Re: [PATCH 1/4] powerpc/topology: Introduce llc_override debug knob
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From: Srikar Dronamraju <hidden>
Date: 2026-10-12 12:15:40
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* Shrikanth Hegde [off-list ref] [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.
quoted hunk ↗ jump to hunk
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