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

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Message-ID: <20200929035010.GB301037@linux.intel.com>
Date:   Tue, 29 Sep 2020 06:50:10 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        linux-sgx@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Jethro Beekman <jethro@...tanix.com>,
        Jordan Hand <jorhand@...ux.microsoft.com>,
        Nathaniel McCallum <npmccallum@...hat.com>,
        Chunyang Hui <sanqian.hcy@...fin.com>,
        Seth Moore <sethmo@...gle.com>, akpm@...ux-foundation.org,
        andriy.shevchenko@...ux.intel.com, asapek@...gle.com,
        cedric.xing@...el.com, chenalexchen@...gle.com,
        conradparker@...gle.com, cyhanish@...gle.com,
        dave.hansen@...el.com, haitao.huang@...el.com,
        josh@...htriplett.org, kai.huang@...el.com, kai.svahn@...el.com,
        kmoy@...gle.com, ludloff@...gle.com, luto@...nel.org,
        nhorman@...hat.com, puiterwijk@...hat.com, rientjes@...gle.com,
        tglx@...utronix.de, yaozhangx@...gle.com
Subject: Re: [PATCH v38 16/24] x86/sgx: Add a page reclaimer

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