[PATCH v3 1/2] powerpc/vcpu: Assume dedicated processors as non-preempt

[Waiman Long]

On 12/5/19 3:32 AM, Srikar Dronamraju wrote:
> With commit 247f2f6f3c70 ("sched/core: Don't schedule threads on pre-empted
> vCPUs"), scheduler avoids preempted vCPUs to schedule tasks on wakeup.
> This leads to wrong choice of CPU, which in-turn leads to larger wakeup
> latencies. Eventually, it leads to performance regression in latency
> sensitive benchmarks like soltp, schbench etc.
>
> On Powerpc, vcpu_is_preempted only looks at yield_count. If the
> yield_count is odd, the vCPU is assumed to be preempted. However
> yield_count is increased whenever LPAR enters CEDE state. So any CPU
> that has entered CEDE state is assumed to be preempted.
>
> Even if vCPU of dedicated LPAR is preempted/donated, it should have
> right of first-use since they are suppose to own the vCPU.
>
> On a Power9 System with 32 cores
>  # lscpu
> Architecture:        ppc64le
> Byte Order:          Little Endian
> CPU(s):              128
> On-line CPU(s) list: 0-127
> Thread(s) per core:  8
> Core(s) per socket:  1
> Socket(s):           16
> NUMA node(s):        2
> Model:               2.2 (pvr 004e 0202)
> Model name:          POWER9 (architected), altivec supported
> Hypervisor vendor:   pHyp
> Virtualization type: para
> L1d cache:           32K
> L1i cache:           32K
> L2 cache:            512K
> L3 cache:            10240K
> NUMA node0 CPU(s):   0-63
> NUMA node1 CPU(s):   64-127
>
>   # perf stat -a -r 5 ./schbench
> v5.4                                     v5.4 + patch
> Latency percentiles (usec)               Latency percentiles (usec)
> 	50.0000th: 45                    	50.0000th: 39
> 	75.0000th: 62                    	75.0000th: 53
> 	90.0000th: 71                    	90.0000th: 67
> 	95.0000th: 77                    	95.0000th: 76
> 	*99.0000th: 91                   	*99.0000th: 89
> 	99.5000th: 707                   	99.5000th: 93
> 	99.9000th: 6920                  	99.9000th: 118
> 	min=0, max=10048                 	min=0, max=211
> Latency percentiles (usec)               Latency percentiles (usec)
> 	50.0000th: 45                    	50.0000th: 34
> 	75.0000th: 61                    	75.0000th: 45
> 	90.0000th: 72                    	90.0000th: 53
> 	95.0000th: 79                    	95.0000th: 56
> 	*99.0000th: 691                  	*99.0000th: 61
> 	99.5000th: 3972                  	99.5000th: 63
> 	99.9000th: 8368                  	99.9000th: 78
> 	min=0, max=16606                 	min=0, max=228
> Latency percentiles (usec)               Latency percentiles (usec)
> 	50.0000th: 45                    	50.0000th: 34
> 	75.0000th: 61                    	75.0000th: 45
> 	90.0000th: 71                    	90.0000th: 53
> 	95.0000th: 77                    	95.0000th: 57
> 	*99.0000th: 106                  	*99.0000th: 63
> 	99.5000th: 2364                  	99.5000th: 68
> 	99.9000th: 7480                  	99.9000th: 100
> 	min=0, max=10001                 	min=0, max=134
> Latency percentiles (usec)               Latency percentiles (usec)
> 	50.0000th: 45                    	50.0000th: 34
> 	75.0000th: 62                    	75.0000th: 46
> 	90.0000th: 72                    	90.0000th: 53
> 	95.0000th: 78                    	95.0000th: 56
> 	*99.0000th: 93                   	*99.0000th: 61
> 	99.5000th: 108                   	99.5000th: 64
> 	99.9000th: 6792                  	99.9000th: 85
> 	min=0, max=17681                 	min=0, max=121
> Latency percentiles (usec)               Latency percentiles (usec)
> 	50.0000th: 46                    	50.0000th: 33
> 	75.0000th: 62                    	75.0000th: 44
> 	90.0000th: 73                    	90.0000th: 51
> 	95.0000th: 79                    	95.0000th: 54
> 	*99.0000th: 113                  	*99.0000th: 61
> 	99.5000th: 2724                  	99.5000th: 64
> 	99.9000th: 6184                  	99.9000th: 82
> 	min=0, max=9887                  	min=0, max=121
>
>  Performance counter stats for 'system wide' (5 runs):
>
> context-switches    43,373  ( +-  0.40% )   44,597 ( +-  0.55% )
> cpu-migrations       1,211  ( +-  5.04% )      220 ( +-  6.23% )
> page-faults         15,983  ( +-  5.21% )   15,360 ( +-  3.38% )
>
> Waiman Long suggested using static_keys.

Since this patch is fixing a performance regression. Maybe we should add

Fixes: 41946c86876e ("locking/core, powerpc: Implement
vcpu_is_preempted(cpu)")

Cheers,
Longman