Re: [PATCH v3 12/19] x86/resctrl: Make resctrl_mounted checks explicit

[Reinette Chatre]

Hi James,

On 4/27/2023 7:19 AM, James Morse wrote:
> Hi Reinette,
> 
> On 01/04/2023 00:28, Reinette Chatre wrote:
>> On 3/20/2023 10:26 AM, James Morse wrote:
>>> The rdt_enable_key is switched when resctrl is mounted, and used to
>>> prevent a second mount of the filesystem. It also enables the
>>> architecture's context switch code.
>>>
>>> This requires another architecture to have the same set of static-keys,
>>> as resctrl depends on them too.
>>>
>>> Make the resctrl_mounted checks explicit: resctrl can keep track of
>>> whether it has been mounted once. This doesn't need to be combined with
>>> whether the arch code is context switching the CLOSID.
>>> Tests against the rdt_mon_enable_key become a test that resctrl is
>>> mounted and that monitoring is enabled.
>>
>> The last sentence above makes the code change hard to follow ...
>> (see below)
>>
>>> This will allow the static-key changing to be moved behind resctrl_arch_
>>> calls.
> 
>>> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
>>> index f38cd2f12285..6279f5c98b39 100644
>>> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
>>> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
>>> @@ -834,7 +834,7 @@ void mbm_handle_overflow(struct work_struct *work)
>>>  
>>>  	mutex_lock(&rdtgroup_mutex);
>>>  
>>> -	if (!static_branch_likely(&rdt_mon_enable_key))
>>> +	if (!resctrl_mounted || !static_branch_likely(&rdt_mon_enable_key))
>>
>> ... considering the text in the changelog the "resctrl_mounted" check seems
>> unnecessary. Looking ahead I wonder if this check would not be more
>> appropriate in patch 15?
> 
> How so?
> 
> This is secretly relying on rdt_mon_enable_key being cleared in rdt_kill_sb() when the
> filesystem is unmounted, otherwise the overflow thread keeps running once the filesystem
> is unmounted.

hmmm ... I do not think my feedback was clear. I understand that this is done
as a prep patch but that was only clear when I read patch 15 because as the
work is presented here it seems unnecessary. 

> 
> I thought it simpler to add all these checks explicitly in one go.
> That makes it simpler to thin out the static keys as their 'and its mounted' behaviour is
> no longer relied on.

Understood. If you want to keep this as a prep patch, could you please update the
changelog to reflect this? The following sentence in the changelog makes this patch
hard to follow since it essentially claims that what this patch does is unnecessary:
"Tests against the rdt_mon_enable_key become a test that resctrl is mounted
and that monitoring is enabled."

I also do still wonder why these resctrl_mounted checks cannot move to patch
15 when they are needed. Adding them there makes it obvious that rdt_mon_enable_key
had a dual purpose that patch 15 splits into two separate checks. 

Reinette