cpufreq: Make drivers using CPUFREQ_ETERNAL specify transition latency

Commit a755d0e2d41b ("cpufreq: Honour transition_latency over transition_delay_us") caused platforms where cpuinfo.transition_latency is CPUFREQ_ETERNAL to get a very large transition latency whereas previously it had been capped at 10 ms (and later at 2 ms). This led to a user-observable regression between 6.6 and 6.12 as described by Shawn: "The dbs sampling_rate was 10000 us on 6.6 and suddently becomes 6442450 us (4294967295 / 1000 * 1.5) on 6.12 for these platforms because the default transition delay was dropped [...]. It slows down dbs governor's reacting to CPU loading change dramatically. Also, as transition_delay_us is used by schedutil governor as rate_limit_us, it shows a negative impact on device idle power consumption, because the device gets slightly less time in the lowest OPP." Evidently, the expectation of the drivers using CPUFREQ_ETERNAL as cpuinfo.transition_latency was that it would be capped by the core, but they may as well return a default transition latency value instead of CPUFREQ_ETERNAL and the core need not do anything with it. Accordingly, introduce CPUFREQ_DEFAULT_TRANSITION_LATENCY_NS and make all of the drivers in question use it instead of CPUFREQ_ETERNAL. Also update the related Rust binding.

Rafael J. Wysocki committed 1 year ago

Commit f97aef01 parent d3f8f8d9 files · +14 −10Browse files

Showing the changes to rust/kernel (1 of 9 files). Show all

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rust/kernel/cpufreq.rs+4−3

@@ -39,7 +39,8 @@ use macros::vtable; · 38 unmodified lines
39	39	const CPUFREQ_NAME_LEN: usize = bindings::CPUFREQ_NAME_LEN as usize;
40	40
41	41	/// Default transition latency value in nanoseconds.
42		−pub const ETERNAL_LATENCY_NS: u32 = bindings::CPUFREQ_ETERNAL as u32;
	42	+pub const DEFAULT_TRANSITION_LATENCY_NS: u32 =
	43	+ bindings::CPUFREQ_DEFAULT_TRANSITION_LATENCY_NS;
43	44
44	45	/// CPU frequency driver flags.
45	46	pub mod flags {
@@ -400,13 +401,13 @@ impl TableBuilder { · 354 unmodified lines
400	401	/// The following example demonstrates how to create a CPU frequency table.
401	402	///
402	403	/// ```
403		−/// use kernel::cpufreq::{ETERNAL_LATENCY_NS, Policy};
	404	+/// use kernel::cpufreq::{DEFAULT_TRANSITION_LATENCY_NS, Policy};
404	405	///
405	406	/// fn update_policy(policy: &mut Policy) {
406	407	/// policy
407	408	/// .set_dvfs_possible_from_any_cpu(true)
408	409	/// .set_fast_switch_possible(true)
409		−/// .set_transition_latency_ns(ETERNAL_LATENCY_NS);
	410	+/// .set_transition_latency_ns(DEFAULT_TRANSITION_LATENCY_NS);
410	411	///
411	412	/// pr_info!("The policy details are: {:?}\n", (policy.cpu(), policy.cur()));
412	413	/// }