Re: ACPI _CST introduced performance regresions on Haswll

["Rafael J. Wysocki" <rafael.j.wysocki@intel.com>]

Hi Mel,

On 10/6/2020 10:36 AM, Mel Gorman wrote:
> Hi Rafael,
>
> Numerous workload regressions have bisected repeatedly to the commit
> 6d4f08a6776 ("intel_idle: Use ACPI _CST on server systems") but only on
> a set of haswell machines that all have the same CPU.
>
> CPU(s):              48
> On-line CPU(s) list: 0-47
> Thread(s) per core:  2
> Core(s) per socket:  12
> Socket(s):           2
> NUMA node(s):        2
> Vendor ID:           GenuineIntel
> CPU family:          6
> Model:               63
> Model name:          Intel(R) Xeon(R) CPU E5-2670 v3 @ 2.30GHz
> Stepping:            2
> CPU MHz:             1200.359
> CPU max MHz:         3100.0000
> CPU min MHz:         1200.0000
>
> As well as being bisected in mainline, backporting the patch to a
> distribution kernel also showed the same type of problem so the patch
> is definitely suspicious. An example comparison showing the performance
> before CST was enabled and recent mainline kernels is as follow
>
> netperf UDP_STREAM
>                                        5.5.0              5.5.0-rc2              5.5.0-rc2                  5.6.0              5.9.0-rc8
>                                      vanilla      sle15-sp2-pre-cst   sle15-sp2-enable-cst                vanilla                vanilla
> Hmean     send-64         203.21 (   0.00%)      206.43 *   1.58%*      176.89 * -12.95%*      181.18 * -10.84%*      194.45 *  -4.31%*
> Hmean     send-128        401.40 (   0.00%)      414.19 *   3.19%*      355.84 * -11.35%*      364.13 *  -9.29%*      387.83 *  -3.38%*
> Hmean     send-256        786.69 (   0.00%)      799.70 (   1.65%)      700.65 * -10.94%*      719.82 *  -8.50%*      756.40 *  -3.85%*
> Hmean     send-1024      3059.57 (   0.00%)     3106.57 *   1.54%*     2659.62 * -13.07%*     2793.58 *  -8.69%*     3006.95 *  -1.72%*
> Hmean     send-2048      5976.66 (   0.00%)     6102.64 (   2.11%)     5249.34 * -12.17%*     5392.04 *  -9.78%*     5805.02 *  -2.87%*
> Hmean     send-3312      9145.09 (   0.00%)     9304.85 *   1.75%*     8197.25 * -10.36%*     8398.36 *  -8.17%*     9120.88 (  -0.26%)
> Hmean     send-4096     10871.63 (   0.00%)    11129.76 *   2.37%*     9667.68 * -11.07%*     9929.70 *  -8.66%*    10863.41 (  -0.08%)
> Hmean     send-8192     17747.35 (   0.00%)    17969.19 (   1.25%)    15652.91 * -11.80%*    16081.20 *  -9.39%*    17316.13 *  -2.43%*
> Hmean     send-16384    29187.16 (   0.00%)    29418.75 *   0.79%*    26296.64 *  -9.90%*    27028.18 *  -7.40%*    26941.26 *  -7.69%*
> Hmean     recv-64         203.21 (   0.00%)      206.43 *   1.58%*      176.89 * -12.95%*      181.18 * -10.84%*      194.45 *  -4.31%*
> Hmean     recv-128        401.40 (   0.00%)      414.19 *   3.19%*      355.84 * -11.35%*      364.13 *  -9.29%*      387.83 *  -3.38%*
> Hmean     recv-256        786.69 (   0.00%)      799.70 (   1.65%)      700.65 * -10.94%*      719.82 *  -8.50%*      756.40 *  -3.85%*
> Hmean     recv-1024      3059.57 (   0.00%)     3106.57 *   1.54%*     2659.62 * -13.07%*     2793.58 *  -8.69%*     3006.95 *  -1.72%*
> Hmean     recv-2048      5976.66 (   0.00%)     6102.64 (   2.11%)     5249.34 * -12.17%*     5392.00 *  -9.78%*     5805.02 *  -2.87%*
> Hmean     recv-3312      9145.09 (   0.00%)     9304.85 *   1.75%*     8197.25 * -10.36%*     8398.36 *  -8.17%*     9120.88 (  -0.26%)
> Hmean     recv-4096     10871.63 (   0.00%)    11129.76 *   2.37%*     9667.68 * -11.07%*     9929.70 *  -8.66%*    10863.38 (  -0.08%)
> Hmean     recv-8192     17747.35 (   0.00%)    17969.19 (   1.25%)    15652.91 * -11.80%*    16081.20 *  -9.39%*    17315.96 *  -2.43%*
> Hmean     recv-16384    29187.13 (   0.00%)    29418.72 *   0.79%*    26296.63 *  -9.90%*    27028.18 *  -7.40%*    26941.23 *  -7.69%*
>
> pre-cst is just before commit 6d4f08a6776 ("intel_idle: Use ACPI _CST on
> server systems") and enable-cst is the commit. It was not fixed by 5.6 or
> 5.9-rc8. A lot of bisections ended up here including kernel compilation,
> tbench, syscall entry/exit microbenchmark, hackbench, Java workloads etc.
>
> What I don't understand is why. The latencies for c-state exit states
> before and after the patch are both as follows
>
> /sys/devices/system/cpu/cpu0/cpuidle/state0/latency:0
> /sys/devices/system/cpu/cpu0/cpuidle/state1/latency:2
> /sys/devices/system/cpu/cpu0/cpuidle/state2/latency:10
> /sys/devices/system/cpu/cpu0/cpuidle/state3/latency:33
> /sys/devices/system/cpu/cpu0/cpuidle/state4/latency:133
>
> Perf profiles did not show up anything interesting. A diff of
> /sys/devices/system/cpu/cpu0/cpuidle/state0/ before and after the patch
> showed up nothing interesting. Any idea why exactly this patch shows up
> as being hazardous on Haswell in particular?
>
Presumably, some of the idle states are disabled by default on the 
affected machines.

Can you check the disable and default_status attributes of each state 
before and after the commit in question?

Cheers!