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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Wed, 5 Jun 2019 20:39:33 +0100 Received: from b03ledav004.gho.boulder.ibm.com (b03ledav004.gho.boulder.ibm.com [9.17.130.235]) by b03cxnp08028.gho.boulder.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x55JdWA320054436 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Wed, 5 Jun 2019 19:39:32 GMT Received: from b03ledav004.gho.boulder.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 91AE178064; Wed, 5 Jun 2019 19:39:32 +0000 (GMT) Received: from b03ledav004.gho.boulder.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 3E52D7805E; Wed, 5 Jun 2019 19:39:31 +0000 (GMT) Received: from maxibm (unknown [9.85.186.195]) by b03ledav004.gho.boulder.ibm.com (Postfix) with ESMTPS; Wed, 5 Jun 2019 19:39:30 +0000 (GMT) Date: Wed, 5 Jun 2019 16:39:27 -0300 From: "Maxiwell S. Garcia" To: David Gibson References: <20190520204340.832-1-maxiwell@linux.ibm.com> <20190520204340.832-2-maxiwell@linux.ibm.com> <20190522232952.GN30423@umbus.fritz.box> <20190523201851.llsufz6dfs3gzyc6@maxibm> <20190530011341.GB2017@umbus.fritz.box> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190530011341.GB2017@umbus.fritz.box> User-Agent: NeoMutt/20180716 X-TM-AS-GCONF: 00 x-cbid: 19060519-0004-0000-0000-00001518ACAA X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00011220; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000286; SDB=6.01213719; UDB=6.00637941; IPR=6.00994793; MB=3.00027198; MTD=3.00000008; XFM=3.00000015; UTC=2019-06-05 19:39:35 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19060519-0005-0000-0000-00008BF412FF Message-Id: <20190605193927.3lek4voay5stxtqt@maxibm> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:, , definitions=2019-06-05_12:, , signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1906050123 X-detected-operating-system: by eggs.gnu.org: GNU/Linux 3.x [generic] X-Received-From: 148.163.158.5 Subject: Re: [Qemu-devel] [Qemu-ppc] [PATCH 1/1] spapr: Do not re-read the clock on pre_save handler on migration X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: qemu-ppc@nongnu.org, qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, May 30, 2019 at 11:13:41AM +1000, David Gibson wrote: > On Thu, May 23, 2019 at 05:18:51PM -0300, Maxiwell S. Garcia wrote: > > On Thu, May 23, 2019 at 09:29:52AM +1000, David Gibson wrote: > > > On Mon, May 20, 2019 at 05:43:40PM -0300, Maxiwell S. Garcia wrote: > > > > This handler was added in the commit: > > > > 42043e4f1241: spapr: clock should count only if vm is running > > > > > > > > In a scenario without migration, this pre_save handler is not > > > > triggered, so the 'stop/cont' commands save and restore the clock > > > > in the function 'cpu_ppc_clock_vm_state_change.' The SW clock > > > > in the guest doesn't know about this pause. > > > > > > > > If the command 'migrate' is called between 'stop' and 'cont', > > > > the pre_save handler re-read the clock, and the SW clock in the > > > > guest will know about the pause between 'stop' and 'migrate.' > > > > If the guest is running a workload like HTC, a side-effect of > > > > this is a lot of process stall messages (with call traces) in > > > > the kernel guest. > > > > > > > > Signed-off-by: Maxiwell S. Garcia > > > > > > What affect will this have on the clock for the case of migrations > > > without a stop/cont around? > > > > The guest timebase is saved when the VM stop running and restored when > > the VM starts running again (cpu_ppc_clock_vm_state_change handler). > > Migrations without stop/cont save the clock when the VM go to the > > FINISH_MIGRATE state. > > Right... which means the clock is effectively stopped for the > migration downtime window while we transfer the final state. That > means the guest clock will drift from wall clock by a couple of > hundred ms across the migration which is not correct. > > > > The complicated thing here is that for > > > *explicit* stops/continues we want to freeze the clock, however for > > > the implicit stop/continue during migration downtime, we want to keep > > > the clock running (logically), so that the guest time of day doesn't > > > get out of sync on migration. > > > > > > > Not sure if the *implicit* word here is about commands from the libvirt > > or any other orchestrator. > > By implicit I mean the stopping of the VM which qemu does to transfer > the final part of the state, rather than because of an explicit > stop/cont command. > > > QEMU itself doesn't know the intent behind the > > command stop/cont. So, If we are using a guest to process a workload and > > the manager tool decide to migrate our VM transparently, it's unpleasant > > to see a lot of process stalls with call traces in the kernel log. > > If you have a lot of process stalls across a migration, that suggests > your permitted downtime window is *way* too long. > I see a difference between live migration and 'cold' migration. In a cold migration scenario (where a user 'stop' the machine, wait an arbitrary time, move it to another server, and run it), the behavior should be the same as executing 'stop/cont' in a guest without migration. This problem also emerges when the 'timeout' flag is used with 'virsh' tool to live-migrate a guest. After the 'timeout', the libvirt sends a 'stop' command to QEMU to suspend the guest before migrate. If the NFS is slow, for example, this guest will wait many minutes to run again. Maybe a solution is to modify the timebase_pre_save handler to know the current vm_state and only save the timebase again when vm_state is not in 'stop' state. What do you think? > > The high-level tools could sync the SW clock with the HW clock if this > > behavior is required, keeping the QEMU stop/cont and stop/migrate/cont > > consistent. > > > > > > --- > > > > hw/ppc/ppc.c | 24 ------------------------ > > > > 1 file changed, 24 deletions(-) > > > > > > > > diff --git a/hw/ppc/ppc.c b/hw/ppc/ppc.c > > > > index ad20584f26..3fb50cbeee 100644 > > > > --- a/hw/ppc/ppc.c > > > > +++ b/hw/ppc/ppc.c > > > > @@ -1056,35 +1056,11 @@ void cpu_ppc_clock_vm_state_change(void *opaque, int running, > > > > } > > > > } > > > > > > > > -/* > > > > - * When migrating, read the clock just before migration, > > > > - * so that the guest clock counts during the events > > > > - * between: > > > > - * > > > > - * * vm_stop() > > > > - * * > > > > - * * pre_save() > > > > - * > > > > - * This reduces clock difference on migration from 5s > > > > - * to 0.1s (when max_downtime == 5s), because sending the > > > > - * final pages of memory (which happens between vm_stop() > > > > - * and pre_save()) takes max_downtime. > > > > > > Urgh.. this comment is confusing - 5s would be a ludicrously long > > > max_downtime by modern standards. > > > > > > > - */ > > > > -static int timebase_pre_save(void *opaque) > > > > -{ > > > > - PPCTimebase *tb = opaque; > > > > - > > > > - timebase_save(tb); > > > > - > > > > - return 0; > > > > -} > > > > - > > > > const VMStateDescription vmstate_ppc_timebase = { > > > > .name = "timebase", > > > > .version_id = 1, > > > > .minimum_version_id = 1, > > > > .minimum_version_id_old = 1, > > > > - .pre_save = timebase_pre_save, > > > > .fields = (VMStateField []) { > > > > VMSTATE_UINT64(guest_timebase, PPCTimebase), > > > > VMSTATE_INT64(time_of_the_day_ns, PPCTimebase), > > > > > > > > > -- > David Gibson | I'll have my music baroque, and my code > david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ > | _way_ _around_! > http://www.ozlabs.org/~dgibson