linux-kernel - Re: [PATCH v2 06/18] x86/resctrl: Allow the allocator to check if a CLOSID can allocate clean RMID

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Message-ID: <7a07defc-356b-6dc2-e3af-c52b1730fc9b@arm.com>
Date:   Mon, 20 Mar 2023 17:12:19 +0000
From:   James Morse <james.morse@....com>
To:     Reinette Chatre <reinette.chatre@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     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,
        xingxin.hx@...nanolis.org, baolin.wang@...ux.alibaba.com,
        Jamie Iles <quic_jiles@...cinc.com>,
        Xin Hao <xhao@...ux.alibaba.com>, peternewman@...gle.com
Subject: Re: [PATCH v2 06/18] x86/resctrl: Allow the allocator to check if a
 CLOSID can allocate clean RMID

Hi Reinette,

On 10/03/2023 19:59, Reinette Chatre wrote:
> On 3/3/2023 10:36 AM, James Morse wrote:
>> On 02/02/2023 23:46, Reinette Chatre wrote:
>>> On 1/13/2023 9:54 AM, James Morse wrote:
> 
> ...
> 
>>>> +bool resctrl_closid_is_dirty(u32 closid)
>>>> +{
>>>> +	struct rmid_entry *entry;
>>>> +	int i;
>>>> +
>>>> +	lockdep_assert_held(&rdtgroup_mutex);
>>>> +
>>>> +	if (!IS_ENABLED(CONFIG_RESCTRL_RMID_DEPENDS_ON_CLOSID))
>>>> +		return false;
>>
>>> Why is a config option chosen? Is this not something that can be set in the
>>> architecture specific code using a global in the form matching existing related
>>> items like "arch_has..." or "arch_needs..."?
>>
>> It doesn't vary by platform, so making it a runtime variable would mean x86 has to carry
>> this extra code around, even though it will never use it. Done like this, the compiler can
>> dead-code eliminate the below checks and embed the constant return value in all the callers.

> This is fair. I missed any other mention of this option in this series so I
> assume this will be a config that will be "select"ed automatically without
> users needing to think about whether it is needed?

Yes, MPAM platforms would unconditionally enable it, as it reflects how MPAM works. [0]
Users would never need to know it exists.


>>>> +	for (i = 0; i < resctrl_arch_system_num_rmid_idx(); i++) {
>>>> +		entry = &rmid_ptrs[i];
>>>> +		if (entry->closid != closid)
>>>> +			continue;
>>>> +
>>>> +		if (entry->busy)
>>>> +			return true;
>>>> +	}
>>>> +
>>>> +	return false;
>>>> +}
>>>
>>> If I understand this correctly resctrl_closid_is_dirty() will return true if
>>> _any_ RMID/PMG associated with a CLOSID is in use. That is, a CLOSID may be
>>> able to support 100s of PMG but if only one of them is busy then the CLOSID
>>> will be considered unusable ("dirty"). It sounds to me that there could be scenarios
>>> when CLOSID could be considered unavailable while there are indeed sufficient
>>> resources?
>>
>> You are right this could happen. I guess the better approach would be to prefer the
>> cleanest CLOSID that has a clean PMG=0. User-space may not be able to allocate all the
>> monitor groups immediately, but that would be preferable to failing the control group
>> creation.
>>
>> But as this code doesn't get built until the MPAM driver is merged, I'd like to keep it to
>> an absolute minimum. This would be more than is needed for MPAM to have close to resctrl
>> feature-parity, so I'd prefer to do this as an improvement once the MPAM driver is upstream.
>>
>> (also in this category is better use of MPAM's monitors and labelling traffic from the iommu)
>>
>>
>>> The function comment states "Determine if clean RMID can be allocate for this
>>> CLOSID." - if I understand correctly it behaves more like "Determine if all
>>> RMID associated with CLOSID are available".
>>
>> Yes, I'll fix that.
> 
> I understand your reasoning for the solution chosen. Would you be ok to expand on
> the function comment more to document the intentions that you summarize above (eg. "This
> is the absolute minimum solution that will be/should be/could be improved ...")?

Sure thing,


Thanks,

James

[0]
https://git.kernel.org/pub/scm/linux/kernel/git/morse/linux.git/tree/drivers/platform/mpam/Kconfig?h=mpam/snapshot/v6.2&id=ef6b11256ba2cceaff846c96183e8eb6019642d7