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

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Date: Fri, 05 Jan 2024 14:13:31 -0600
From: "Haitao Huang" <haitao.huang@...ux.intel.com>
To: jarkko@...nel.org, dave.hansen@...ux.intel.com, tj@...nel.org,
 mkoutny@...e.com, linux-kernel@...r.kernel.org, linux-sgx@...r.kernel.org,
 x86@...nel.org, cgroups@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
 bp@...en8.de, hpa@...or.com, sohil.mehta@...el.com, "Dave Hansen"
 <dave.hansen@...el.com>
Cc: zhiquan1.li@...el.com, kristen@...ux.intel.com, seanjc@...gle.com,
 zhanb@...rosoft.com, anakrish@...rosoft.com, mikko.ylinen@...ux.intel.com,
 yangjie@...rosoft.com
Subject: Re: [PATCH v6 00/12] Add Cgroup support for SGX EPC memory

On Fri, 05 Jan 2024 12:29:05 -0600, Dave Hansen <dave.hansen@...el.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