Re: [PATCH] powercap: intel_rapl: Fix setting of Power Limit 4 to 0

[srinivas pandruvada]

Hi Rui,

On Wed, 2023-09-06 at 02:20 +0000, Zhang, Rui wrote:
> Hi, Srinivas,
> 
> Thanks for the patch.
> 
> On Tue, 2023-09-05 at 17:06 -0700, Srinivas Pandruvada wrote:
> > System runs at minimum performance, once powercap RAPL package
> > domain
> > "enabled" flag is toggled.
> > 
> > Setting RAPL package domain enabled flag to 0, results in setting
> > of
> > power limit 4 (PL4) MSR 0x601 to 0.
> 
> This is the bug. Setting enabled flag should only affect the Enable
> bit
> but not the other bits.
> 
> >  This implies disabling PL4 limit.
> > The PL4 limit controls the peak power. This can significantly
> > change
> > the performance. Even worse, when the enabled flag is set to 1
> > again.
> > This will set PL4 MSR value to 0x01, which means reduce peak power
> > to
> > 0.125W. This will force the system to run at the lowest possible
> > performance.
> > 
> > This is caused by a change which assumes that there is an enable
> > bit
> > in the PL4 MSR like other power limits.
> 
> MSR RAPL doesn't have PL4 enable bit, but TPMI RAPL does have per
> power
> limit enable bit.
That is correct, but not sure that in practice used or not.

> 
> > 
> > In functions set_floor_freq_default() and rapl_remove_package(),
> > call
> > rapl_write_pl_data with PL_ENABLE and PL_CLAMP for only power limit
> > 1
> > and 2. Similarly don't read PL_ENABLE for PL4 to check the presence
> > of
> > power limit 4.
> 
> IMO, the rootcause is that we expose an non-exits PL4_ENABLE
> primitive
> for MSR Interface, and even worse we expose it wrongly that the
> primitive uses the power limit bits.
> 
> Commit 9050a9cd5e4c ("powercap: intel_rapl: Cleanup Power Limits
> support") pokes the MSR interface PL4_ENABLE primitive and exposes
> this
> bug.
> 
> Sumeet is testing the below fix right now, and I suppose he will give
> some update soon.
> 
> thanks,
> rui
> 
> diff --git a/drivers/powercap/intel_rapl_common.c
> b/drivers/powercap/intel_rapl_common.c
> index 8fac57b28f8a..d407f918876f 100644
> --- a/drivers/powercap/intel_rapl_common.c
> +++ b/drivers/powercap/intel_rapl_common.c
> @@ -658,8 +658,6 @@ static struct rapl_primitive_info
> rpi_msr[NR_RAPL_PRIMITIVES] = {
>                             RAPL_DOMAIN_REG_LIMIT, ARBITRARY_UNIT,
> 0),
>         [PL2_CLAMP] = PRIMITIVE_INFO_INIT(PL2_CLAMP,
> POWER_LIMIT2_CLAMP, 48,
>                             RAPL_DOMAIN_REG_LIMIT, ARBITRARY_UNIT,
> 0),
> -       [PL4_ENABLE] = PRIMITIVE_INFO_INIT(PL4_ENABLE,
> POWER_LIMIT4_MASK, 0,
> -                               RAPL_DOMAIN_REG_PL4, ARBITRARY_UNIT,
> 0),
>         [TIME_WINDOW1] = PRIMITIVE_INFO_INIT(TIME_WINDOW1,
> TIME_WINDOW1_MASK, 17,
>                             RAPL_DOMAIN_REG_LIMIT, TIME_UNIT, 0),
>         [TIME_WINDOW2] = PRIMITIVE_INFO_INIT(TIME_WINDOW2,
> TIME_WINDOW2_MASK, 49,
> 
> 
Let me try this, but this is not enough. The enable/disable is also
gets checked for presence of PL4.

Thanks,
Srinivas

>