From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCHv2] KVM: optimize apic interrupt delivery Date: Wed, 12 Sep 2012 08:13:54 -0700 Message-ID: <20120912151354.GO4257@linux.vnet.ibm.com> References: <20120911130225.GN20907@redhat.com> <20120911141023.GB26031@redhat.com> <20120911171300.GJ4257@linux.vnet.ibm.com> <20120911223337.GA28821@redhat.com> <20120912010334.GK4257@linux.vnet.ibm.com> <50503D92.7090108@redhat.com> <20120912123441.GQ20907@redhat.com> <505081E9.8080505@redhat.com> <20120912124426.GR20907@redhat.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Avi Kivity , "Michael S. Tsirkin" , kvm@vger.kernel.org, mtosatti@redhat.com To: Gleb Natapov Return-path: Received: from e33.co.us.ibm.com ([32.97.110.151]:41756 "EHLO e33.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759641Ab2ILPQp (ORCPT ); Wed, 12 Sep 2012 11:16:45 -0400 Received: from /spool/local by e33.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 12 Sep 2012 09:16:44 -0600 Received: from d03relay03.boulder.ibm.com (d03relay03.boulder.ibm.com [9.17.195.228]) by d03dlp02.boulder.ibm.com (Postfix) with ESMTP id 2BF6D3E4007B for ; Wed, 12 Sep 2012 09:14:02 -0600 (MDT) Received: from d03av01.boulder.ibm.com (d03av01.boulder.ibm.com [9.17.195.167]) by d03relay03.boulder.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id q8CFDxUi222086 for ; Wed, 12 Sep 2012 09:14:00 -0600 Received: from d03av01.boulder.ibm.com (loopback [127.0.0.1]) by d03av01.boulder.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id q8CFDvul005299 for ; Wed, 12 Sep 2012 09:13:58 -0600 Content-Disposition: inline In-Reply-To: <20120912124426.GR20907@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On Wed, Sep 12, 2012 at 03:44:26PM +0300, Gleb Natapov wrote: > On Wed, Sep 12, 2012 at 03:36:57PM +0300, Avi Kivity wrote: > > On 09/12/2012 03:34 PM, Gleb Natapov wrote: > > > On Wed, Sep 12, 2012 at 10:45:22AM +0300, Avi Kivity wrote: > > >> On 09/12/2012 04:03 AM, Paul E. McKenney wrote: > > >> >> > > Paul, I'd like to check something with you here: > > >> >> > > this function can be triggered by userspace, > > >> >> > > any number of times; we allocate > > >> >> > > a 2K chunk of memory that is later freed by > > >> >> > > kfree_rcu. > > >> >> > > > > >> >> > > Is there a risk of DOS if RCU is delayed while > > >> >> > > lots of memory is queued up in this way? > > >> >> > > If yes is this a generic problem with kfree_rcu > > >> >> > > that should be addressed in core kernel? > > >> >> > > > >> >> > There is indeed a risk. > > >> >> > > >> >> In our case it's a 2K object. Is it a practical risk? > > >> > > > >> > How many kfree_rcu()s per second can a given user cause to happen? > > >> > > >> Not much more than a few hundred thousand per second per process (normal > > >> operation is zero). > > >> > > > I managed to do 21466 per second. > > > > Strange, why so slow? > > > Because ftrace buffer overflows :) With bigger buffer I get 169940. Ah, good, should not be a problem. In contrast, if you ran kfree_rcu() in a tight loop, you could probably do in excess of 100M per CPU per second. Now -that- might be a problem. Well, it -might- be a problem if you somehow figured out how to allocate memory that quickly in a steady-state manner. ;-) > > >> Good idea. Michael, is should be easy to modify kvm-unit-tests to write > > >> to the APIC ID register in a loop. > > >> > > > I did. Memory consumption does not grow on otherwise idle host. Very good -- the checks in __call_rcu(), which is common code invoked by kfree_rcu(), seem to be doing their job, then. These do keep a per-CPU counter, which can be adjusted via rcutree.blimit, which defaults to taking evasive action if more than 10K callbacks are waiting on a given CPU. My concern was that you might be overrunning that limit in way less than a grace period (as in about a hundred microseconds. My concern was of course unfounded -- you take several grace periods in push 10K callbacks through. Thanx, Paul > > Ok, thanks. > > > > > > -- > > error compiling committee.c: too many arguments to function > > -- > Gleb. >