From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A909CC04AB5 for ; Thu, 6 Jun 2019 17:32:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8CA642083D for ; Thu, 6 Jun 2019 17:32:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728183AbfFFRco (ORCPT ); Thu, 6 Jun 2019 13:32:44 -0400 Received: from mga12.intel.com ([192.55.52.136]:8560 "EHLO mga12.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726924AbfFFRco (ORCPT ); Thu, 6 Jun 2019 13:32:44 -0400 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from orsmga007.jf.intel.com ([10.7.209.58]) by fmsmga106.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 06 Jun 2019 10:32:43 -0700 X-ExtLoop1: 1 Received: from sjchrist-coffee.jf.intel.com (HELO linux.intel.com) ([10.54.74.36]) by orsmga007.jf.intel.com with ESMTP; 06 Jun 2019 10:32:43 -0700 Date: Thu, 6 Jun 2019 10:32:43 -0700 From: Sean Christopherson To: Andy Lutomirski Cc: Jarkko Sakkinen , linux-sgx@vger.kernel.org, Dave Hansen , Cedric Xing , Jethro Beekman , "Dr . Greg Wettstein" Subject: Re: [PATCH 5/7] x86/sgx: Add flag to zero added region instead of copying from source Message-ID: <20190606173243.GE23169@linux.intel.com> References: <20190605194845.926-1-sean.j.christopherson@intel.com> <20190605194845.926-6-sean.j.christopherson@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-sgx-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-sgx@vger.kernel.org On Thu, Jun 06, 2019 at 10:20:38AM -0700, Andy Lutomirski wrote: > On Wed, Jun 5, 2019 at 12:49 PM Sean Christopherson > wrote: > > > > For some enclaves, e.g. an enclave with a small code footprint and a > > large working set, the vast majority of pages added to the enclave are > > zero pages. Introduce a flag to denote such zero pages. The major > > benefit of the flag will be realized in a future patch to use Linux's > > actual zero page as the source, as opposed to explicitly zeroing the > > enclave's backing memory. > > > > I feel like I probably asked this at some point, but why is there a > workqueue here at all? Performance. A while back I wrote a patch set to remove the worker queue and discovered that it tanks enclave build time when the enclave is being hosted by a Golang application. Here's a snippet from a mail discussing the code. The bad news is that I don't think we can remove the add page worker as applications with userspace schedulers, e.g. Go's M:N scheduler, can see a 10x or more throughput improvement when using the worker queue. I did a bit of digging for the Golang case to make sure I wasn't doing something horribly stupid/naive and found that it's a generic issue in Golang with blocking (or just long-running) system calls. Because Golang multiplexes Goroutines on top of OS threads, blocking syscalls introduce latency and context switching overhead, e.g. Go's scheduler will spin up a new OS thread to service other Goroutines after it realizes the syscall has blocked, and will later destroy one of the OS threads so that it doesn't build up too many unused. IIRC, the scenario is spinning up several goroutines, each building an enclave. I played around with adding a flag to do a synchronous EADD but didn't see a meaningful change in performance for the simple case. Supporting both the worker queue and direct paths was complex enough that I decided it wasn't worth the trouble for initial upstreaming.