linux-kernel - Re: [PATCH v1 1/6] x86/resctrl: Move __resctrl_sched

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Date: Sun, 7 Apr 2024 12:21:17 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Peter Newman <peternewman@...gle.com>
CC: Fenghua Yu <fenghua.yu@...el.com>, James Morse <james.morse@....com>,
	Stephane Eranian <eranian@...gle.com>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen
	<dave.hansen@...ux.intel.com>, <x86@...nel.org>, "H. Peter Anvin"
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	<beaub@...ux.microsoft.com>, "Matthew Wilcox (Oracle)" <willy@...radead.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 1/6] x86/resctrl: Move __resctrl_sched_in() out-of-line

Hi Peter,

On 4/5/2024 3:04 PM, Peter Newman wrote:
> Hi Reinette,
> 
> On Thu, Apr 4, 2024 at 4:09 PM Reinette Chatre
> <reinette.chatre@...el.com> wrote:
>>
>> Hi Peter,
>>
>> On 3/25/2024 10:27 AM, Peter Newman wrote:
>>> __resctrl_sched_in() is unable to dereference a struct rdtgroup pointer
>>> when defined inline because rdtgroup is a private structure defined in
>>> internal.h.
>>
>> Being inline has nothing to do with whether it can reference a struct rdtgroup
>> pointer, no?
> 
> No, but it has a lot to do with whether it can de-reference a struct
> rdtgroup pointer in order to obtain a CLOSID or RMID, as doing so
> would pull the definitions for struct rdtgroup and struct mongroup
> into an external header. Before doing so, I would want to make sure
> implementing  __resctrl_sched_in() inline is actually adding value.

I expect that each architecture would need architecture specific task
switching code and by pointing to rdtgroup from the task_struct every
architecture would need to know how to reach the CLOSID/RMID within.

Having architectures reach into the private fs data is not ideal
so we should consider to make just a portion of rdtgroup information
required to support switching code accessible to the architectures, not the
entire rdtgroup and mongroup structures.

..

>>> +static inline void resctrl_sched_in(struct task_struct *tsk)
>>> +{
>>> +#ifdef CONFIG_X86_CPU_RESCTRL
>>> +     if (static_branch_likely(&rdt_enable_key))
>>> +             __resctrl_sched_in(tsk);
>>> +#endif
>>> +}
>>> +
>>
>> include/linux/resctrl.h should rather be divided to accommodate code
>> as below:
>>
>> #ifdef CONFIG_X86_CPU_RESCTRL
>>
>> static inline void resctrl_sched_in(struct task_struct *tsk)
>> {
>>         if (static_branch_likely(&rdt_enable_key))
>>                 __resctrl_sched_in(tsk);
>> }
>>
>> #else
>>
>> static inline void resctrl_sched_in(struct task_struct *tsk) {}
>>
>> #endif
>>
>> so that core code does not get anything unnecessary when CONFIG_X86_CPU_RESCTRL
>> is not set.
> 
> Will do.

I think this needs more thought. rdt_enable_key is x86 specific now and should not
be in the fs code. Every architecture will have its own task switch code, with
__resctrl_sched_in() belonging to x86 and thus not something to be directly called
from the fs code.

Reinette