linux-kernel - Re: Re: [PATCH v1 2/3] arm64: idle: Cache AMU counters before entering idle

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Date: Mon, 11 Mar 2024 11:27:27 -0700
From: Vanshidhar Konda <vanshikonda@...amperecomputing.com>
To: "lihuisong (C)" <lihuisong@...wei.com>
Cc: Beata Michalska <beata.michalska@....com>, 
	Ionela Voinescu <ionela.voinescu@....com>, linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, rafael@...nel.org, sumitg@...dia.com, zengheng4@...wei.com, 
	yang@...amperecomputing.com, will@...nel.org, sudeep.holla@....com, liuyonglong@...wei.com, 
	zhanjie9@...ilicon.com, linux-acpi@...r.kernel.org
Subject: Re: Re: [PATCH v1 2/3] arm64: idle: Cache AMU counters before
 entering idle

On Thu, Mar 07, 2024 at 11:17:26AM +0800, lihuisong (C) wrote:
>
>在 2024/3/1 0:25, Vanshidhar Konda 写道:
>>AMU counters do not increment while a CPU is in idle. Saving the value
>>of the core and constant counters prior to invoking WFI allows FIE to
>>compute the frequency of a CPU that is idle.
>>
>>Signed-off-by: Vanshidhar Konda <vanshikonda@...amperecomputing.com>
>>---
>>  arch/arm64/kernel/idle.c     | 10 ++++++++++
>>  arch/arm64/kernel/topology.c | 14 ++++++++------
>>  2 files changed, 18 insertions(+), 6 deletions(-)
>>
>>diff --git a/arch/arm64/kernel/idle.c b/arch/arm64/kernel/idle.c
>>index 05cfb347ec26..5ed2e57188a8 100644
>>--- a/arch/arm64/kernel/idle.c
>>+++ b/arch/arm64/kernel/idle.c
>>@@ -26,6 +26,16 @@ void __cpuidle cpu_do_idle(void)
>>  	arm_cpuidle_save_irq_context(&context);
>>+#ifdef CONFIG_ARM64_AMU_EXTN
>>+	/* Update the AMU counters before entering WFI. The cached AMU counter
>>+	 * value is used to determine CPU frequency while the CPU is idle
>>+	 * without needing to wake up the CPU.
>>+	 */
>>+
>>+	if (cpu_has_amu_feat(smp_processor_id()))
>>+		update_freq_counters_refs();
>>+#endif
>The below point I has mentioned in [1].
>This is just for the WFI state.
>What about other deeper idle states, like retention and power down?
>The path to enter idle state is different for them. We should do this 
>for all idle states.
>

Yes. That makes sense. I'll account for them in the next version of the
patch. I'll work on the next version of the patch based on the updated
patch from @Beata.

Thanks,
Vanshi

>>+
>>  	dsb(sy);
>>  	wfi();
>>diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
>>index db8d14525cf4..8905eb0c681f 100644
>>--- a/arch/arm64/kernel/topology.c
>>+++ b/arch/arm64/kernel/topology.c
>>@@ -240,13 +240,15 @@ unsigned int arch_freq_get_on_cpu(int cpu)
>>  	} while (read_seqcount_retry(&cpu_sample->seq, seq));
>>  	/*
>>-	 * Bail on invalid count and when the last update was too long ago,
>>-	 * which covers idle and NOHZ full CPUs.
>>+	 * Bail on invalid count and when the last update was too long ago.
>>+	 * This covers idle, NOHZ full and isolated CPUs.
>>+	 *
>>+	 * Idle CPUs don't need to be measured because AMU counters stop
>>+	 * incrementing during WFI/WFE.
>>  	 */
>>-	if (!delta_const_cnt || ((jiffies - last) > MAX_SAMPLE_AGE)) {
>>-		if (!(housekeeping_cpu(cpu, HK_TYPE_TICK) && idle_cpu(cpu)))
>>-			goto fallback;
>>-	}
>>+	if (!delta_const_cnt ||
>>+	    ((jiffies - last) > MAX_SAMPLE_AGE && !idle_cpu(cpu)))
>>+		goto fallback;
>>  	/*
>>  	 * CPU frequency = reference perf (in Hz) * (/\ delivered) / (/\ reference)
>[1] https://lore.kernel.org/linux-arm-kernel/20231212072617.14756-1-lihuisong@huawei.com/
>