RE: [PATCH v10 00/12] AMD broadcast TLB invalidation

[Michael Kelley <mhklinux@outlook.com>]

From: riel@surriel.com <riel@surriel.com> Sent: Tuesday, February 11, 2025 1:08 PM
> 
> Add support for broadcast TLB invalidation using AMD's INVLPGB instruction.
> 
> This allows the kernel to invalidate TLB entries on remote CPUs without
> needing to send IPIs, without having to wait for remote CPUs to handle
> those interrupts, and with less interruption to what was running on
> those CPUs.
> 
> Because x86 PCID space is limited, and there are some very large
> systems out there, broadcast TLB invalidation is only used for
> processes that are active on 3 or more CPUs, with the threshold
> being gradually increased the more the PCID space gets exhausted.
> 
> Combined with the removal of unnecessary lru_add_drain calls
> (see https://lkml.org/lkml/2024/12/19/1388) this results in a
> nice performance boost for the will-it-scale tlb_flush2_threads
> test on an AMD Milan system with 36 cores:
> 
> - vanilla kernel:           527k loops/second
> - lru_add_drain removal:    731k loops/second
> - only INVLPGB:             527k loops/second
> - lru_add_drain + INVLPGB: 1157k loops/second
> 
> Profiling with only the INVLPGB changes showed while
> TLB invalidation went down from 40% of the total CPU
> time to only around 4% of CPU time, the contention
> simply moved to the LRU lock.
> 
> Fixing both at the same time about doubles the
> number of iterations per second from this case.
> 
> Some numbers closer to real world performance
> can be found at Phoronix, thanks to Michael:
> 
> https://www.phoronix.com/news/AMD-INVLPGB-Linux-Benefits
> 
> My current plan is to implement support for Intel's RAR
> (Remote Action Request) TLB flushing in a follow-up series,
> after this thing has been merged into -tip. Making things
> any larger would just be unwieldy for reviewers.
> 
> v10:
>  - simplify partial pages with min(nr, 1) in the invlpgb loop (Peter)
>  - document x86 paravirt, AMD invlpgb, and ARM64 flush without IPI (Brendan)
>  - remove IS_ENABLED(CONFIG_X86_BROADCAST_TLB_FLUSH) (Brendan)
>  - various cleanups (Brendan)
> v9:
>  - print warning when start or end address was rounded (Peter)
>  - in the reclaim code, tlbsync at context switch time (Peter)
>  - fix !CONFIG_CPU_SUP_AMD compile error in arch_tlbbatch_add_pending (Jan)
> v8:
>  - round start & end to handle non-page-aligned callers (Steven & Jan)
>  - fix up changelog & add tested-by tags (Manali)
> v7:
>  - a few small code cleanups (Nadav)
>  - fix spurious VM_WARN_ON_ONCE in mm_global_asid
>  - code simplifications & better barriers (Peter & Dave)
> v6:
>  - fix info->end check in flush_tlb_kernel_range (Michael)
>  - disable broadcast TLB flushing on 32 bit x86
> v5:
>  - use byte assembly for compatibility with older toolchains (Borislav, Michael)
>  - ensure a panic on an invalid number of extra pages (Dave, Tom)
>  - add cant_migrate() assertion to tlbsync (Jann)
>  - a bunch more cleanups (Nadav)
>  - key TCE enabling off X86_FEATURE_TCE (Andrew)
>  - fix a race between reclaim and ASID transition (Jann)
> v4:
>  - Use only bitmaps to track free global ASIDs (Nadav)
>  - Improved AMD initialization (Borislav & Tom)
>  - Various naming and documentation improvements (Peter, Nadav, Tom, Dave)
>  - Fixes for subtle race conditions (Jann)
> v3:
>  - Remove paravirt tlb_remove_table call (thank you Qi Zheng)
>  - More suggested cleanups and changelog fixes by Peter and Nadav
> v2:
>  - Apply suggestions by Peter and Borislav (thank you!)
>  - Fix bug in arch_tlbbatch_flush, where we need to do both
>    the TLBSYNC, and flush the CPUs that are in the cpumask.
>  - Some updates to comments and changelogs based on questions.
> 

Tested this series in an Azure Confidential VM based on SEV-SNP,
which is running on Hyper-V and exposes INVLPGB in the guest VM.
I applied the patches to 6.13.0 with one minor fixup, but did not
include the patch to remove unnecessary lru_add_drain calls.
I also added some custom telemetry to see when INVLPGB is
being used vs. Hyper-V's paravirt hypercalls for TLB flushing.

I did not see any problems. The custom telemetry looked about
as I expected, showing a mix of INVLPGB and the PV hypercalls.
So for the series:

Tested-by: Michael Kelley <mhklinux@outlook.com>