[RFC PATCH v1 0/2] Don't broadcast TLBI if mm was only active on local CPU

[Huang, Ying]

Hi, Ryan,

Ryan Roberts <ryan.roberts at arm.com> writes:

> Hi All,
>
> This is an RFC for my implementation of an idea from James Morse to avoid
> broadcasting TBLIs to remote CPUs if it can be proven that no remote CPU could
> have ever observed the pgtable entry for the TLB entry that is being
> invalidated. It turns out that x86 does something similar in principle.
>
> The primary feedback I'm looking for is; is this actually correct and safe?
> James and I both believe it to be, but it would be useful to get further
> validation.
>
> Beyond that, the next question is; does it actually improve performance?
> stress-ng's --tlb-shootdown stressor suggests yes; as concurrency increases, we
> do a much better job of sustaining the overall number of "tlb shootdowns per
> second" after the change:
>
> +------------+--------------------------+--------------------------+--------------------------+
> |            |     Baseline (v6.15)     |        tlbi local        |        Improvement       |
> +------------+-------------+------------+-------------+------------+-------------+------------+
> | nr_threads |     ops/sec |    ops/sec |     ops/sec |    ops/sec |     ops/sec |    ops/sec |
> |            | (real time) | (cpu time) | (real time) | (cpu time) | (real time) | (cpu time) |
> +------------+-------------+------------+-------------+------------+-------------+------------+
> |          1 |        9109 |       2573 |        8903 |       3653 |         -2% |        42% |
> |          4 |        8115 |       1299 |        9892 |       1059 |         22% |       -18% |
> |          8 |        5119 |        477 |       11854 |       1265 |        132% |       165% |
> |         16 |        4796 |        286 |       14176 |        821 |        196% |       187% |
> |         32 |        1593 |         38 |       15328 |        474 |        862% |      1147% |
> |         64 |        1486 |         19 |        8096 |        131 |        445% |       589% |
> |        128 |        1315 |         16 |        8257 |        145 |        528% |       806% |
> +------------+-------------+------------+-------------+------------+-------------+------------+
>
> But looking at real-world benchmarks, I haven't yet found anything where it
> makes a huge difference; When compiling the kernel, it reduces kernel time by
> ~2.2%, but overall wall time remains the same. I'd be interested in any
> suggestions for workloads where this might prove valuable.
>
> All mm selftests have been run and no regressions are observed. Applies on
> v6.17-rc3.

Thanks for working on this.

Several previous TLBI broadcast optimization have been tried before,
Cced the original authors for discussion.  Some workloads show good
improvement,

https://lore.kernel.org/lkml/20190617143255.10462-1-indou.takao@jp.fujitsu.com/
https://lore.kernel.org/all/20200203201745.29986-1-aarcange@redhat.com/

Especially in the following mail,

https://lore.kernel.org/all/20200314031609.GB2250@redhat.com/

---
Best Regards,
Huang, Ying