Subject: Re: [PATCH v2 2/2] selftest/cpuidle: Add support for cpuidle latency measurement

From: Gautham R Shenoy <[email-masked]>
Date: Mon, 20 Jul 2020 11:22:42 +0530

Hi Pratik,

On Fri, Jul 17, 2020 at 02:48:01PM +0530, Pratik Rajesh Sampat wrote:
> This patch adds support to trace IPI based and timer based wakeup
> latency from idle states
> 
> Latches onto the test-cpuidle_latency kernel module using the debugfs
> interface to send IPIs or schedule a timer based event, which in-turn
> populates the debugfs with the latency measurements.
> 
> Currently for the IPI and timer tests; first disable all idle states
> and then test for latency measurements incrementally enabling each state
> 
> Signed-off-by: Pratik Rajesh Sampat <[email-masked]>

A few comments below.

> ---
>  tools/testing/selftests/Makefile           |   1 +
>  tools/testing/selftests/cpuidle/Makefile   |   6 +
>  tools/testing/selftests/cpuidle/cpuidle.sh | 257 +++++++++++++++++++++
>  tools/testing/selftests/cpuidle/settings   |   1 +
>  4 files changed, 265 insertions(+)
>  create mode 100644 tools/testing/selftests/cpuidle/Makefile
>  create mode 100755 tools/testing/selftests/cpuidle/cpuidle.sh
>  create mode 100644 tools/testing/selftests/cpuidle/settings
> 

[..skip..]

> +
> +ins_mod()
> +{
> +	if [ ! -f "$MODULE" ]; then
> +		printf "$MODULE module does not exist. Exitting\n"

If the module has been compiled into the kernel (due to a
localyesconfig, for instance), then it is unlikely that we will find
it in /lib/modules. Perhaps you want to check if the debugfs
directories created by the module exist, and if so, print a message
saying that the modules is already loaded or some such?

> +		exit $ksft_skip
> +	fi
> +	printf "Inserting $MODULE module\n\n"
> +	insmod $MODULE
> +	if [ $? != 0 ]; then
> +		printf "Insmod $MODULE failed\n"
> +		exit $ksft_skip
> +	fi
> +}
> +
> +compute_average()
> +{
> +	arr=("$@")
> +	sum=0
> +	size=${#arr[@]}
> +	for i in "${arr[@]}"
> +	do
> +		sum=$((sum + i))
> +	done
> +	avg=$((sum/size))

It would be good to assert that "size" isn't 0 here.

> +}
> +
> +# Disable all stop states
> +disable_idle()
> +{
> +	for ((cpu=0; cpu<NUM_CPUS; cpu++))
> +	do
> +		for ((state=0; state<NUM_STATES; state++))
> +		do
> +			echo 1 > /sys/devices/system/cpu/cpu$cpu/cpuidle/state$state/disable

So, on offlined CPUs, we won't see
/sys/devices/system/cpu/cpu$cpu/cpuidle/state$state directory. You
should probably perform this operation only on online CPUs.

> +		done
> +	done
> +}
> +
> +# Perform operation on each CPU for the given state
> +# $1 - Operation: enable (0) / disable (1)
> +# $2 - State to enable
> +op_state()
> +{
> +	for ((cpu=0; cpu<NUM_CPUS; cpu++))
> +	do
> +		echo $1 > /sys/devices/system/cpu/cpu$cpu/cpuidle/state$2/disable

Ditto

> +	done
> +}

This is a helper function. For better readability of the main code you
can define the following wrappers and use them.

cpuidle_enable_state()
{
	state=$1
	op_state 1 $state
}

cpuidle_disable_state()
{
	state=$1
	op_state 0 $state

}

> +

[..snip..]

> +run_ipi_tests()
> +{
> +        extract_latency
> +        disable_idle
> +        declare -a avg_arr
> +        echo -e "--IPI Latency Test---" >> $LOG
> +
> +		echo -e "--Baseline IPI Latency measurement: CPU Busy--" >> $LOG
> +		printf "%s %10s %12s\n" "SRC_CPU" "DEST_CPU" "IPI_Latency(ns)" >> $LOG
> +		for ((cpu=0; cpu<NUM_CPUS; cpu++))
> +		do
> +			ipi_test_once "baseline" $cpu
> +			printf "%-3s %10s %12s\n" $src_cpu $cpu $ipi_latency >> $LOG
> +			avg_arr+=($ipi_latency)
> +		done
> +		compute_average "${avg_arr[@]}"
> +		echo -e "Baseline Average IPI latency(ns): $avg" >> $LOG
> +
> +        for ((state=0; state<NUM_STATES; state++))
> +        do
> +			unset avg_arr
> +			echo -e "---Enabling state: $state---" >> $LOG
> +			op_state 0 $state
> +			printf "%s %10s %12s\n" "SRC_CPU" "DEST_CPU" "IPI_Latency(ns)" >> $LOG
> +			for ((cpu=0; cpu<NUM_CPUS; cpu++))
> +			do

If a CPU is offline, then we should skip it here.

> +				# Running IPI test and logging results
> +				sleep 1
> +				ipi_test_once "test" $cpu
> +				printf "%-3s %10s %12s\n" $src_cpu $cpu $ipi_latency >> $LOG
> +				avg_arr+=($ipi_latency)
> +			done
> +			compute_average "${avg_arr[@]}"
> +			echo -e "Expected IPI latency(ns): ${latency_arr[$state]}" >> $LOG
> +			echo -e "Observed Average IPI latency(ns): $avg" >> $LOG
> +			op_state 1 $state
> +        done
> +}
> +
> +# Extract the residency in microseconds and convert to nanoseconds.
> +# Add 100 ns so that the timer stays for a little longer than the residency
> +extract_residency()
> +{
> +	for ((state=0; state<NUM_STATES; state++))
> +	do
> +		residency=$(($(cat /sys/devices/system/cpu/cpu0/cpuidle/state$state/residency) * 1000 + 200))
> +		residency_arr+=($residency)
> +	done
> +}
> +
> +# Run the Timeout test
> +# $1 run for baseline - busy cpu or regular environment
> +# $2 destination cpu
> +# $3 timeout
> +timeout_test_once()
> +{
> +	dest_cpu=$2
> +	if [ "$1" = "baseline" ]; then
> +		# Keep the CPU busy
> +		taskset -c $dest_cpu cat /dev/random > /dev/null &
> +		task_pid=$!
> +		# Wait for the workload to achieve 100% CPU usage
> +		sleep 1
> +	fi
> +	taskset -c $dest_cpu echo $3 > /sys/kernel/debug/latency_test/timeout_expected_ns
> +	# Wait for the result to populate
> +	sleep 0.1
> +	timeout_diff=$(cat /sys/kernel/debug/latency_test/timeout_diff_ns)
> +	src_cpu=$(cat /sys/kernel/debug/latency_test/timeout_cpu_src)
> +	if [ "$1" = "baseline" ]; then
> +		kill $task_pid
> +		wait $task_pid 2>/dev/null
> +	fi
> +}
> +
> +run_timeout_tests()
> +{
> +	extract_residency
> +	disable_idle
> +	declare -a avg_arr
> +	echo -e "\n--Timeout Latency Test--" >> $LOG
> +
> +	echo -e "--Baseline Timeout Latency measurement: CPU Busy--" >> $LOG
> +	printf "%s %10s %10s\n" "Wakeup_src" "Baseline_delay(ns)">> $LOG
> +	for ((cpu=0; cpu<NUM_CPUS; cpu++))
> +	do

Again only perform this on online CPUs.

> +		timeout_test_once "baseline" $cpu ${residency_arr[0]}
> +		printf "%-3s %13s\n" $src_cpu $timeout_diff >> $LOG
> +		avg_arr+=($timeout_diff)
> +	done
> +	compute_average "${avg_arr[@]}"
> +	echo -e "Baseline Average timeout diff(ns): $avg" >> $LOG
> +
> +	for ((state=0; state<NUM_STATES; state++))
> +	do

> +		echo -e "---Enabling state: $state---" >> $LOG
> +		op_state 0 $state
> +		printf "%s %10s %10s\n" "Wakeup_src" "Baseline_delay(ns)" "Delay(ns)" >> $LOG
> +		unset avg_arr
> +		for ((cpu=0; cpu<NUM_CPUS; cpu++))
> +		do

Ditto.

> +			timeout_test_once "test" $cpu ${residency_arr[$state]}
> +			printf "%-3s %13s %18s\n" $src_cpu $baseline_timeout_diff $timeout_diff >> $LOG
> +			avg_arr+=($timeout_diff)
> +		done
> +		compute_average "${avg_arr[@]}"
> +		echo -e "Expected timeout(ns): ${residency_arr[$state]}" >> $LOG
> +		echo -e "Observed Average timeout diff(ns): $avg" >> $LOG
> +		op_state 1 $state
> +	done
> +}
> +

--
Thanks and Regards
gautham.