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

Haitao Huang
2023-11-26T16:27:48Z

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

> On Mon, 2023-10-30 at 11:20 -0700, Haitao Huang wrote:
>> From: Sean Christopherson <sean.j.christopherson@intel.com>
>>
>> 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@intel.com>
>> Co-developed-by: Kristen Carlson Accardi <kristen@linux.intel.com>
>> Signed-off-by: Kristen Carlson Accardi <kristen@linux.intel.com>
>> Co-developed-by: Haitao Huang <haitao.huang@linux.intel.com>
>> Signed-off-by: Haitao Huang <haitao.huang@linux.intel.com>
>> Cc: Sean Christopherson <seanjc@google.com>
>> ---
>>
>
> [...]
>
>> +/**
>> + * 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