From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755161AbdKATh4 (ORCPT ); Wed, 1 Nov 2017 15:37:56 -0400 Received: from out4-smtp.messagingengine.com ([66.111.4.28]:35963 "EHLO out4-smtp.messagingengine.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755111AbdKAThx (ORCPT ); Wed, 1 Nov 2017 15:37:53 -0400 X-ME-Sender: Message-Id: <1509565071.2650718.1158454064.7E910622@webmail.messagingengine.com> From: Colin Walters To: Shawn Landden Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" X-Mailer: MessagingEngine.com Webmail Interface - ajax-66b6e65c Subject: Re: [RFC] EPOLL_KILLME: New flag to epoll_wait() that subscribes process to death row (new syscall) Date: Wed, 01 Nov 2017 15:37:51 -0400 In-Reply-To: References: <20171101053244.5218-1-slandden@gmail.com> <1509549397.2561228.1158168688.4CFA4326@webmail.messagingengine.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from quoted-printable to 8bit by nfs id vA1JcmcF024074 On Wed, Nov 1, 2017, at 03:02 PM, Shawn Landden wrote: > > This solves the fact that epoll_pwait() already is a 6 argument (maximum allowed) syscall. But what if the process has multiple epoll() instances in multiple threads? Well, that's a subset of the general question of - what is the interaction of this system call and threading?  It looks like you've prototyped this out in userspace with systemd, but from a quick glance at the current git, systemd's threading is limited doing sync()/fsync() and gethostbyname() async. But languages with a GC tend to at least use a background thread for that, and of course lots of modern userspace makes heavy use of multithreading (or variants like goroutines). A common pattern though is to have a "main thread" that acts as a control point and runs the mainloop (particularly for anything with a GUI). That's going to be the thing calling prctl(SET_IDLE) - but I think its idle state should implicitly affect the whole process, since for a lot of apps those other threads are going to just be "background". It'd probably then be an error to use prctl(SET_IDLE) in more than one thread ever? (Although that might break in golang due to the way goroutines can be migrated across threads) That'd probably be a good "generality test" - what would it take to have this system call be used for a simple golang webserver app that's e.g. socket activated by systemd, or a Kubernetes service? Or another really interesting case would be qemu; make it easy to flag VMs as always having this state (most of my testing VMs are like this; it's OK if they get destroyed, I just reinitialize them from the gold state). Going back to threading - a tricky thing we should handle in general is when userspace libraries create threads that are unknown to the app; the "async gethostbyname()" is a good example. To be conservative we'd likely need to "fail non-idle", but figure out some way tell the kernel for e.g. GC threads that they're still idle.