Re: [PATCH v4 12/24] x86/resctrl: Make resctrl_arch_rmid_read() retry when it is interrupted

[Peter Newman]

Hi James,

On Thu, May 25, 2023 at 8:03 PM James Morse <james.morse@xxxxxxx> wrote:
> +interrupted:
> +       am = get_arch_mbm_state(hw_dom, rmid, eventid);
> +       if (am)
> +               start_msr_val = atomic64_read(&am->prev_msr);
> +
>         ret = __rmid_read(rmid, eventid, &msr_val);
>         if (ret)
>                 return ret;
>
>         am = get_arch_mbm_state(hw_dom, rmid, eventid);
>         if (am) {
> -               am->chunks += mbm_overflow_count(am->prev_msr, msr_val,
> -                                                hw_res->mbm_width);
> -               chunks = get_corrected_mbm_count(rmid, am->chunks);
> -               am->prev_msr = msr_val;
> +               old_msr_val = atomic64_cmpxchg(&am->prev_msr, start_msr_val,
> +                                              msr_val);
> +               if (old_msr_val != start_msr_val)
> +                       goto interrupted;
> +
> +               chunks = mbm_overflow_count(start_msr_val, msr_val,
> +                                           hw_res->mbm_width);
> +               atomic64_add(chunks, &am->chunks);
> +
> +               chunks = get_corrected_mbm_count(rmid,
> +                                                atomic64_read(&am->chunks));
>         } else {
>                 chunks = msr_val;
>         }

It looks like if __rmid_read() is interrupted by an occupancy counter
read between writing QM_EVTSEL and reading QM_CTR, it will not perform
any update to am->prev_msr, and the interrupted read will return the
same counter value as in the interrupting read.

Maybe there's something you can create to check that's updated unconditionally?

-Peter