From mboxrd@z Thu Jan 1 00:00:00 1970 From: Srivatsa Vaddagiri Subject: Re: [PATCH] qemu-kvm: response to SIGUSR1 to start/stop a VCPU (v2) Date: Wed, 1 Dec 2010 23:30:40 +0530 Message-ID: <20101201180040.GH8073@linux.vnet.ibm.com> References: <1290530963-3448-1-git-send-email-aliguori@us.ibm.com> <4CECCA39.4060702@redhat.com> <4CED1A23.9030607@linux.vnet.ibm.com> <4CED1FD3.1000801@redhat.com> <20101201123742.GA3780@linux.vnet.ibm.com> <4CF6460C.5070604@redhat.com> <20101201161221.GA8073@linux.vnet.ibm.com> <1291220718.32004.1696.camel@laptop> <20101201172953.GF8073@linux.vnet.ibm.com> <1291225502.32004.1787.camel@laptop> Reply-To: vatsa@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Avi Kivity , Anthony Liguori , qemu-devel@nongnu.org, kvm@vger.kernel.org, Chris Wright , Ingo Molnar , Mike Galbraith To: Peter Zijlstra Return-path: Received: from e31.co.us.ibm.com ([32.97.110.149]:46283 "EHLO e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750857Ab0LASAs (ORCPT ); Wed, 1 Dec 2010 13:00:48 -0500 Received: from d03relay03.boulder.ibm.com (d03relay03.boulder.ibm.com [9.17.195.228]) by e31.co.us.ibm.com (8.14.4/8.13.1) with ESMTP id oB1HlL6w011892 for ; Wed, 1 Dec 2010 10:47:21 -0700 Received: from d03av02.boulder.ibm.com (d03av02.boulder.ibm.com [9.17.195.168]) by d03relay03.boulder.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id oB1I0ldm076528 for ; Wed, 1 Dec 2010 11:00:47 -0700 Received: from d03av02.boulder.ibm.com (loopback [127.0.0.1]) by d03av02.boulder.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id oB1I0hYm028969 for ; Wed, 1 Dec 2010 11:00:46 -0700 Content-Disposition: inline In-Reply-To: <1291225502.32004.1787.camel@laptop> Sender: kvm-owner@vger.kernel.org List-ID: On Wed, Dec 01, 2010 at 06:45:02PM +0100, Peter Zijlstra wrote: > On Wed, 2010-12-01 at 22:59 +0530, Srivatsa Vaddagiri wrote: > > > > yield_task_fair(...) > > { > > > > + ideal_runtime = sched_slice(cfs_rq, curr); > > + delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime; > > + rem_time_slice = ideal_runtime - delta_exec; > > + > > + current->donate_time += rem_time_slice > some_threshold ? > > + some_threshold : rem_time_slice; > > > > ... > > } > > > > > > sched_slice(...) > > { > > slice = ... > > > > + slice += current->donate_time; > > > > } > > > > or something close to it. I am bit reluctant to go that route myself, unless the > > fairness issue with plain yield is quite bad. > > That really won't do anything. You need to adjust both tasks their > vruntime. We are dealing with just one task here (the task that is yielding). After recording how much timeslice we are "giving up" in current->donate_time (donate_time is perhaps not the right name to use), we adjust the yielding task's vruntime as per existing logic (for ex: to make it go to back of runqueue). When the yielding tasks gets to run again, lock is hopefully available for it to grab, we let it run longer than the default sched_slice() to compensate for what time it gave up previously to other threads in same runqueue. This ensures that because of yielding upon lock contention, we are not leaking bandwidth in favor of other guests. Again I don't know how much of fairness issue this is in practice, so unless we see some numbers I'd prefer sticking to plain yield() upon lock-contention (for unmodified guests that is). > Also, I really wouldn't touch the yield() implementation, nor > would I expose any such time donation crap to userspace. - vatsa