linux-kernel - Re: [PATCH v6 09/12] x86/mm: enable broadcast TLB invalidation for multi-threaded processes

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Message-ID: <20250121095507.GB5388@noisy.programming.kicks-ass.net>
Date: Tue, 21 Jan 2025 10:55:07 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Rik van Riel <riel@...riel.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, bp@...en8.de,
	dave.hansen@...ux.intel.com, zhengqi.arch@...edance.com,
	nadav.amit@...il.com, thomas.lendacky@....com, kernel-team@...a.com,
	linux-mm@...ck.org, akpm@...ux-foundation.org, jannh@...gle.com,
	mhklinux@...look.com, andrew.cooper3@...rix.com
Subject: Re: [PATCH v6 09/12] x86/mm: enable broadcast TLB invalidation for
 multi-threaded processes

On Sun, Jan 19, 2025 at 09:40:17PM -0500, Rik van Riel wrote:
> +/*
> + * Figure out whether to assign a global ASID to a process.
> + * We vary the threshold by how empty or full global ASID space is.
> + * 1/4 full: >= 4 active threads
> + * 1/2 full: >= 8 active threads
> + * 3/4 full: >= 16 active threads
> + * 7/8 full: >= 32 active threads
> + * etc
> + *
> + * This way we should never exhaust the global ASID space, even on very
> + * large systems, and the processes with the largest number of active
> + * threads should be able to use broadcast TLB invalidation.
> + */
> +#define HALFFULL_THRESHOLD 8
> +static bool meets_global_asid_threshold(struct mm_struct *mm)
> +{
> +	int avail = global_asid_available;
> +	int threshold = HALFFULL_THRESHOLD;
> +
> +	if (!avail)
> +		return false;
> +
> +	if (avail > MAX_ASID_AVAILABLE * 3 / 4) {
> +		threshold = HALFFULL_THRESHOLD / 4;
> +	} else if (avail > MAX_ASID_AVAILABLE / 2) {
> +		threshold = HALFFULL_THRESHOLD / 2;
> +	} else if (avail < MAX_ASID_AVAILABLE / 3) {
> +		do {
> +			avail *= 2;
> +			threshold *= 2;
> +		} while ((avail + threshold) < MAX_ASID_AVAILABLE / 2);
> +	}
> +
> +	return mm_active_cpus_exceeds(mm, threshold);
> +}

I'm still very much disliking this. Why do we need this? Yes, running
out of ASID space is a pain, but this increasing threshold also makes
things behave weird.

Suppose our most used processes starts slow, and ends up not getting an
ASID because too much irrelevant crap gets started before it spawns
enough threads and then no longer qualifies.

Can't we just start with a very simple constant test and poke at things
if/when its found to not work?