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

From: Gautham R Shenoy <[email-masked]>
Date: Thu, 5 Dec 2019 10:37:54 +0530

Hi Srikar,

On Wed, Dec 04, 2019 at 07:14:58PM +0530, 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.

The regression in the latency sensitive benchmarks is due to
preferring potentially busy vCPUs over vCPUs in the CEDE state.

> 
> 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)
> 	49.0000th: 47                           	50.0000th: 33
> 	74.0000th: 64                           	75.0000th: 44
> 	89.0000th: 76                           	90.0000th: 50
> 	94.0000th: 83                           	95.0000th: 53
> 	*98.0000th: 103                         	*99.0000th: 57
> 	98.5000th: 2124                         	99.5000th: 59
> 	98.9000th: 7976                         	99.9000th: 83
> 	min=-1, max=10519                       	min=0, max=117
> Latency percentiles (usec)                      Latency percentiles (usec)
> 	49.0000th: 45                           	50.0000th: 34
> 	74.0000th: 61                           	75.0000th: 45
> 	89.0000th: 70                           	90.0000th: 52
> 	94.0000th: 77                           	95.0000th: 56
> 	*98.0000th: 504                         	*99.0000th: 62
> 	98.5000th: 4012                         	99.5000th: 64
> 	98.9000th: 8168                         	99.9000th: 79
> 	min=-1, max=14500                       	min=0, max=123
> Latency percentiles (usec)                      Latency percentiles (usec)
> 	49.0000th: 48                           	50.0000th: 35
> 	74.0000th: 65                           	75.0000th: 47
> 	89.0000th: 76                           	90.0000th: 55
> 	94.0000th: 82                           	95.0000th: 59
> 	*98.0000th: 1098                        	*99.0000th: 67
> 	98.5000th: 3988                         	99.5000th: 71
> 	98.9000th: 9360                         	99.9000th: 98
> 	min=-1, max=19283                       	min=0, max=137
> Latency percentiles (usec)                      Latency percentiles (usec)
> 	49.0000th: 46                           	50.0000th: 35
> 	74.0000th: 63                           	75.0000th: 46
> 	89.0000th: 73                           	90.0000th: 53
> 	94.0000th: 78                           	95.0000th: 57
> 	*98.0000th: 113                         	*99.0000th: 63
> 	98.5000th: 2316                         	99.5000th: 65
> 	98.9000th: 7704                         	99.9000th: 83
> 	min=-1, max=17976                       	min=0, max=139
> Latency percentiles (usec)                      Latency percentiles (usec)
> 	49.0000th: 46                           	50.0000th: 34
> 	74.0000th: 62                           	75.0000th: 46
> 	89.0000th: 73                           	90.0000th: 53
> 	94.0000th: 79                           	95.0000th: 57
> 	*98.0000th: 97                          	*99.0000th: 64
> 	98.5000th: 1398                         	99.5000th: 70
> 	98.9000th: 8136                         	99.9000th: 100
> 	min=-1, max=10008                       	min=0, max=142
> 
> Performance counter stats for 'system wide' (4 runs):
> 
> context-switches       42,604 ( +-  0.87% )       45,397 ( +-  0.25% )
> cpu-migrations          0,195 ( +-  2.70% )          230 ( +-  7.23% )
> page-faults            16,783 ( +- 14.87% )       16,781 ( +-  9.77% )
> 
> Waiman Long suggested using static_keys.

This needs to be Cc'ed to the stable kernel 4.18+

Reviewed-by: Gautham R. Shenoy <[email-masked]>

> 
> Reported-by: Parth Shah <[email-masked]>
> Reported-by: Ihor Pasichnyk <[email-masked]>
> Cc: Parth Shah <[email-masked]>
> Cc: Ihor Pasichnyk <[email-masked]>
> Cc: Juri Lelli <[email-masked]>
> Cc: Waiman Long <[email-masked]>
> Signed-off-by: Srikar Dronamraju <[email-masked]>
> ---
>  arch/powerpc/include/asm/spinlock.h | 5 +++--
>  arch/powerpc/mm/numa.c              | 4 ++++
>  2 files changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/spinlock.h b/arch/powerpc/include/asm/spinlock.h
> index e9a960e28f3c..866f6ca0427a 100644
> --- a/arch/powerpc/include/asm/spinlock.h
> +++ b/arch/powerpc/include/asm/spinlock.h
> @@ -35,11 +35,12 @@
>  #define LOCK_TOKEN	1
>  #endif
> 
> -#ifdef CONFIG_PPC_PSERIES
> +#if defined(CONFIG_PPC_PSERIES) && defined(CONFIG_PPC_SPLPAR)
> +DECLARE_STATIC_KEY_FALSE(shared_processor);
>  #define vcpu_is_preempted vcpu_is_preempted
>  static inline bool vcpu_is_preempted(int cpu)
>  {
> -	if (!firmware_has_feature(FW_FEATURE_SPLPAR))
> +	if (!static_branch_unlikely(&shared_processor))
>  		return false;
>  	return !!(be32_to_cpu(lppaca_of(cpu).yield_count) & 1);
>  }
> diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
> index 50d68d21ddcc..ffb971f3a63c 100644
> --- a/arch/powerpc/mm/numa.c
> +++ b/arch/powerpc/mm/numa.c
> @@ -1568,9 +1568,13 @@ int prrn_is_enabled(void)
>  	return prrn_enabled;
>  }
> 
> +DEFINE_STATIC_KEY_FALSE(shared_processor);
> +EXPORT_SYMBOL_GPL(shared_processor);
> +
>  void __init shared_proc_topology_init(void)
>  {
>  	if (lppaca_shared_proc(get_lppaca())) {
> +		static_branch_enable(&shared_processor);
>  		bitmap_fill(cpumask_bits(&cpu_associativity_changes_mask),
>  			    nr_cpumask_bits);
>  		numa_update_cpu_topology(false);
> -- 
> 2.18.1
>