Messages in this thread | | | Subject | Re: [PATCH v2 6/6] arm64: use activity monitors for frequency invariance | From | Valentin Schneider <> | Date | Wed, 29 Jan 2020 17:13:53 +0000 |
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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)
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