Re: [PATCH 0/8] x86, sched: Dynamic ITMT core ranking support and some yak shaving

[Tim Chen]

On Wed, 2024-12-11 at 18:55 +0000, K Prateek Nayak wrote:
> The ITMT infrastructure currently assumes ITMT rankings to be static and
> is set correctly prior to enabling ITMT support which allows the CPU
> with the highest core ranking to be cached as the "asym_prefer_cpu" in
> the sched_group struct. However, with the introduction of Preferred Core
> support in amd-pstate, these rankings can change at runtime.
> 
> This series adds support for dynamic ranking in generic scheduler layer
> without the need to rebuild the sched domain hierarchy and fixes an
> issue with x86_die_flags() on AMD systems that support Preferred Core
> ranking with some yak shaving done along the way.
> 
> Patch 1 to 4 are independent cleanup around ITMT infrastructure, removal
> of x86_smt_flags wrapper, and moving the "sched_itmt_enabled" sysctl to
> debugfs.
> 
> Patch 5 adds the SD_ASYM_PACKING flag to the PKG domain on all ITMT
> enabled systems. The rationale behind the addition is elaborates in the
> same. One open question remains is for Intel processors with multiple
> Tiles in a PKG which advertises itself as multiple LLCs in a PKG and
> supports ITMT - is it okay to set SD_ASYM_PACKING for PKG domain on
> these processors?

After talking to my colleagues Ricardo and Srinivas, we think that this
should be fine for Intel CPUs.

Tim

> 
> Patch 6 and 7 are independent possible micro-optimizations discovered
> when auditing update_sg_lb_stats()
> 
> Patch 8 uncaches the asym_prefer_cpu from the sched_group struct and
> finds it during load balancing in update_sg_lb_stats() before it is used
> to make any scheduling decisions. This is the simplest approach; an
> alternate approach would be to move the asym_prefer_cpu to
> sched_domain_shared and allow the first load balancing instance post a
> priority change to update the cached asym_prefer_cpu. On systems with
> static priorities, this would allow benefits of caching while on systems
> with dynamic priorities, it'll reduce the overhead of finding
> "asym_prefer_cpu" each time update_sg_lb_stats() is called however the
> benefits come with added code complexity which is why Patch 8 is marked
> as an RFC.
> 
> This series is based on
> 
>   git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git sched/core 
> 
> at commit 2a77e4be12cb ("sched/fair: Untangle NEXT_BUDDY and
> pick_next_task()") and is a spiritual successor to a previous attempt
> at fixing the x86_die_flags() on Preferred Core enabled system by Mario
> that can be found at
> https://lore.kernel.org/lkml/20241203201129.31957-1-mario.limonciello@xxxxxxx/
> 
> ---
> K Prateek Nayak (8):
>   x86/itmt: Convert "sysctl_sched_itmt_enabled" to boolean
>   x86/itmt: Use guard() for itmt_update_mutex
>   x86/itmt: Move the "sched_itmt_enabled" sysctl to debugfs
>   x86/topology: Remove x86_smt_flags and use cpu_smt_flags directly
>   x86/topology: Use x86_sched_itmt_flags for PKG domain unconditionally
>   sched/fair: Do not compute NUMA Balancing stats unnecessarily during
>     lb
>   sched/fair: Do not compute overloaded status unnecessarily during lb
>   sched/fair: Uncache asym_prefer_cpu and find it during
>     update_sd_lb_stats()
> 
>  arch/x86/include/asm/topology.h |  4 +-
>  arch/x86/kernel/itmt.c          | 81 ++++++++++++++-------------------
>  arch/x86/kernel/smpboot.c       | 19 +-------
>  kernel/sched/fair.c             | 41 +++++++++++++----
>  kernel/sched/sched.h            |  1 -
>  kernel/sched/topology.c         | 15 +-----
>  6 files changed, 69 insertions(+), 92 deletions(-)
> 
> 
> base-commit: 2a77e4be12cb58bbf774e7c717c8bb80e128b7a4