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
