linux-kernel - Re: [PATCH v1 24/31] x86/resctrl: Move get_config

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Date: Thu, 11 Apr 2024 10:41:03 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Dave Martin <Dave.Martin@....com>
CC: James Morse <james.morse@....com>, <x86@...nel.org>,
	<linux-kernel@...r.kernel.org>, Fenghua Yu <fenghua.yu@...el.com>, "Thomas
 Gleixner" <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "Borislav
 Petkov" <bp@...en8.de>, H Peter Anvin <hpa@...or.com>, Babu Moger
	<Babu.Moger@....com>, <shameerali.kolothum.thodi@...wei.com>, "D Scott
 Phillips OS" <scott@...amperecomputing.com>, <carl@...amperecomputing.com>,
	<lcherian@...vell.com>, <bobo.shaobowang@...wei.com>,
	<tan.shaopeng@...itsu.com>, <baolin.wang@...ux.alibaba.com>, Jamie Iles
	<quic_jiles@...cinc.com>, Xin Hao <xhao@...ux.alibaba.com>,
	<peternewman@...gle.com>, <dfustini@...libre.com>, <amitsinght@...vell.com>,
	David Hildenbrand <david@...hat.com>, Rex Nie <rex.nie@...uarmicro.com>
Subject: Re: [PATCH v1 24/31] x86/resctrl: Move get_config_index() to a header

Hi Dave,

On 4/11/2024 7:25 AM, Dave Martin wrote:
> On Mon, Apr 08, 2024 at 08:25:26PM -0700, Reinette Chatre wrote:
>> Hi James,
>>
>> On 3/21/2024 9:50 AM, James Morse wrote:
>>> get_config_index() is used by the architecture specific code to map a
>>> CLOSID+type pair to an index in the configuration arrays.
>>>
>>> MPAM needs to do this too to preserve the ABI to user-space, there is
>>> no reason to do it differently.
>>>
>>> Move the helper to a header file.
>>>
>>> Signed-off-by: James Morse <james.morse@....com>
>>> ---
>>>  arch/x86/kernel/cpu/resctrl/ctrlmondata.c | 19 +++----------------
>>>  include/linux/resctrl.h                   | 15 +++++++++++++++
>>>  2 files changed, 18 insertions(+), 16 deletions(-)
> 
> [...]
> 
>>> diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
>>> index 3de5bc63ace0..73c111963433 100644
>>> --- a/include/linux/resctrl.h
>>> +++ b/include/linux/resctrl.h
>>> @@ -258,6 +258,21 @@ bool resctrl_arch_is_evt_configurable(enum resctrl_event_id evt);
>>>  void resctrl_arch_mon_event_config_write(void *info);
>>>  void resctrl_arch_mon_event_config_read(void *info);
>>>  
>>> +/* For use by arch code to remap resctrl's smaller CDP CLOSID range */
>>> +static inline u32 resctrl_get_config_index(u32 closid,
>>> +					   enum resctrl_conf_type type)
>>> +{
>>> +	switch (type) {
>>> +	default:
>>> +	case CDP_NONE:
>>> +		return closid;
>>> +	case CDP_CODE:
>>> +			return (closid * 2) + 1;
>>> +	case CDP_DATA:
>>> +			return (closid * 2);
>>> +	}
>>> +}
>>
>> (please check the tabs)
> 
> Noted.  I also see that redundant parentheses seem spuriously added
> compared with the original version of this moved code.  I can make a
> note to drop them if you prefer.
> 
>> This change is unexpected to me. Could you please elaborate how
>> MPAM's variant of CDP works?
>>
>> Thank you very much.
>>
>> Reinette
> 
> Note: I haven't discussed this specifically with James, so the following
> is my best guess at the rationale...  With that in mind:
> 
> For MPAM, CDP isn't a special mode; instead, the PARTIDs for
> instructions and data are always configured independently in the CPU.
> If resctrl is not configured for CDP, we simply program the same PARTID
> value both for instructions and data on task switch.
> 
> For a given resctrl control group we could pick two random unrelated
> PARTIDs, but there seems to be no advantage in doing that since resctrl
> enables cdp globally or not, and we would require more effort to
> translate resctrl closids to PARTIDs if we didn't pair the IDs up
> systematically.
> 
> (See [1], [2] in James' snapshot, which illustrate how he proposes
> to do it.)
> 
> 
> So, we may as well stick with the same scheme already established for
> x86: nothing forces us to do that, but it looks simpler than the
> alternatives.  I think that's the idea, anyway.
> 
> Then, if the same scheme is used by multiple arches (and 100% of the
> arches currently known to resctrl), it probably makes sense to share the
> definition of the mapping at least as a default for arches that don't
> have their own different ways of doing it.
> 
> Does this make sense?

It does, thank you very much.

> 
> I can recommend adding some of this rationale to the commit message
> if it helps (and assuming I'm right!)

Sounds good, thank you.

Reinette