Re: [PATCH v38 16/24] x86/sgx: Add a page reclaimer

Jarkko Sakkinen
2020-09-29T03:50:24Z

On Mon, Sep 28, 2020 at 06:14:39PM -0700, Sean Christopherson wrote:
> On Tue, Sep 22, 2020 at 05:03:23PM +0300, Jarkko Sakkinen wrote:
> > On Tue, Sep 22, 2020 at 12:45:38PM +0200, Borislav Petkov wrote:
> > > > + spin_lock(&sgx_active_page_list_lock);
> > > > + for (i = 0; i < SGX_NR_TO_SCAN; i++) {
> > > > + if (list_empty(&sgx_active_page_list))
> > >
> > > Isn't it enough to do this once, i.e., not in the loop? You're holding
> > > sgx_active_page_list_lock...
>
> Argh, I missed this until I looked at Jarkko's updated tree.
>
> The reason for checking list_empty() on every iteration is that the loop is
> greedy, i.e. it tries to grab and reclaim up to 16 (SGX_NR_TO_SCAN) EPC pages
> at a time.
>
> > I think that would make sense. Distantly analogous to the EINIT
> > discussion. Too complex code for yet to be known problem workloads I'd
> > say.
>
> Nooooo. Please no.

I added this comment in the beginning of the sgx_reclaim_pages() based
on your response:

/*
* Take a fixed number of pages from the head of the active page pool and
* reclaim them to the enclave's private shmem files. Skip the pages, which have
* been accessed since the last scan. Move those pages to the tail of active
* page pool so that the pages get scanned in LRU like fashion.
*
* Batch process a chunk of pages (at the moment 16) 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.
*/

And reverted reclaimer patch as it was. Do you have anything in mind
that I should add or modify in it?

/Jarkko