Re: [PATCH] x86/mm/tlb: avoid reading mm_tlb_gen when possible

Peter Zijlstra
2022-03-28T10:36:25Z

On Tue, Mar 22, 2022 at 10:07:57PM +0000, Nadav Amit wrote:
> From: Nadav Amit <namit@vmware.com>
>
> On extreme TLB shootdown storms, the mm's tlb_gen cacheline is highly
> contended and reading it should (arguably) be avoided as much as
> possible.
>
> Currently, flush_tlb_func() reads the mm's tlb_gen unconditionally,
> even when it is not necessary (e.g., the mm was already switched).
> This is wasteful.
>
> Moreover, one of the existing optimizations is to read mm's tlb_gen to
> see if there are additional in-flight TLB invalidations and flush the
> entire TLB in such a case. However, if the request's tlb_gen was already
> flushed, the benefit of checking the mm's tlb_gen is likely to be offset
> by the overhead of the check itself.
>
> Running will-it-scale with tlb_flush1_threads show a considerable
> benefit on 56-core Skylake (up to +24%):
>
> threads Baseline (v5.17+) +Patch
> 1 159960 160202
> 5 310808 308378 (-0.7%)
> 10 479110 490728
> 15 526771 562528
> 20 534495 587316
> 25 547462 628296
> 30 579616 666313
> 35 594134 701814
> 40 612288 732967
> 45 617517 749727
> 50 637476 735497
> 55 614363 778913 (+24%)
>

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>