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

Mathieu Desnoyers
2025-10-31T15:14:04Z

On 2025-10-31 11:06, Thomas Gleixner wrote:
> On Thu, Oct 30 2025 at 10:23, Mathieu Desnoyers wrote:
>> On 2025-10-29 09:09, Thomas Gleixner wrote:
>>> 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.
>
> At this point yes, but when all bits are in place then the lock fields
> end up in a different cache line.
>
> The false sharing issue vs. *pcpu and max_cids is minor, but I can move
> the low frequency modified members past the work, so it does not matter
> at all. The work stuff is rarely used, so there is no point to worry
> about the occasional cache line contention on that.

If we have alignment requirements on fields that matter for performance,
I recommend using __cacheline_group_{begin,end}_aligned() to make this
explicit. See include/linux/cache.h.

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.