Re: [PATCH v12 09/14] x86/sgx: Implement async reclamation for cgroup

Haitao Huang
2024-04-24T00:26:20Z

On Tue, 23 Apr 2024 17:13:15 -0500, Huang, Kai <kai.huang@intel.com> wrote:

> On Tue, 2024-04-23 at 10:30 -0500, Haitao Huang wrote:
>> > > It's a workaround because you use the capacity==0 but it does not
>> really
>> > > mean to disable the misc cgroup for specific resource IIUC.
>> >
>> > Please read the comment around @misc_res_capacity again:
>> >
>> > * Miscellaneous resources capacity for the entire machine. 0
>> capacity
>> > * means resource is not initialized or not present in the host.
>> >
>>
>> I mentioned this in earlier email. I think this means no SGX EPC. It
>> doesnot mean sgx epc cgroup not enabled. That's also consistent with the
>> behavior try_charge() fails if capacity is zero.
>
> OK. To me the "capacity" is purely the concept of cgroup, so it must be
> interpreted within the scope of "cgroup". If cgroup, in our case, SGX
> cgroup, is disabled, then whether "leaving the capacity to reflect the
> presence of hardware resource" doesn't really matter.
> So what you are saying is that, the kernel must initialize the capacity
> of
> some MISC resource once it is added as valid type.
> And you must initialize the "capacity" even MISC cgroup is disabled
> entirely by kernel commandline, in which case, IIUC, misc.capacity is not
> even going to show in the /fs.
>
> If this is your point, then your patch:
>
> cgroup/misc: Add SGX EPC resource type
>
> is already broken, because you added the new type w/o initializing the
> capacity.
>
> Please fix that up.
>
>>
>> > >
>> > > There is explicit way for user to disable misc without setting
>> capacity> > to
>> > > zero.
>> >
>> > Which way are you talking about?
>>
>> Echo "-misc" to cgroup.subtree_control at root level for example still
>> shows non-zero sgx_epc capacity.
>
> I guess "having to disable all MISC resources just in order to disable
> SGX
> EPC cgroup" is a brilliant idea.
>
> You can easily disable the entire MISC cgroup by commandline for that
> purpose if that's acceptable.
>
> And I have no idea why "still showing non-zero EPC capacity" is important
> if SGX cgroup cannot be supported at all.
>

Okay, all I'm trying to say is we should care about consistency in code
and don't want SGX do something different. Mixing "disable" with
"capacity==0" causes inconsistencies AFAICS:

1) The try_charge() API currently returns error when capacity is zero. So
it appears not to mean that the cgroup is disabled otherwise it should
return success.

2) The current explicit way ("-misc") to disable misc still shows non-zero
entries in misc.capacity. (At least for v2 cgroup, it does when I tested).
Maybe this is not important but I just don't feel good about this
inconsistency.

For now I'll just do BUG_ON() unless there are more strong opinions one
way or the other.

BR
Haitao