Re: [PATCH v1 05/31] x86/resctrl: Remove rdtgroup from update_cpu_closid_rmid()

[Reinette Chatre]

Hi Dave,

On 4/12/2024 9:12 AM, Dave Martin wrote:
> On Mon, Apr 08, 2024 at 08:16:08PM -0700, Reinette Chatre wrote:
>> Hi James,
>>
>> On 3/21/2024 9:50 AM, James Morse wrote:
>>> update_cpu_closid_rmid() takes a struct rdtgroup as an argument, which
>>> it uses to update the local CPUs default pqr values. This is a problem
>>> once the resctrl parts move out to /fs/, as the arch code cannot
>>> poke around inside struct rdtgroup.
>>>
>>> Rename update_cpu_closid_rmid() as resctrl_arch_sync_cpus_defaults()
>>> to be used as the target of an IPI, and pass the effective CLOSID
>>> and RMID in a new struct.
>>>
>>> Signed-off-by: James Morse <james.morse@xxxxxxx>
>>> ---
>>>  arch/x86/kernel/cpu/resctrl/rdtgroup.c | 19 +++++++++++++++----
>>>  include/linux/resctrl.h                | 11 +++++++++++
>>>  2 files changed, 26 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>> index 5d2c1ce5b6b1..18f097fce51e 100644
>>> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>> @@ -341,13 +341,13 @@ static int rdtgroup_cpus_show(struct kernfs_open_file *of,
>>>   * from update_closid_rmid() is protected against __switch_to() because
>>>   * preemption is disabled.
>>>   */
>>> -static void update_cpu_closid_rmid(void *info)
>>> +void resctrl_arch_sync_cpu_defaults(void *info)
>>>  {
>>> -	struct rdtgroup *r = info;
>>> +	struct resctrl_cpu_sync *r = info;
>>>  
>>>  	if (r) {
>>>  		this_cpu_write(pqr_state.default_closid, r->closid);
>>> -		this_cpu_write(pqr_state.default_rmid, r->mon.rmid);
>>> +		this_cpu_write(pqr_state.default_rmid, r->rmid);
>>>  	}
>>>  
>>>  	/*
>>> @@ -362,11 +362,22 @@ static void update_cpu_closid_rmid(void *info)
>>>   * Update the PGR_ASSOC MSR on all cpus in @cpu_mask,
>>>   *
>>>   * Per task closids/rmids must have been set up before calling this function.
>>> + * @r may be NULL.
>>>   */
>>>  static void
>>>  update_closid_rmid(const struct cpumask *cpu_mask, struct rdtgroup *r)
>>>  {
>>> -	on_each_cpu_mask(cpu_mask, update_cpu_closid_rmid, r, 1);
>>> +	struct resctrl_cpu_sync defaults;
>>> +	struct resctrl_cpu_sync *defaults_p = NULL;
>>
>> Please maintain reverse fir order.
> 
> Or, more tersely as follows?
> 
> 	struct resctrl_cpu_sync defaults, *defaults_p = NULL;

Sure.

> 
> "Reverse fir order" seems to be documented as a preference rather than a
> rule.

This does not seem to be a place that warrants an exception to this
preference. Note how this function is not consistent with any other
in the file.

> The declarations can be swapped, but defaults_p is in some sense a weak
> pointer to defaults, so it feels a bit strange to declare them backwards.
> 
> Alternatively, could we rename defaults_p to p?  Given the size of this
> function I don't think that impacts clarity.

Do you imply that this would maintain the order in this patch? It does
not look to me that it would but I may be looking wrong.

sidenote: the "on_each_cpu_mask()" in update_closid_rmid() can be on
one line.

> 
> I'll wait for your opinion on this.
> 
> 

..

>>> + * struct resctrl_cpu_sync, or NULL.
>>> + */
>>
>> Updating the CPU's defaults is not the primary goal of this function and because
>> of that I do not think this should be the focus with the main goal (updating
>> RMID and CLOSID on CPU) ignored. Specifically, this function only updates
>> the defaults if *info is set but it _always_ ensures CPU is running with
>> appropriate CLOSID/RMID (which may or may not be from a CPU default).
>>
>> I think resctrl_arch_sync_cpu_closid_rmid() may be more appropriate
>> and the comment needs to elaborate what the function does.
>>
>>> +void resctrl_arch_sync_cpu_defaults(void *info);
> 
> That seems reasonable, and follows the original naming and what the
> code does:
> 
> What about:
> 
> /**
>  * resctrl_arch_sync_cpu_defaults() - Refresh the CPU's CLOSID and RMID.
>  *				      Call via IPI.

Did you intend to change function name?

How about "Refresh the CPU's ..." -> "Refresh this CPU's ..." I think it
makes it more obvious how this function is called.

>  * @info:	If non-NULL, a pointer to a struct resctrl_cpu_sync specifying
>  *		the new CLOSID and RMID for tasks in the default resctrl ctrl
>  *		and mon group when running on this CPU.  If NULL, the default
>  *		CLOSID and RMID are not changed.

"If NULL, this CPU is not re-assigned to a different group." ?

>  *
>  * This is how reassignment of CPUs and/or tasks to different resctrl groups
>  * is propagated when requested by the resctrl fs core code.

Could you please use imperative tone here?  For example, "Propagates reassignment
of CPUs and/or tasks to different resctrl groups."

>  *
>  * This function should typically record the per-cpu defaults specified by

"should" sounds like there may be cases when this is not done? Maybe just
"Records the per-CPU defaults specified ..."

>  * @info (if any), and then reconfigure the CPU's hardware CLOSID and RMID
>  * for subsequent execution based on @current, in the same way as during a
>  * task switch.
>  */
> 
> ...?

Reinette