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

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