Re: [patch V4 16/20] sched/mmcid: Provide new scheduler CID mechanism

[Mathieu Desnoyers]

On 2025-11-16 15:49, Thomas Gleixner wrote:
[...]
> The current upstream implementation tries to keep the CID with the task
> even in overcommit situations, which complicates task migration. It also
> has to do the CID space consolidation work from a task work in the exit to
> user space path. As that work is assigned to a random task related to a MM
> this can inflict unwanted exit latencies.
>
> Implement the context switch parts of a strict ownership mechanism to
> address this.
>
> This removes most of the work from the task which schedules out. Only
> during transitioning from per CPU to per task ownership it is required to
> drop the CID when leaving the CPU to prevent CID space exhaustion. Other
> than that scheduling out is just a single check and branch.

Same comment as for prior patch.

[...]

>   struct mm_mm_cid {
> +	/* Hotpath read mostly members */
>   	struct mm_cid_pcpu	__percpu *pcpu;
> +	unsigned int		percpu;
> +	unsigned int		transit;

[...]

> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -10495,6 +10495,7 @@ void mm_init_cid(struct mm_struct *mm, s
>   		per_cpu_ptr(pcpu, cpu)->cid = MM_CID_UNSET;
>   
>   	mm->mm_cid.max_cids = 0;
> +	mm->mm_cid.percpu = 0;

I'm unsure where it appears (maybe later in the series ?), but
I would have expected:

        mm->mm_cid.transit = 0;

here.

Other than those comments:

Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com