Re: [PATCH v6 00/12] Add Cgroup support for SGX EPC memory

Haitao Huang
2024-01-05T20:13:36Z

On Fri, 05 Jan 2024 12:29:05 -0600, Dave Hansen <dave.hansen@intel.com>
wrote:

> There's very little about how the LRU design came to be in this cover
> letter. Let's add some details.
>
> How's this?
>
> Writing this up, I'm a lot more convinced that this series is, in
> general, taking the right approach. I honestly don't see any other
> alternatives. As much as I'd love to do something stupidly simple like
> just killing enclaves at the moment they hit the limit, that would be a
> horrid experience for users _and_ a departure from what the existing
> reclaim support does.
>
> That said, there's still a lot of work do to do refactor this series.
> It's in need of some love to make it more clear what is going on and to
> making the eventual switch over to per-cgroup LRUs more gradual. Each
> patch in the series is still doing way too much, _especially_ in patch
> 10.
>
> ==
>
> The existing EPC memory management aims to be a miniature version of the
> core VM where EPC memory can be overcommitted and reclaimed. EPC
> allocations can wait for reclaim. The alternative to waiting would have
> been to send a signal and let the enclave die.
>
> This series attempts to implement that same logic for cgroups, for the
> same reasons: it's preferable to wait for memory to become available and
> let reclaim happen than to do things that are fatal to enclaves.
>
> There is currently a global reclaimable page SGX LRU list. That list
> (and the existing scanning algorithm) is essentially useless for doing
> reclaim when a cgroup hits its limit because the cgroup's pages are
> scattered around that LRU. It is unspeakably inefficient to scan a
> linked list with millions of entries for what could be dozens of pages
> from a cgroup that needs reclaim.
>
> Even if unspeakably slow reclaim was accepted, the existing scanning
> algorithm only picks a few pages off the head of the global LRU. It
> would either need to hold the list locks for unreasonable amounts of
> time, or be taught to scan the list in pieces, which has its own
> challenges.
>
> tl;dr: An cgroup hitting its limit should be as similar as possible to
> the system running out of EPC memory. The only two choices to implement
> that are nasty changes the existing LRU scanning algorithm, or to add
> new LRUs. The result: Add a new LRU for each cgroup and scans those
> instead. Replace the existing global cgroup with the root cgroup's LRU
> (only when this new support is compiled in, obviously).
>

I'll add this to the cover letter as a section justifying the LRU design
for per-cgroup reclaiming.
Thank you very much.

Haitao