Re: [PATCH 1/4] powerpc/topology: Introduce llc_override debug knob

[Srikar Dronamraju <srikar@linux.ibm.com>]

* Shrikanth Hegde <sshegde@linux.ibm.com> [2026-10-12 15:50:34]:

Hi Shrikanth,

> 
> As a result, it may be enabled but has no room to operate.
> Hence provide a debugfs option one can set to try experiment their
> workload with cache aware scheduling. This also provides ways to explore
> it on system with SMT=4 to see if there are any opportunities
> to improve or uncover bugs if there is any.
> 
> This does not change the hardware cache topology. It only changes the
> sched-domain level at which SD_SHARE_LLC is set.
> 
> Initially start with,
> - New debugs file called llc_override. It can be set/get on
>   /sys/kernel/debug/powerpc/llc_override
> - Adds parser logic for the debug knob.
> 

This is a good way to allow experimenting to know if a software override of
the LLC esp given the latency numbers within hemisphere and chip.

> There is no point in setting NUMA as the LLC domain. So give option
> until pkg to experiment with.
> 
> Actual flag manipulations and rebuilding sched domains are introduced
> in subsequent patches.
> 
> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
> ---
> PS: Split is deliberate. If squashing all this into one patch helps,
>     let me know.
> 
>  arch/powerpc/kernel/smp.c | 90 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 90 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> index 6a5a5469aaae..433757511adb 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -36,6 +36,7 @@
>  #include <linux/pgtable.h>
>  #include <linux/clockchips.h>
>  #include <linux/kexec.h>
> +#include <linux/debugfs.h>
>  
>  #include <asm/ptrace.h>
>  #include <linux/atomic.h>
> @@ -1826,3 +1827,92 @@ void __noreturn arch_cpu_idle_dead(void)
>  }
>  
>  #endif
> +
> +enum llc_override_values {
> +	LLC_OVERRIDE_DISABLED,
> +	LLC_OVERRIDE_SMT,
> +	LLC_OVERRIDE_MC,
> +	LLC_OVERRIDE_PKG,
> +};
> +

For the current experiments, would it help to keep it simple and have just
SMT and MC. I mean we can include disabled and pkg based on our experiments.


-- 
Thanks and Regards
Srikar Dronamraju