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

[Haitao Huang]

On Mon, 20 Nov 2023 11:45:46 +0800, Huang, Kai <kai.huang@xxxxxxxxx> wrote:

> On Mon, 2023-10-30 at 11:20 -0700, Haitao Huang wrote:
> > From: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> >
> > To prepare for per-cgroup reclamation, separate the top-level reclaim
> > function, sgx_reclaim_epc_pages(), into two separate functions:
> >
> > - sgx_isolate_epc_pages() scans and isolates reclaimable pages from a  
> > given LRU list.
> > - sgx_do_epc_reclamation() performs the real reclamation for the  
> > already isolated pages.
> >
> > Create a new function, sgx_reclaim_epc_pages_global(), calling those two
> > in succession, to replace the original sgx_reclaim_epc_pages(). The
> > above two functions will serve as building blocks for the reclamation
> > flows in later EPC cgroup implementation.
> >
> > sgx_do_epc_reclamation() returns the number of reclaimed pages. The EPC
> > cgroup will use the result to track reclaiming progress.
> >
> > sgx_isolate_epc_pages() returns the additional number of pages to scan
> > for current epoch of reclamation. The EPC cgroup will use the result to
> > determine if more scanning to be done in LRUs in its children groups.
>
> This changelog says nothing about "why", but only mentions the  
> "implementation".
>
> For instance, assuming we need to reclaim @npages_to_reclaim from the
> @epc_cgrp_to_reclaim and its descendants, why cannot we do:
>
> 	for_each_cgroup_and_descendants(&epc_cgrp_to_reclaim, &epc_cgrp) {
> 		if (npages_to_reclaim <= 0)
> 			return;
>
> 		npages_to_reclaim -= sgx_reclaim_pages_lru(&epc_cgrp->lru,
> 					npages_to_reclaim);
> 	}
>
> Is there any difference to have "isolate" + "reclaim"?

This is to optimize "reclaim". See how etrack was done in sgx_encl_ewb.
Here we just follow the same design as ksgxd for each reclamation cycle.

> > Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> > Co-developed-by: Kristen Carlson Accardi <kristen@xxxxxxxxxxxxxxx>
> > Signed-off-by: Kristen Carlson Accardi <kristen@xxxxxxxxxxxxxxx>
> > Co-developed-by: Haitao Huang <haitao.huang@xxxxxxxxxxxxxxx>
> > Signed-off-by: Haitao Huang <haitao.huang@xxxxxxxxxxxxxxx>
> > Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
> > ---
>
> [...]
>
> > +/**
> > + * sgx_do_epc_reclamation() - Perform reclamation for isolated EPC  
> > pages.
> > + * @iso:		List of isolated pages for reclamation
> > + *
> > + * Take a list of EPC pages and reclaim them to the enclave's private  
> > shmem files.  Do not
> > + * reclaim the pages that have been accessed since the last scan, and  
> > move each of those pages
> > + * to the tail of its tracking LRU list.
> > + *
> > + * Limit the number of pages to be processed up to SGX_NR_TO_SCAN_MAX  
> > per call in order to
> > + * degrade amount of IPI's and ETRACK's potentially required.  
> > sgx_encl_ewb() does degrade a bit
> > + * among the HW threads with three stage EWB pipeline (EWB, ETRACK +  
> > EWB and IPI + EWB) but not
> > + * sufficiently. Reclaiming one page at a time would also be  
> > problematic as it would increase
> > + * the lock contention too much, which would halt forward progress.
>
> This is kinda optimization, correct?  Is there any real performance data  
> to
> justify this?

The above sentences were there originally. This optimization was justified.

> If this optimization is useful, shouldn't we bring this
> optimization to the current sgx_reclaim_pages() instead, e.g., just  
> increase
> SGX_NR_TO_SCAN (16) to SGX_NR_TO_SCAN_MAX (32)?

SGX_NR_TO_SCAN_MAX might be designed earlier for other reasons I don't  
know. Currently it is really the buffer size allocated in  
sgx_reclaim_pages(). Both cgroup and ksgxd scan 16 pages a time.Maybe we  
should just use SGX_NR_TO_SCAN. No _MAX needed. The point was to batch  
reclamation to certain number to minimize impact of EWB pipeline. 16 was  
the original design.

Thanks
Haitao