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

[Nadav Amit]

> On 30 Dec 2024, at 19:53, Rik van Riel <riel@xxxxxxxxxxx> wrote:
> 
> +/*
> + * Figure out whether to assign a broadcast (global) ASID to a process.
> + * We vary the threshold by how empty or full broadcast 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 broadcast 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_broadcast_asid_threshold(struct mm_struct *mm)
> +{
> +	int avail = broadcast_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);
> +}

Rik,

I thought about it further and I am not sure this approach is so great.
It reminds me the technique of eating chocolate forever: each day eat
half of the previous day. It works in theory, but less in practice.

IOW, I mean it seems likely that early processes would get and hog all
broadcast ASIDs. It seems necessary to be able to revoke broadcast ASIDs,
although I understand it can be complicated.

Do you have any other resource in mind that Linux manages in a similar
way (avoids revoking)?