Re: [PATCH] arm_mpam: Try reading again if MPAM instance returns not ready

[Zeng Heng]

On 2025/9/20 0:11, James Morse wrote:
> Hi Zeng,
>
> On 16/09/2025 14:17, Zeng Heng wrote:
> > After updating the monitor configuration, the first read of the monitoring
> > result requires waiting for the "not ready" duration before an effective
> > value can be obtained.
>
> May need to wait - some platforms need to do this, some don't.
> Yours is the first I've heard of that does this!

I'm afraid similar platforms do exist. As long as one component has more
than one MSC, after first updating the component’s monitor every MSC
instance needs to wait for MAX_NRDY_USEC us before reading the monitor
result.

In fact, most platforms don’t have nearly as many performance monitors
as PARTIDs, so the monitors often have to be time-shared, which making
the problem even more pronounced.

> > Because a component consists of multiple MPAM instances, after updating the
> > configuration of each instance, should wait for the "not ready" period of
> > per single instance before the valid monitoring value can be obtained, not
> > just wait for once interval per component.
>
> I'm really trying to avoid that ... if you have ~200 MSC pretending to be one thing, you'd
> wait 200x the maximum period. On systems with CMN, the number of MSC scales with the
> number of CPUs, so 200x isn't totally crazy.
> > I think the real problem here is the driver doesn't go on to 
reconfigure MSC-2 if MSC-1
> returned not-ready, meaning the "I'll only wait once" logic kicks in and returns not-ready
> to the user. (which is presumably what you're seeing?)

Yes, exactly.

> Does this solve your problem?:
> -----------------%<-----------------
> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index 404bd4c1fd5e..2f39d0339349 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -1395,7 +1395,7 @@ static void __ris_msmon_read(void *arg)
>
>   static int _msmon_read(struct mpam_component *comp, struct mon_read *arg)
>   {
> -       int err, idx;
> +       int err, any_err = 0, idx;
>          struct mpam_msc *msc;
>          struct mpam_vmsc *vmsc;
>          struct mpam_msc_ris *ris;
> @@ -1412,15 +1412,19 @@ static int _msmon_read(struct mpam_component *comp, stru
> ct mon_read *arg)
>                                                      true);
>                          if (!err && arg->err)
>                                  err = arg->err;
> +
> +                       /*
> +                        * Save one error to be returned to the caller, but
> +                        * keep reading counters so that the get reprogrammed.
> +                        * On platforms with NRDY this lets us wait once.
> +                        */
>                          if (err)
> -                               break;
> +                               any_err = err;
>                  }
> -               if (err)
> -                       break;
>          }
>          srcu_read_unlock(&mpam_srcu, idx);
>
> -       return err;
> +       return any_err;
>   }
>
>   int mpam_msmon_read(struct mpam_component *comp, struct mon_cfg *ctx,
> -----------------%<-----------------

I agree with this modification: Reconfigure all MSCs first, then if any
of them returns EBUSY, wait just once for MAX_NRDY_USEC and re-read
monitor result, this guarantees that the monitor result is valid.



Thanks,
Zeng Heng