Re: [RFC PATCH v3 01/13] lib/sbm: Introduce helpers for architectures to configure LLC properties

[Shrikanth Hegde <sshegde@linux.ibm.com>]

Hi Prateek.

I ran the series on 320 CPU power11 system today.
hackbench shows high regression across the different number of groups and
it is more severe at lower number of groups. I haven't run anything else yet.

I haven't taken a look at the patches yet. Post your talk today will try to
go through and see if i can spot something.

On 10/2/26 12:58 AM, K Prateek Nayak wrote:
> Subsequent commits will add arch/ hooks to configure the sparsebitmask
> (sbm) topology - number of sbm instance and the maximum CPUs that a
> single instance can represent.
> 
> Introduce sbm_set_topology() that helps architectures configure the sbm
> topology. Also add arch_sbm_cpu_instance_id() which will be used to map
> CPUs to the sbm instances during init to keep CPUs on the same instance
> on the same sbm leaf.
> 
> Individual arch/ users will be added one at a time in the subsequent
> commits.
> 
> Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
> ---
>   include/linux/sbm.h |  8 ++++++++
>   lib/Makefile        |  2 +-
>   lib/sbm.c           | 27 +++++++++++++++++++++++++++
>   3 files changed, 36 insertions(+), 1 deletion(-)
>   create mode 100644 include/linux/sbm.h
>   create mode 100644 lib/sbm.c
> 
> diff --git a/include/linux/sbm.h b/include/linux/sbm.h
> new file mode 100644
> index 000000000000..adac12ed233a
> --- /dev/null
> +++ b/include/linux/sbm.h
> @@ -0,0 +1,8 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _LINUX_SBM_H
> +#define _LINUX_SBM_H
> +
> +int arch_sbm_cpu_instance_id(int cpu);
> +void sbm_set_topology(int num_instances, int max_threads_per_instance);
> +
> +#endif /* _LINUX_SBM_H */
> diff --git a/lib/Makefile b/lib/Makefile
> index dfab958327c5..7776330f5b25 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -40,7 +40,7 @@ lib-y := ctype.o string.o vsprintf.o cmdline.o \
>   	 is_single_threaded.o plist.o decompress.o kobject_uevent.o \
>   	 earlycpio.o seq_buf.o siphash.o dec_and_lock.o \
>   	 nmi_backtrace.o win_minmax.o memcat_p.o \
> -	 buildid.o objpool.o iomem_copy.o sys_info.o
> +	 buildid.o objpool.o iomem_copy.o sys_info.o sbm.o
>   
>   lib-$(CONFIG_UNION_FIND) += union_find.o
>   lib-$(CONFIG_PRINTK) += dump_stack.o
> diff --git a/lib/sbm.c b/lib/sbm.c
> new file mode 100644
> index 000000000000..82280e3306de
> --- /dev/null
> +++ b/lib/sbm.c
> @@ -0,0 +1,27 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#include <linux/sbm.h>
> +#include <linux/init.h>
> +#include <linux/printk.h>
> +
> +static int sbm_max_threads_per_instance = -1;
> +static int sbm_num_instance = -1;
> +
> +/*
> + * In absence of an arch definition, consider all CPUs to
> + * belong to the same sbm instance.
> + */
> +int __weak arch_sbm_cpu_instance_id(int cpu)
> +{
> +	return 0;
> +}
> +
> +void __init sbm_set_topology(int num_instances, int max_threads_per_instance)
> +{
> +	sbm_max_threads_per_instance = max_threads_per_instance;
> +	sbm_num_instance = num_instances;
> +
> +	pr_info("sbm topology set to %d instance(s) with a maximum of %d thread(s) per instance\n",
> +		sbm_num_instance,
> +		sbm_max_threads_per_instance);
> +}

nit: Stray empty line at the end.

> +