Date: Fri, 12 Jan 2024 11:07:09 -0600
From: "Haitao Huang" <haitao.huang@...ux.intel.com>
To: "Mehta, Sohil" <sohil.mehta@...el.com>, "jarkko@...nel.org"
 <jarkko@...nel.org>, "x86@...nel.org" <x86@...nel.org>,
 "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
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 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "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>, "Zhang, Bo" <zhanb@...rosoft.com>,
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 <anakrish@...rosoft.com>, "sean.j.christopherson@...el.com"
 <sean.j.christopherson@...el.com>, "Li, Zhiquan1" <zhiquan1.li@...el.com>,
 "yangjie@...rosoft.com" <yangjie@...rosoft.com>
Subject: Re: [PATCH v6 09/12] x86/sgx: Restructure top-level EPC reclaim
 function

On Sun, 17 Dec 2023 19:44:56 -0600, Huang, Kai <kai.huang@...el.com> wrote:

>
>> >
>> > The point is, with or w/o this patch, you can only reclaim 16 EPC  
>> pages
>> > in one
>> > function call (as you have said you are going to remove
>> > SGX_NR_TO_SCAN_MAX,
>> > which is a cipher to both of us).  The only difference I can see is,
>> > with this
>> > patch, you can have multiple calls of "isolate" and then call the
>> > "do_reclaim"
>> > once.
>> >
>> > But what's the good of having the "isolate" if the "do_reclaim" can  
>> only
>> > reclaim
>> > 16 pages anyway?
>> >
>> > Back to my last reply, are you afraid of any LRU has less than 16  
>> pages
>> > to
>> > "isolate", therefore you need to loop LRUs of descendants to get 16?
>> > Cause I
>> > really cannot think of any other reason why you are doing this.
>> >
>> >
>>
>> I think I see your point. By capping pages reclaimed per cycle to 16,
>> there is not much difference even if those 16 pages are spread in  
>> separate
>> LRUs . The difference is only significant when we ever raise that cap.  
>> To
>> preserve the current behavior of ewb loops independent on number of LRUs
>> to loop through for each reclaiming cycle, regardless of the exact value
>> of the page cap, I would still think current approach in the patch is
>> reasonable choice.  What do you think?
>
> To me I won't bother to do that.  Having less than 16 pages in one LRU is
> *extremely rare* that should never happen in practice.  It's pointless  
> to make
> such code adjustment at this stage.
>
> Let's focus on enabling functionality first.  When you have some real
> performance issue that is related to this, we can come back then.
>

I have done some rethinking about this and realize this does save quite  
some significant work: without breaking out isolation part from  
sgx_reclaim_pages(), I can remove the changes to use a list for isolated  
pages, and no need to introduce "state" such as RECLAIM_IN_PROGRESS. About  
1/3 of changes for per-cgroup reclamation will be gone.

So I think I'll go this route now. The only downside may be performance if  
a enclave spreads its pages in different cgroups and even that is minimum  
impact as we limit reclamation to 16 pages a time. Let me know if someone  
feel strongly we need dealt with that and see some other potential issues  
I may have missed.

Thanks

Haitao

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