Re: [PATCH 1/3] x86/resctrl: Display the number of available CLOSIDs

[Reinette Chatre]

Hi Haifeng,

On 1/24/2024 11:44 PM, Haifeng Xu wrote:
> On 2024/1/25 06:23, Reinette Chatre wrote:
>> On 1/23/2024 1:20 AM, Haifeng Xu wrote:
>>> We can know the number of CLOSIDs for each rdt resource, for example:
>>>
>>> cat /sys/fs/resctrl/info/L3/num_closids
>>> cat /sys/fs/resctrl/info/MB/num_closids
>>> ...
>>>
>>> The number of available CLOSIDs is the minimal value of them. When users
>>> try to create new control groups, to avoid running out of CLOSIDs, they
>>> have to traverse /sys/fs/resctrl/ and count the number of directories.
>>>
>>> To make things more easier, add a RFTYPE_TOP_INFO file 'free_closids'
>>> that tells users how many free closids are left.
>>
>> I do not see this as a change that benefits the kernel or user space.
>> It sounds to me as though user space is planning some behavior based
>> on what it knows about the current kernel internals and requesting
>> more information to peek into these internals to make it easier
>> to do so. The kernel can always choose to do things different
>> internally, but it is required to maintain a consistent interface to
>> user space. We should thus always take great care with new interfaces.
>>
>> From what I can tell user space intends to use this "free_closids"
>> to mean "how many more control resource groups can be created". This
>> is not a contract that I think we should enter into. There has been
>> discussions aiming to disconnect the number of resource groups
>> from the number of closids (effectively letting resource groups
>> with the same resource allocations share a closid). 
> 
> Is this feature merged into latest kernel or just discussions?
> Could you please provide more details?
> Last time, you mentioned that a monitoring group can be moved
> from one control group to another.

It was the original proposal that started a discussion [1]. The discussion
about the problem needing solved instead led to the feature where a monitoring
group can be moved from one control group to another. This solution only works
for AMD and Intel though.

> This is something
>> that the kernel may still do at some point but sharing "free_closids"
>> knowing that user space intends to use it as a "number of resource groups
>> remaining" counter would make future enhancements like this difficult.
> 
> OK, thanks.
> 
>>
>> Could you please provide more detail in why this is required? User
>> space should not need to keep track to know how many groups can be
>> created, creating a new group will fail with ENOSPC if no more
>> groups can be created.
>>
> 
> User space reports alerts when failing to create new groups. If no one tell them that
> closids aren't enough, they will keep trying to create new groups and the number of alerts
> could be very high.

When user space receives the "no space available" it should be expected that
any more attempts to create new groups will also fail (until a resource group is
removed). It is futile for user space to keep trying in this scenario.

> 
> So we want to know how many closids are available, if it's zero, we give up creating
> new control groups and those alerts will disappear.
> 
> Maybe user behavoirs can be ajusted. There is no need to create too many groups, especially
> for those groups with same resource. Or as you mentioned above, we can reuse closid.

Reinette

[1] https://lore.kernel.org/lkml/CALPaoCj-zav4x6H3ffXo_O+RFan8Qb-uLy-DdtkaQTfuxY4a0w@xxxxxxxxxxxxxx/