, cpu_samples) = {
> +	.seq = SEQCNT_ZERO(cpu_samples.seq)
> +};
>  static cpumask_var_t amu_fie_cpus;
>  
>  void update_freq_counters_refs(void)
>  {
> -	this_cpu_write(arch_core_cycles_prev, read_corecnt());
> -	this_cpu_write(arch_const_cycles_prev, read_constcnt());
> +	struct amu_counters *cpu_sample = this_cpu_ptr(&cpu_samples);
> +	u64 core_cnt, const_cnt, delta_core_cnt, delta_const_cnt;
> +
> +	const_cnt = read_constcnt();
> +	core_cnt = read_corecnt();
> +
> +	if (unlikely(core_cnt < cpu_sample->core_cnt) ||
> +	    unlikely(const_cnt < cpu_sample->const_cnt)) {
> +		WARN(1, "AMU counter values should be monotonic.\n");
> +		cpu_sample->delta_const_cnt = 0;
> +		cpu_sample->delta_core_cnt = 0;
Not sure if zero-ing is really necessary here
> +		return;
> +	}
> +
> +	delta_core_cnt = core_cnt - cpu_sample->core_cnt;
> +	delta_const_cnt = const_cnt - cpu_sample->const_cnt;
> +
> +	cpu_sample->core_cnt = core_cnt;
> +	cpu_sample->const_cnt = const_cnt;
> +
> +	raw_write_seqcount_begin(&cpu_sample->seq);
> +	cpu_sample->last_update = jiffies;
> +	cpu_sample->delta_const_cnt = delta_const_cnt;
> +	cpu_sample->delta_core_cnt = delta_core_cnt;
> +	raw_write_seqcount_end(&cpu_sample->seq);
>  }
>  
>  static inline bool freq_counters_valid(int cpu)
> @@ -108,8 +144,7 @@ static inline bool freq_counters_valid(int cpu)
>  		return false;
>  	}
>  
> -	if (unlikely(!per_cpu(arch_const_cycles_prev, cpu) ||
> -		     !per_cpu(arch_core_cycles_prev, cpu))) {
> +	if (unlikely(per_cpu_ptr(&cpu_samples, cpu) == NULL)) {
>  		pr_debug("CPU%d: cycle counters are not enabled.\n", cpu);
>  		return false;
>  	}
> @@ -152,19 +187,15 @@ void freq_inv_set_max_ratio(int cpu, u64 max_rate)
>  
>  static void amu_scale_freq_tick(void)
>  {
> -	u64 prev_core_cnt, prev_const_cnt;
> -	u64 core_cnt, const_cnt, scale;
> -
> -	prev_const_cnt = this_cpu_read(arch_const_cycles_prev);
> -	prev_core_cnt = this_cpu_read(arch_core_cycles_prev);
> +	struct amu_counters *cpu_sample = this_cpu_ptr(&cpu_samples);
> +	u64 delta_core_cnt, delta_const_cnt, scale;
>  
>  	update_freq_counters_refs();
>  
> -	const_cnt = this_cpu_read(arch_const_cycles_prev);
> -	core_cnt = this_cpu_read(arch_core_cycles_prev);
> +	delta_const_cnt = cpu_sample->delta_const_cnt;
> +	delta_core_cnt = cpu_sample->delta_core_cnt;
>  
> -	if (unlikely(core_cnt <= prev_core_cnt ||
> -		     const_cnt <= prev_const_cnt))
> +	if ((delta_const_cnt == 0) || (delta_core_cnt == 0))
>  		return;
>  
>  	/*
> @@ -175,15 +206,62 @@ static void amu_scale_freq_tick(void)
>  	 * See validate_cpu_freq_invariance_counters() for details on
>  	 * arch_max_freq_scale and the use of SCHED_CAPACITY_SHIFT.
>  	 */
> -	scale = core_cnt - prev_core_cnt;
> +	scale = delta_core_cnt;
>  	scale *= this_cpu_read(arch_max_freq_scale);
>  	scale = div64_u64(scale >> SCHED_CAPACITY_SHIFT,
> -			  const_cnt - prev_const_cnt);
> +			  delta_const_cnt);
>  
>  	scale = min_t(unsigned long, scale, SCHED_CAPACITY_SCALE);
>  	this_cpu_write(arch_freq_scale, (unsigned long)scale);
>  }
>  
> +/*
> + * Discard samples older than the define maximum sample age of 20ms. There
> + * is no point in sending IPIs in such a case. If the scheduler tick was
> + * not running then the CPU is either idle or isolated.
> + */
> +#define MAX_SAMPLE_AGE	((unsigned long)HZ / 50)
This depends on the config, so for HZ_1000 it will indeed give 20ms,
for CONFIG_250 that will be 5ms. It might be better to set it to number of
expected missed ticks instead ? Or amend the comment.
> +
> +unsigned int arch_freq_get_on_cpu(int cpu)
> +{
> +	struct amu_counters *cpu_sample = per_cpu_ptr(&cpu_samples, cpu);
> +	u64 delta_const_cnt, delta_core_cnt;
> +	unsigned int seq, freq;
> +	unsigned long last;
> +
> +	if (!freq_counters_valid(cpu))
> +		goto fallback;
> +
> +	do {
> +		seq = raw_read_seqcount_begin(&cpu_sample->seq);
> +		last = cpu_sample->last_update;
> +		delta_core_cnt = cpu_sample->delta_core_cnt;
> +		delta_const_cnt = cpu_sample->delta_const_cnt;
> +	} while (read_seqcount_retry(&cpu_sample->seq, seq));
> +
This seems to be taken from APERF/MPERF relevant code. Including the comments.
> +	/*
> +	 * Bail on invalid count and when the last update was too long ago,
> +	 * which covers idle and NOHZ full CPUs.
> +	 */
> +	if (!delta_const_cnt || ((jiffies - last) > MAX_SAMPLE_AGE)) {
Shouldn't the first condition (non-zero increase of cnt_cycles counter)
disqualify the sample taken altogether ?
> +		if (!(housekeeping_cpu(cpu, HK_TYPE_TICK) && idle_cpu(cpu)))
> +			goto fallback;
Not entirely convinced that this condition is what is expected ?
For housekeeping cpu that is not idle it will still resolve to AMU counters,
not sure if that's what was intended ?
Also, for cases when given cpufreq policy spans more than a single core, the
frequency might be queried based on relevant CPU that might have seen the tick
within specified timeframe (see [1])

> +	}
> +
> +	/*
> +	 * CPU frequency = reference perf (in Hz) * (/\ delivered) / (/\ reference)
> +	 * AMU reference performance counter increment rate is equal to the rate
> +	 * of increment of the System counter, CNTPCT_EL0 and can be used to
> +	 * compute the CPU frequency.
> +	 */
> +	return div64_u64((delta_core_cnt * (arch_timer_get_rate() / HZ)),
/HZ/HZ_PER_KHZ ?
> +			 delta_const_cnt);
> +
> +fallback:
> +	freq = cpufreq_quick_get(cpu);
> +	return freq ? freq : cpufreq_get_hw_max_freq(cpu);
If the arch specific code cannot determine the frequency it should actually make
it clear by returning '0' instead of trying to patch things up by itself (?)

Overall I'd prefer to revive [1] and amened it accordingly instead.

---
[1] https://lore.kernel.org/all/20231127160838.1403404-1-beata.michalska@xxxxxxx/
---
Best Regards
Beata
> +}
> +
>  static struct scale_freq_data amu_sfd = {
>  	.source = SCALE_FREQ_SOURCE_ARCH,
>  	.set_freq_scale = amu_scale_freq_tick,
> -- 
> 2.43.1
>