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

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Message-ID: <aLcfvIfFb6xD-NXp@arm.com>
Date: Tue, 2 Sep 2025 17:47:56 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>,
	James Morse <james.morse@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1 0/2] Don't broadcast TLBI if mm was only active on
 local CPU

On Fri, Aug 29, 2025 at 04:35:06PM +0100, Ryan Roberts wrote:
> 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.

I suspect it's highly dependent on hardware and how it handles the DVM
messages. There were some old proposals from Fujitsu:

https://lore.kernel.org/linux-arm-kernel/20190617143255.10462-1-indou.takao@jp.fujitsu.com/

Christoph Lameter (Ampere) also followed with some refactoring in this
area to allow a boot-configurable way to do TLBI via IS ops or IPI:

https://lore.kernel.org/linux-arm-kernel/20231207035703.158053467@gentwo.org/

(for some reason, the patches did not make it to the list, I have them
in my inbox if you are interested)

I don't remember any real-world workload, more like hand-crafted
mprotect() loops.

Anyway, I think the approach in your series doesn't have downsides, it's
fairly clean and addresses some low-hanging fruits. For multi-threaded
workloads where a flush_tlb_mm() is cheaper than a series of per-page
TLBIs, I think we can wait for that hardware to be phased out. The TLBI
range operations should significantly reduce the DVM messages between
CPUs.

-- 
Catalin