Re: [patch V3 13/20] sched/mmcid: Provide precomputed maximal value

Mathieu Desnoyers
2025-10-30T14:23:14Z

On 2025-10-29 09:09, Thomas Gleixner wrote:
> Reading mm::mm_users and mm:::mm_cid::nr_cpus_allowed everytime to compute

every time

> + * @max_cids: The exclusive maximum CID value for allocation and convergance

convergence

> + * @users: The number of tasks sharing this MM. Seperate from mm::mm_users

Separate

> + * as that is modified by mmget()/mm_put() by other entities which
> + * do not actually share the MM.
> * @lock: Spinlock to protect all fields except @pcpu. It also protects
> * the MM cid cpumask and the MM cidmask bitmap.
> */
> struct mm_mm_cid {
> struct mm_cid_pcpu __percpu *pcpu;
> + unsigned int max_cids;
> unsigned int nr_cpus_allowed;
> + unsigned int users;

I suspect this reintroduces false-sharing between the "users"
and "lock" fields (updated every time a thread is forked/exits)
and load of the pcpu pointer which is pretty much immutable.
This will slow down accesses to the percpu data in the scheduler
fast path.

> raw_spinlock_t lock;
> }____cacheline_aligned_in_smp;
> #else /* CONFIG_SCHED_MM_CID */

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.