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Date: Thu, 5 Dec 2019 08:48:03 -0500
From: Phil Auld <pauld@redhat.com>
To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Subject: Re: [PATCH v3 1/2] powerpc/vcpu: Assume dedicated processors as
 non-preempt
Message-ID: <20191205134803.GA18693@lorien.usersys.redhat.com>
References: <20191205083218.25824-1-srikar@linux.vnet.ibm.com>
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Cc: Juri Lelli <juri.lelli@redhat.com>, Parth Shah <parth@linux.ibm.com>,
 "Gautham R . Shenoy" <ego@linux.vnet.ibm.com>,
 Ihor Pasichnyk <Ihor.Pasichnyk@ibm.com>, Waiman Long <longman@redhat.com>,
 linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
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On Thu, Dec 05, 2019 at 02:02:17PM +0530 Srikar Dronamraju wrote:
> With commit 247f2f6f3c70 ("sched/core: Don't schedule threads on pre-empt=
ed
> 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.
>=20
> 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.
>=20
> Even if vCPU of dedicated LPAR is preempted/donated, it should have
> right of first-use since they are suppose to own the vCPU.
>=20
> 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
>=20
>   # perf stat -a -r 5 ./schbench
> v5.4                                     v5.4 + patch
> Latency percentiles (usec)               Latency percentiles (usec)
> =0950.0000th: 45                    =0950.0000th: 39
> =0975.0000th: 62                    =0975.0000th: 53
> =0990.0000th: 71                    =0990.0000th: 67
> =0995.0000th: 77                    =0995.0000th: 76
> =09*99.0000th: 91                   =09*99.0000th: 89
> =0999.5000th: 707                   =0999.5000th: 93
> =0999.9000th: 6920                  =0999.9000th: 118
> =09min=3D0, max=3D10048                 =09min=3D0, max=3D211
> Latency percentiles (usec)               Latency percentiles (usec)
> =0950.0000th: 45                    =0950.0000th: 34
> =0975.0000th: 61                    =0975.0000th: 45
> =0990.0000th: 72                    =0990.0000th: 53
> =0995.0000th: 79                    =0995.0000th: 56
> =09*99.0000th: 691                  =09*99.0000th: 61
> =0999.5000th: 3972                  =0999.5000th: 63
> =0999.9000th: 8368                  =0999.9000th: 78
> =09min=3D0, max=3D16606                 =09min=3D0, max=3D228
> Latency percentiles (usec)               Latency percentiles (usec)
> =0950.0000th: 45                    =0950.0000th: 34
> =0975.0000th: 61                    =0975.0000th: 45
> =0990.0000th: 71                    =0990.0000th: 53
> =0995.0000th: 77                    =0995.0000th: 57
> =09*99.0000th: 106                  =09*99.0000th: 63
> =0999.5000th: 2364                  =0999.5000th: 68
> =0999.9000th: 7480                  =0999.9000th: 100
> =09min=3D0, max=3D10001                 =09min=3D0, max=3D134
> Latency percentiles (usec)               Latency percentiles (usec)
> =0950.0000th: 45                    =0950.0000th: 34
> =0975.0000th: 62                    =0975.0000th: 46
> =0990.0000th: 72                    =0990.0000th: 53
> =0995.0000th: 78                    =0995.0000th: 56
> =09*99.0000th: 93                   =09*99.0000th: 61
> =0999.5000th: 108                   =0999.5000th: 64
> =0999.9000th: 6792                  =0999.9000th: 85
> =09min=3D0, max=3D17681                 =09min=3D0, max=3D121
> Latency percentiles (usec)               Latency percentiles (usec)
> =0950.0000th: 46                    =0950.0000th: 33
> =0975.0000th: 62                    =0975.0000th: 44
> =0990.0000th: 73                    =0990.0000th: 51
> =0995.0000th: 79                    =0995.0000th: 54
> =09*99.0000th: 113                  =09*99.0000th: 61
> =0999.5000th: 2724                  =0999.5000th: 64
> =0999.9000th: 6184                  =0999.9000th: 82
> =09min=3D0, max=3D9887                  =09min=3D0, max=3D121
>=20
>  Performance counter stats for 'system wide' (5 runs):
>=20
> 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% )
>=20
> Waiman Long suggested using static_keys.
>=20
> Reported-by: Parth Shah <parth@linux.ibm.com>
> Reported-by: Ihor Pasichnyk <Ihor.Pasichnyk@ibm.com>
> Cc: Parth Shah <parth@linux.ibm.com>
> Cc: Ihor Pasichnyk <Ihor.Pasichnyk@ibm.com>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Phil Auld <pauld@redhat.com>
> Cc: Waiman Long <longman@redhat.com>
> Cc: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
> Tested-by: Juri Lelli <juri.lelli@redhat.com>
> Ack-by: Waiman Long <longman@redhat.com>
> Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
> Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> ---
> Changelog v1 (https://patchwork.ozlabs.org/patch/1204190/) ->v3:
> Code is now under CONFIG_PPC_SPLPAR as it depends on CONFIG_PPC_PSERIES.
> This was suggested by Waiman Long.
>=20
>  arch/powerpc/include/asm/spinlock.h | 5 +++--
>  arch/powerpc/mm/numa.c              | 4 ++++
>  2 files changed, 7 insertions(+), 2 deletions(-)
>=20
> diff --git a/arch/powerpc/include/asm/spinlock.h b/arch/powerpc/include/a=
sm/spinlock.h
> index e9a960e28f3c..de817c25deff 100644
> --- a/arch/powerpc/include/asm/spinlock.h
> +++ b/arch/powerpc/include/asm/spinlock.h
> @@ -35,11 +35,12 @@
>  #define LOCK_TOKEN=091
>  #endif
> =20
> -#ifdef CONFIG_PPC_PSERIES
> +#ifdef CONFIG_PPC_SPLPAR
> +DECLARE_STATIC_KEY_FALSE(shared_processor);
>  #define vcpu_is_preempted vcpu_is_preempted
>  static inline bool vcpu_is_preempted(int cpu)
>  {
> -=09if (!firmware_has_feature(FW_FEATURE_SPLPAR))
> +=09if (!static_branch_unlikely(&shared_processor))
>  =09=09return false;
>  =09return !!(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)
>  =09return prrn_enabled;
>  }
> =20
> +DEFINE_STATIC_KEY_FALSE(shared_processor);
> +EXPORT_SYMBOL_GPL(shared_processor);
> +
>  void __init shared_proc_topology_init(void)
>  {
>  =09if (lppaca_shared_proc(get_lppaca())) {
> +=09=09static_branch_enable(&shared_processor);
>  =09=09bitmap_fill(cpumask_bits(&cpu_associativity_changes_mask),
>  =09=09=09    nr_cpumask_bits);
>  =09=09numa_update_cpu_topology(false);
> --=20
> 2.18.1
>

This looks good to me, thanks Srikar.

Acked-by: Phil Auld <pauld@redhat.com>
--=20