> 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)?
