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

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Date: Tue, 23 Apr 2024 19:26:15 -0500
From: "Haitao Huang" <haitao.huang@...ux.intel.com>
To: "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>,
 "jarkko@...nel.org" <jarkko@...nel.org>, "x86@...nel.org" <x86@...nel.org>,
 "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
 "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>, "hpa@...or.com"
 <hpa@...or.com>, "mingo@...hat.com" <mingo@...hat.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "mkoutny@...e.com" <mkoutny@...e.com>, "Mehta, Sohil" <sohil.mehta@...el.com>,
 "tj@...nel.org" <tj@...nel.org>, "tim.c.chen@...ux.intel.com"
 <tim.c.chen@...ux.intel.com>, "tglx@...utronix.de" <tglx@...utronix.de>,
 "bp@...en8.de" <bp@...en8.de>, "Huang, Kai" <kai.huang@...el.com>
Cc: "mikko.ylinen@...ux.intel.com" <mikko.ylinen@...ux.intel.com>,
 "seanjc@...gle.com" <seanjc@...gle.com>, "anakrish@...rosoft.com"
 <anakrish@...rosoft.com>, "Zhang, Bo" <zhanb@...rosoft.com>,
 "kristen@...ux.intel.com" <kristen@...ux.intel.com>, "yangjie@...rosoft.com"
 <yangjie@...rosoft.com>, "Li, Zhiquan1" <zhiquan1.li@...el.com>,
 "chrisyan@...rosoft.com" <chrisyan@...rosoft.com>
Subject: Re: [PATCH v12 09/14] x86/sgx: Implement async reclamation for cgroup

On Tue, 23 Apr 2024 17:13:15 -0500, Huang, Kai <kai.huang@...el.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