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

[Sean Christopherson]

On Tue, Sep 29, 2020 at 06:50:10AM +0300, Jarkko Sakkinen wrote:
> 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?

Nope, can't think of anything.