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SubjectRe: ACPI _CST introduced performance regresions on Haswll
On Wed, Oct 07, 2020 at 05:45:30PM +0200, Rafael J. Wysocki wrote:
> > pre-cst is just before your patch
> > enable-cst is your patch that was bisected
> > enable-cst-no-hsx-acpi is your patch with use_acpi disabled
> > 5.9-rc8-vanilla is what it sounds like
> > 5.9-rc8-no-hsx-acpi disables use_acpi
> >
> > The enable-cst-no-hsx-acpi result indicates that use_acpi was the issue for
> > Haswell (at least these machines). Looking just at 5.9-rc8-vanillaa might
> > have been misleading because its performance is not far off the baseline
> > due to unrelated changes that mostly offset the performance penalty.
> >
> > The key question is -- how appropriate would it be to disable acpi for
> > Haswell? Would that be generally safe or could it hide other surprises?
> >
> It should be safe, but let's try to do something more fine-grained.
>
> There is the CPUIDLE_FLAG_ALWAYS_ENABLE flag that is set for C1E.  Can you
> please try to set it for C6 in hsw_cstates instead of clearing use_acpi in
> idle_cpu_hsx and retest?
>

Performance-wise, always enabling C6 helps but it may be specific to
this workload. Looking across all tested kernels I get;

netperf-udp
5.5.0 5.5.0-rc2 5.5.0-rc2 5.9.0-rc8 5.9.0-rc8 5.9.0-rc8
vanilla pre-cst enable-cst vanilla disable-acpi enable-c6
Hmean send-64 196.31 ( 0.00%) 208.56 * 6.24%* 181.15 * -7.72%* 199.84 * 1.80%* 235.09 * 19.76%* 234.79 * 19.60%*
Hmean send-128 391.75 ( 0.00%) 408.13 * 4.18%* 359.92 * -8.12%* 396.81 ( 1.29%) 469.44 * 19.83%* 465.55 * 18.84%*
Hmean send-256 776.38 ( 0.00%) 798.39 * 2.84%* 707.31 * -8.90%* 781.63 ( 0.68%) 917.19 * 18.14%* 905.06 * 16.57%*
Hmean send-1024 3019.64 ( 0.00%) 3099.00 * 2.63%* 2756.32 * -8.72%* 3017.06 ( -0.09%) 3509.84 * 16.23%* 3532.85 * 17.00%*
Hmean send-2048 5790.31 ( 0.00%) 6209.53 * 7.24%* 5394.42 * -6.84%* 5846.11 ( 0.96%) 6861.93 * 18.51%* 6852.08 * 18.34%*
Hmean send-3312 8909.98 ( 0.00%) 9483.92 * 6.44%* 8332.35 * -6.48%* 9047.52 * 1.54%* 10677.93 * 19.84%* 10509.41 * 17.95%*
Hmean send-4096 10517.63 ( 0.00%) 11044.19 * 5.01%* 9851.70 * -6.33%* 10914.24 * 3.77%* 12719.58 * 20.94%* 12731.06 * 21.04%*
Hmean send-8192 17355.48 ( 0.00%) 18344.50 * 5.70%* 15844.38 * -8.71%* 17690.46 ( 1.93%) 20777.97 * 19.72%* 20220.24 * 16.51%*
Hmean send-16384 28585.78 ( 0.00%) 28950.90 ( 1.28%) 25946.88 * -9.23%* 26643.69 * -6.79%* 30891.89 * 8.07%* 30701.46 * 7.40%*

The difference between always using ACPI and force enabling C6 is
negligible in this case but more on that later

netperf-udp
5.9.0-rc8 5.9.0-rc8
disable-acpi enable-c6
Hmean send-64 235.09 ( 0.00%) 234.79 ( -0.13%)
Hmean send-128 469.44 ( 0.00%) 465.55 ( -0.83%)
Hmean send-256 917.19 ( 0.00%) 905.06 ( -1.32%)
Hmean send-1024 3509.84 ( 0.00%) 3532.85 ( 0.66%)
Hmean send-2048 6861.93 ( 0.00%) 6852.08 ( -0.14%)
Hmean send-3312 10677.93 ( 0.00%) 10509.41 * -1.58%*
Hmean send-4096 12719.58 ( 0.00%) 12731.06 ( 0.09%)
Hmean send-8192 20777.97 ( 0.00%) 20220.24 * -2.68%*
Hmean send-16384 30891.89 ( 0.00%) 30701.46 ( -0.62%)

The default status and enabled states differ.

For 5.9-rc8 vanilla, the default and disabled status for cstates are

./5.9.0-rc8-vanilla/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state0/disable:0
./5.9.0-rc8-vanilla/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state1/disable:0
./5.9.0-rc8-vanilla/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state2/disable:0
./5.9.0-rc8-vanilla/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state3/disable:1
./5.9.0-rc8-vanilla/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state4/disable:1
./5.9.0-rc8-vanilla/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state0/default_status:enabled
./5.9.0-rc8-vanilla/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state1/default_status:enabled
./5.9.0-rc8-vanilla/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state2/default_status:enabled
./5.9.0-rc8-vanilla/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state3/default_status:disabled
./5.9.0-rc8-vanilla/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state4/default_status:disabled

For use_acpi == false, all c-states are enabled

./5.9.0-rc8-disable-acpi/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state0/disable:0
./5.9.0-rc8-disable-acpi/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state1/disable:0
./5.9.0-rc8-disable-acpi/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state2/disable:0
./5.9.0-rc8-disable-acpi/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state3/disable:0
./5.9.0-rc8-disable-acpi/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state4/disable:0
./5.9.0-rc8-disable-acpi/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state0/default_status:enabled
./5.9.0-rc8-disable-acpi/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state1/default_status:enabled
./5.9.0-rc8-disable-acpi/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state2/default_status:enabled
./5.9.0-rc8-disable-acpi/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state3/default_status:enabled
./5.9.0-rc8-disable-acpi/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state4/default_status:enabled

Force enabling C6

./5.9.0-rc8-enable-c6/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state0/disable:0
./5.9.0-rc8-enable-c6/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state1/disable:0
./5.9.0-rc8-enable-c6/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state2/disable:0
./5.9.0-rc8-enable-c6/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state3/disable:1
./5.9.0-rc8-enable-c6/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state4/disable:0
./5.9.0-rc8-enable-c6/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state0/default_status:enabled
./5.9.0-rc8-enable-c6/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state1/default_status:enabled
./5.9.0-rc8-enable-c6/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state2/default_status:enabled
./5.9.0-rc8-enable-c6/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state3/default_status:disabled
./5.9.0-rc8-enable-c6/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state4/default_status:enabled

Note that as expected, C3 remains disabled when only C6 is forced (state3
== c3, state4 == c6). While this particular workload does not appear to
care as it does not remain idle for long, the exit latency difference
between c3 and c6 is large so potentially a workload that idles for short
durations that are somewhere between c1e and c3 exit latency might take
a larger penalty exiting from c6 state if the deeper c-state is selected
for idling.

./5.9.0-rc8-enable-c6/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state0/residency:0
./5.9.0-rc8-enable-c6/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state1/residency:2
./5.9.0-rc8-enable-c6/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state2/residency:20
./5.9.0-rc8-enable-c6/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state3/residency:100
./5.9.0-rc8-enable-c6/iter-0/sys/devices/system/cpu/cpu0/cpuidle/state4/residency:400

--
Mel Gorman
SUSE Labs

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