Re: [PATCH v12 08/14] x86/sgx: Add basic EPC reclamation flow for cgroup

[Haitao Huang]

On Wed, 17 Apr 2024 18:51:28 -0500, Huang, Kai <kai.huang@xxxxxxxxx> wrote:

> On 16/04/2024 3:20 pm, Haitao Huang wrote:
> > From: Kristen Carlson Accardi <kristen@xxxxxxxxxxxxxxx>
> >  Currently in the EPC page allocation, the kernel simply fails the
> > allocation when the current EPC cgroup fails to charge due to its usage
> > reaching limit.  This is not ideal. When that happens, a better way is
> > to reclaim EPC page(s) from the current EPC cgroup (and/or its
> > descendants) to reduce its usage so the new allocation can succeed.
> >  Add the basic building blocks to support per-cgroup reclamation.
> >  Currently the kernel only has one place to reclaim EPC pages: the  
> > global
> > EPC LRU list.  To support the "per-cgroup" EPC reclaim, maintain an LRU
> > list for each EPC cgroup, and introduce a "cgroup" variant function to
> > reclaim EPC pages from a given EPC cgroup and its descendants.
> >  Currently the kernel does the global EPC reclaim in sgx_reclaim_page().
> > It always tries to reclaim EPC pages in batch of SGX_NR_TO_SCAN (16)
> > pages.  Specifically, it always "scans", or "isolates" SGX_NR_TO_SCAN
> > pages from the global LRU, and then tries to reclaim these pages at once
> > for better performance.
> >  Implement the "cgroup" variant EPC reclaim in a similar way, but keep
> > the implementation simple: 1) change sgx_reclaim_pages() to take an LRU
> > as input, and return the pages that are "scanned" and attempted for
> > reclamation (but not necessarily reclaimed successfully); 2) loop the
> > given EPC cgroup and its descendants and do the new sgx_reclaim_pages()
> > until SGX_NR_TO_SCAN pages are "scanned".
> >  This implementation, encapsulated in sgx_cgroup_reclaim_pages(), always
> > tries to reclaim SGX_NR_TO_SCAN pages from the LRU of the given EPC
> > cgroup, and only moves to its descendants when there's no enough
> > reclaimable EPC pages to "scan" in its LRU.  It should be enough for
> > most cases.
> >  Note, this simple implementation doesn't _exactly_ mimic the current
> > global EPC reclaim (which always tries to do the actual reclaim in batch
> > of SGX_NR_TO_SCAN pages): when LRUs have less than SGX_NR_TO_SCAN
> > reclaimable pages, the actual reclaim of EPC pages will be split into
> > smaller batches _across_ multiple LRUs with each being smaller than
> > SGX_NR_TO_SCAN pages.
> >  A more precise way to mimic the current global EPC reclaim would be to
> > have a new function to only "scan" (or "isolate") SGX_NR_TO_SCAN pages
> > _across_ the given EPC cgroup _AND_ its descendants, and then do the
> > actual reclaim in one batch.  But this is unnecessarily complicated at
> > this stage.
> >  Alternatively, the current sgx_reclaim_pages() could be changed to
> > return the actual "reclaimed" pages, but not "scanned" pages. However,
> > the reclamation is a lengthy process, forcing a successful reclamation
> > of predetermined number of pages may block the caller for too long. And
> > that may not be acceptable in some synchronous contexts, e.g., in
> > serving an ioctl().
> >  With this building block in place, add synchronous reclamation support
> > in sgx_cgroup_try_charge(): trigger a call to
> > sgx_cgroup_reclaim_pages() if the cgroup reaches its limit and the
> > caller allows synchronous reclaim as indicated by s newly added
> > parameter.
> >  A later patch will add support for asynchronous reclamation reusing
> > sgx_cgroup_reclaim_pages().
> >  Note all reclaimable EPC pages are still tracked in the global LRU thus
> > no per-cgroup reclamation is actually active at the moment. Per-cgroup
> > tracking and reclamation will be turned on in the end after all
> > necessary infrastructure is in place.
>
> Nit:
>
> "all necessary infrastructures are in place", or, "all necessary  
> building blocks are in place".
>
> ?
>
> >  Co-developed-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> > Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> > Signed-off-by: Kristen Carlson Accardi <kristen@xxxxxxxxxxxxxxx>
> > Co-developed-by: Haitao Huang <haitao.huang@xxxxxxxxxxxxxxx>
> > Signed-off-by: Haitao Huang <haitao.huang@xxxxxxxxxxxxxxx>
> > Tested-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
> > ---
>
> Reviewed-by: Kai Huang <kai.huang@xxxxxxxxx>

Thanks

> More nitpickings below:
>
> [...]
>
> > -static inline int sgx_cgroup_try_charge(struct sgx_cgroup *sgx_cg)
> > +static inline int sgx_cgroup_try_charge(struct sgx_cgroup *sgx_cg,  
> > enum sgx_reclaim reclaim)
>
> Let's still wrap the text on 80-character basis.
>
> I guess most people are more used to that.
>
> [...]
>
> > -		epc_page = list_first_entry_or_null(&sgx_global_lru.reclaimable,
> > -						    struct sgx_epc_page, list);
> > +		epc_page = list_first_entry_or_null(&lru->reclaimable, struct  
> > sgx_epc_page, list);
>
> Ditto.

Actually I changed to 100 char width based on comments from Jarkko IIRC.
I don't have personal preference, but will not change back to 80 unless  
Jarkko also agrees.

Thanks
Haitao