Re: [RFC 2/2] selftest/cpuidle: Add support for cpuidle latency measurement

Pratik Sampat
2021-03-22T13:11:37Z

Hi Doug,

On 20/03/21 8:34 pm, Doug Smythies wrote:
> On Wed, Mar 17, 2021 at 11:44 PM Pratik Sampat <psampat@linux.ibm.com> wrote:
>> Hi Doug,
>> Thanks for trying these patches out.
>>
>> On 18/03/21 2:30 am, Doug Smythies wrote:
>>> Hi Pratik,
>>>
>>> It just so happens that I have been trying Artem's version this last
>>> week, so I tried yours.
>>>
>>> On Mon, Mar 15, 2021 at 4:49 AM Pratik Rajesh Sampat
>>> <psampat@linux.ibm.com> wrote:
>>> ...
> ...
>>> Other notes:
>>>
>>> No idle state for CPU 0 ever gets disabled.
>>> I assume this is because CPU 0 can never be offline,
>>> so that bit of code (Disable all stop states) doesn't find its state.
>>> By the way, processor = Intel i5-9600K
>> I had tried these patches on an IBM POWER 9 processor and disabling CPU0's idle
>> state works there. However, it does make sense for some processors to treat CPU
>> 0 differently.
>> Maybe I could write in a case if idle state disabling fails for a CPU then we
>> just skip it?
> I didn't try it, I just did a hack so I could continue for this reply.

Sure. In subsequent version I could write something to cleanly handle online
fail checks, maybe even specifically for CPU0.

>>> The system is left with all idle states disabled, well not for CPU 0
>>> as per the above comment. The suggestion is to restore them,
>>> otherwise my processor hogs 42 watts instead of 2.
>>>
>>> My results are highly variable per test.
>> Question: Do you notice high variability with IPI test, Timer test or both?
> The IPI test has less variability than the Timer test.
>
>> I can think of two reasons for high run to run variance:
>>
>> 1. If you observe variance in timer tests, then I believe there could a
>> mechanism of "C-state pre-wake" on some Intel machines at play here, which can
>> pre-wake a CPU from an idle state when timers are armed. I'm not sure if the
>> Intel platform that you're running on does that or not.
>>
>> Artem had described this behavior to me a while ago and I think his wult page
>> describes this behavior in more detail:
>> https://intel.github.io/wult/#c-state-pre-wake
> Yes, I have reviewed all the references.
> And yes, I think my processors have the pre-wake stuff.
>
> I do not have the proper hardware to do the Artem pre-wake workaround
> method, but might buy it in future.

That explains the variability that we are seeing in the Timer tests on the
Intel processor you've tried on.
Also based on the data pasted below, it means that the IPI tests are more
reliable than Timers.

Maybe it would be better to not run the Timer test on Intel platforms that
support this pre-wakeup feature?
However, I don't know how (or if) Intel exposes this information to the
userspace and if other platforms like AMD also have this feature in some form.

Another way of solving this problem could be to have the timer test as an
optional parameter in the selftest for people to use while also printing a
disclaimer for x86 users about the potential hardware design?

--
Thanks
Pratik

>> 2. I have noticed variability in results when there are kernel book-keeping or
>> jitter tasks scheduled from time to time on an otherwise idle core.
>> In the full per-CPU logs at tools/testing/selftests/cpuidle/cpuidle.log can you
>> spot any obvious outliers per-CPU state?
> Yes.
> I'll just paste in an example cpuidle.log file having used the -v option
> below, along with my hack job diff.
>
> doug@s19:~/temp-k-git/linux/tools/testing/selftests/cpuidle$ cat
> cpuidle.log.v3-1
> --IPI Latency Test---
> --Baseline IPI Latency measurement: CPU Busy--
> SRC_CPU DEST_CPU IPI_Latency(ns)
> 0 0 140
> 0 1 632
> 0 2 675
> 0 3 671
> 0 4 675
> 0 5 767
> 0 6 653
> 0 7 826
> 0 8 819
> 0 9 615
> 0 10 758
> 0 11 758
> Baseline Avg IPI latency(ns): 665
> ---Enabling state: 0---
> SRC_CPU DEST_CPU IPI_Latency(ns)
> 0 0 76
> 0 1 484
> 0 2 494
> 0 3 539
> 0 4 498
> 0 5 491
> 0 6 474
> 0 7 434
> 0 8 544
> 0 9 476
> 0 10 447
> 0 11 467
> Expected IPI latency(ns): 0
> Observed Avg IPI latency(ns) - State 0: 452
> ---Enabling state: 1---
> SRC_CPU DEST_CPU IPI_Latency(ns)
> 0 0 72
> 0 1 1081
> 0 2 821
> 0 3 1486
> 0 4 1022
> 0 5 960
> 0 6 1634
> 0 7 933
> 0 8 1032
> 0 9 1046
> 0 10 1430
> 0 11 1338
> Expected IPI latency(ns): 1000
> Observed Avg IPI latency(ns) - State 1: 1071
> ---Enabling state: 2---
> SRC_CPU DEST_CPU IPI_Latency(ns)
> 0 0 264
> 0 1 30836
> 0 2 30562
> 0 3 30748
> 0 4 35286
> 0 5 30978
> 0 6 1952
> 0 7 36066
> 0 8 30670
> 0 9 30605
> 0 10 30635
> 0 11 35423
> Expected IPI latency(ns): 120000
> Observed Avg IPI latency(ns) - State 2: 27002
> ---Enabling state: 3---
> SRC_CPU DEST_CPU IPI_Latency(ns)
> 0 0 71
> 0 1 30853
> 0 2 32095
> 0 3 32661
> 0 4 30230
> 0 5 34348
> 0 6 2012
> 0 7 30816
> 0 8 30908
> 0 9 31130
> 0 10 34150
> 0 11 32050
> Expected IPI latency(ns): 1034000
> Observed Avg IPI latency(ns) - State 3: 26777
>
> --Timeout Latency Test--
> --Baseline Timeout Latency measurement: CPU Busy--
> Wakeup_src Baseline_delay(ns)
> 0 453
> 1 568
> 2 387
> 3 337
> 4 433
> 5 579
> 6 330
> 7 400
> 8 561
> 9 544
> 10 569
> 11 523
> Baseline Avg timeout diff(ns): 473
> ---Enabling state: 0---
> Wakeup_src Baseline_delay(ns) Delay(ns)
> 0 399
> 1 388
> 2 352
> 3 385
> 4 334
> 5 415
> 6 320
> 7 356
> 8 401
> 9 379
> 10 339
> 11 384
> Expected timeout(ns): 200
> Observed Avg timeout diff(ns) - State 0: 371
> ---Enabling state: 1---
> Wakeup_src Baseline_delay(ns) Delay(ns)
> 0 666
> 1 575
> 2 608
> 3 590
> 4 608
> 5 552
> 6 582
> 7 593
> 8 597
> 9 587
> 10 588
> 11 610
> Expected timeout(ns): 1200
> Observed Avg timeout diff(ns) - State 1: 596
> ---Enabling state: 2---
> Wakeup_src Baseline_delay(ns) Delay(ns)
> 0 36386
> 1 1069
> 2 866
> 3 884
> 4 850
> 5 55642
> 6 408082
> 7 1184
> 8 406075
> 9 406830
> 10 414105
> 11 406594
> Expected timeout(ns): 360200
> Observed Avg timeout diff(ns) - State 2: 178213
> ---Enabling state: 3---
> Wakeup_src Baseline_delay(ns) Delay(ns)
> 0 406049
> 1 913
> 2 410134
> 3 921
> 4 406237
> 5 950
> 6 407181
> 7 920
> 8 407678
> 9 894
> 10 406320
> 11 304161
> Expected timeout(ns): 3102200
> Observed Avg timeout diff(ns) - State 3: 229363
>
>
> My hack job, (CPUs always online):
>
> diff --git a/tools/testing/selftests/cpuidle/cpuidle.sh
> b/tools/testing/selftests/cpuidle/cpuidle.sh
> index de5141d5b76b..70bdacda5e91 100755
> --- a/tools/testing/selftests/cpuidle/cpuidle.sh
> +++ b/tools/testing/selftests/cpuidle/cpuidle.sh
> @@ -86,10 +86,6 @@ disable_idle()
> {
> for ((cpu=0; cpu<NUM_CPUS; cpu++))
> do
> - local cpu_status=$(cpu_is_online $cpu)
> - if [ $cpu_status == 0 ]; then
> - continue
> - fi
> for ((state=0; state<NUM_STATES; state++))
> do
> echo 1 >
> /sys/devices/system/cpu/cpu$cpu/cpuidle/state$state/disable
> @@ -104,10 +100,6 @@ op_state()
> {
> for ((cpu=0; cpu<NUM_CPUS; cpu++))
> do
> - local cpu_status=$(cpu_is_online $cpu)
> - if [ $cpu_status == 0 ]; then
> - continue
> - fi
> echo $1 >
> /sys/devices/system/cpu/cpu$cpu/cpuidle/state$2/disable
> done
> }
> @@ -124,17 +116,6 @@ cpuidle_disable_state()
> op_state 1 $state
> }
>
> -cpu_is_online()
> -{
> - cpu=$1
> - if [ ! -f "/sys/devices/system/cpu/cpu$cpu/online" ]; then
> - echo 0
> - return
> - fi
> - status=$(cat /sys/devices/system/cpu/cpu$cpu/online)
> - echo $status
> -}
> -
> # Extract latency in microseconds and convert to nanoseconds
> extract_latency()
> {
> @@ -179,10 +160,6 @@ run_ipi_tests()
> printf "%s %10s %12s\n" "SRC_CPU" "DEST_CPU" "IPI_Latency(ns)" >> $LOG
> for ((cpu=0; cpu<NUM_CPUS; cpu+=SMT))
> do
> - local cpu_status=$(cpu_is_online $cpu)
> - if [ $cpu_status == 0 ]; then
> - continue
> - fi
> ipi_test_once "baseline" $cpu
> printf "%-3s %10s %12s\n" $src_cpu $cpu $ipi_latency >> $LOG
> avg_arr+=($ipi_latency)
> @@ -198,10 +175,6 @@ run_ipi_tests()
> printf "%s %10s %12s\n" "SRC_CPU" "DEST_CPU"
> "IPI_Latency(ns)" >> $LOG
> for ((cpu=0; cpu<NUM_CPUS; cpu+=SMT))
> do
> - local cpu_status=$(cpu_is_online $cpu)
> - if [ $cpu_status == 0 ]; then
> - continue
> - fi
> # Running IPI test and logging results
> sleep 1
> ipi_test_once "test" $cpu
> @@ -262,10 +235,6 @@ run_timeout_tests()
> printf "%s %10s %10s\n" "Wakeup_src" "Baseline_delay(ns)">> $LOG
> for ((cpu=0; cpu<NUM_CPUS; cpu+=SMT))
> do
> - local cpu_status=$(cpu_is_online $cpu)
> - if [ $cpu_status == 0 ]; then
> - continue
> - fi
> timeout_test_once "baseline" $cpu 1000000
> printf "%-3s %13s\n" $src_cpu $timeout_diff >> $LOG
> avg_arr+=($timeout_diff)
> @@ -281,10 +250,6 @@ run_timeout_tests()
> printf "%s %10s %10s\n" "Wakeup_src"
> "Baseline_delay(ns)" "Delay(ns)" >> $LOG
> for ((cpu=0; cpu<NUM_CPUS; cpu+=SMT))
> do
> - local cpu_status=$(cpu_is_online $cpu)
> - if [ $cpu_status == 0 ]; then
> - continue
> - fi
> timeout_test_once "test" $cpu 1000000
> printf "%-3s %13s %18s\n" $src_cpu
> $baseline_timeout_diff $timeout_diff >> $LOG
> avg_arr+=($timeout_diff)
> @@ -314,3 +279,7 @@ run_timeout_tests
> printf "Removing $MODULE module\n"
> printf "Full Output logged at: $LOG\n"
> rmmod $MODULE
> +
> +printf "enabling idle states\n"
> +
> +echo 0 | tee /sys/devices/system/cpu/cpu*/cpuidle/state*/disable
> (END)