Re: [PATCH 02/11] powerpc/smp: Merge Power9 topology with Power topology

[Gautham R Shenoy <ego@linux.vnet.ibm.com>]

Hi Srikar,

On Tue, Jul 14, 2020 at 10:06:15AM +0530, Srikar Dronamraju wrote:
> A new sched_domain_topology_level was added just for Power9. However the
> same can be achieved by merging powerpc_topology with power9_topology
> and makes the code more simpler especially when adding a new sched
> domain.
> 
> Cc: linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
> Cc: Michael Ellerman <michaele@au1.ibm.com>
> Cc: Nick Piggin <npiggin@au1.ibm.com>
> Cc: Oliver OHalloran <oliveroh@au1.ibm.com>
> Cc: Nathan Lynch <nathanl@linux.ibm.com>
> Cc: Michael Neuling <mikey@linux.ibm.com>
> Cc: Anton Blanchard <anton@au1.ibm.com>
> Cc: Gautham R Shenoy <ego@linux.vnet.ibm.com>
> Cc: Vaidyanathan Srinivasan <svaidy@linux.ibm.com>
> Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> ---
>  arch/powerpc/kernel/smp.c | 33 ++++++++++-----------------------
>  1 file changed, 10 insertions(+), 23 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> index 680c0edcc59d..069ea4b21c6d 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -1315,7 +1315,7 @@ int setup_profiling_timer(unsigned int multiplier)
>  }
> 
>  #ifdef CONFIG_SCHED_SMT
> -/* cpumask of CPUs with asymetric SMT dependancy */
> +/* cpumask of CPUs with asymmetric SMT dependency */
>  static int powerpc_smt_flags(void)
>  {
>  	int flags = SD_SHARE_CPUCAPACITY | SD_SHARE_PKG_RESOURCES;
> @@ -1328,14 +1328,6 @@ static int powerpc_smt_flags(void)
>  }
>  #endif
> 
> -static struct sched_domain_topology_level powerpc_topology[] = {
> -#ifdef CONFIG_SCHED_SMT
> -	{ cpu_smt_mask, powerpc_smt_flags, SD_INIT_NAME(SMT) },
> -#endif
> -	{ cpu_cpu_mask, SD_INIT_NAME(DIE) },
> -	{ NULL, },
> -};
> -
>  /*
>   * P9 has a slightly odd architecture where pairs of cores share an L2 cache.
>   * This topology makes it *much* cheaper to migrate tasks between adjacent cores
> @@ -1353,7 +1345,13 @@ static int powerpc_shared_cache_flags(void)
>   */
>  static const struct cpumask *shared_cache_mask(int cpu)
>  {
> -	return cpu_l2_cache_mask(cpu);
> +	if (shared_caches)
> +		return cpu_l2_cache_mask(cpu);
> +
> +	if (has_big_cores)
> +		return cpu_smallcore_mask(cpu);
> +
> +	return cpu_smt_mask(cpu);
>  }
> 
>  #ifdef CONFIG_SCHED_SMT
> @@ -1363,7 +1361,7 @@ static const struct cpumask *smallcore_smt_mask(int cpu)
>  }
>  #endif
> 
> -static struct sched_domain_topology_level power9_topology[] = {
> +static struct sched_domain_topology_level powerpc_topology[] = {


>  #ifdef CONFIG_SCHED_SMT
>  	{ cpu_smt_mask, powerpc_smt_flags, SD_INIT_NAME(SMT) },
>  #endif
> @@ -1388,21 +1386,10 @@ void __init smp_cpus_done(unsigned int max_cpus)
>  #ifdef CONFIG_SCHED_SMT
>  	if (has_big_cores) {
>  		pr_info("Big cores detected but using small core scheduling\n");
I> -		power9_topology[0].mask = smallcore_smt_mask;
>  		powerpc_topology[0].mask = smallcore_smt_mask;
>  	}
>  #endif
> -	/*
> -	 * If any CPU detects that it's sharing a cache with another CPU then
> -	 * use the deeper topology that is aware of this sharing.
> -	 */
> -	if (shared_caches) {
> -		pr_info("Using shared cache scheduler topology\n");
> -		set_sched_topology(power9_topology);
> -	} else {
> -		pr_info("Using standard scheduler topology\n");
> -		set_sched_topology(powerpc_topology);


Ok, so we will go with the three level topology by default (SMT,
CACHE, DIE) and will rely on the sched-domain creation code to
degenerate CACHE domain in case SMT and CACHE have the same set of
CPUs (POWER8 for eg).

From a cleanup perspective this is better, since we won't have to
worry about defining multiple topology structures, but from a
performance point of view, wouldn't we now pay an extra penalty of
degenerating the CACHE domains on POWER8 kind of systems, each time
when a CPU comes online ?

Do we know how bad it is ? If the degeneration takes a few extra
microseconds, that should be ok I suppose.

--
Thanks and Regards
gautham.