Re: [PATCH v6 09/12] x86/sgx: Restructure top-level EPC reclaim function

[Haitao Huang]

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