Re: [PATCH] sched/mmcid: Fix affinity-triggered switch back to per-task mode

[Mathieu Desnoyers]

On 2026-09-15 11:59, Hui Su wrote:
> MM CID switches to per-CPU ownership when the number of users of an
> mm exceeds the maximum number of CIDs. When the affinity union grows,
> the per-CPU threshold is recalculated and the MM can switch back to
> per-task ownership through deferred work.
>
> mm_cid_calc_pcpu_thrs() incorrectly uses
> min(nr_cpus_allowed, users) to calculate that switch-back threshold.
> This makes pcpu_thrs no larger than users, so the
> users < pcpu_thrs check in mm_update_cpus_allowed() can never succeed.
>
> For example, with three users restricted to two CPUs, the MM enters
> per-CPU mode. On an eight-CPU system, widening the affinity union to
> all eight CPUs should produce a threshold of four, but the old
> calculation produces three and returns early. The MM therefore remains
> in per-CPU mode even though the widened CPU set makes per-task
> ownership preferable.
>
> Calculate pcpu_thrs from nr_cpus_allowed directly. The users bound in
> __mm_update_max_cids() still controls when per-CPU mode is entered, so
> this does not change the threshold calculated when switching from
> per-task to per-CPU ownership.
>
> Tested with an x86_64 QEMU reproducer using three users, an initial
> two-CPU affinity, and an eight-CPU affinity expansion. The baseline
> did not invoke mm_cid_work_fn() after the affinity expansion. The
> fixed kernel invoked the deferred work once and completed the switch
> back to per-task mode.

I think you're onto something.

We should cover those scenarios with selftests in
tools/testing/selftests/rseq/.

Thomas, I recall you had tests for those state transitions when
rewriting rseq mm_cid. Were those local or did they make it upstream ?

Thanks,

Mathieu

> Fixes: fbd0e71dc370 ("sched/mmcid: Provide CID ownership mode fixup functions")
> Signed-off-by: Hui Su <sh_def@xxxxxxx>
> ---
>   kernel/sched/core.c | 5 ++---
>   1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 7885ff76e69f..0280ed132d0a 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -10845,11 +10845,10 @@ static void __mm_update_max_cids(struct mm_mm_cid *mc)
>   
>   static inline unsigned int mm_cid_calc_pcpu_thrs(struct mm_mm_cid *mc)
>   {
> -	unsigned int opt_cids;
> +	unsigned int nr_cpus = mc->nr_cpus_allowed;
>   
> -	opt_cids = min(mc->nr_cpus_allowed, mc->users);
>   	/* Has to be at least 1 because 0 indicates PCPU mode off */
> -	return max(min(opt_cids - opt_cids / 4, num_possible_cpus() / 2), 1);
> +	return max(min(nr_cpus - nr_cpus / 4, num_possible_cpus() / 2), 1);
>   }
>   
>   static bool mm_update_max_cids(struct mm_struct *mm)
>
> base-commit: 587858367581b9c55c3690f4e63382ad622719d4


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