From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:54898) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cjn02-0002kp-5l for qemu-devel@nongnu.org; Fri, 03 Mar 2017 08:11:59 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cjn00-0007pW-8M for qemu-devel@nongnu.org; Fri, 03 Mar 2017 08:11:58 -0500 Received: from mx1.redhat.com ([209.132.183.28]:53672) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1cjn00-0007oz-1a for qemu-devel@nongnu.org; Fri, 03 Mar 2017 08:11:56 -0500 Date: Fri, 3 Mar 2017 13:11:51 +0000 From: "Dr. David Alan Gilbert" Message-ID: <20170303131150.GC2439@work-vm> References: <33183CC9F5247A488A2544077AF19020DA1D01C2@DGGEMA505-MBX.china.huawei.com> <20170303120054.GB2439@work-vm> <10d66d73-a269-acb4-bc3d-2250793cba8e@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <10d66d73-a269-acb4-bc3d-2250793cba8e@redhat.com> Subject: Re: [Qemu-devel] [Bug?] BQL about live migration List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini Cc: "Gonglei (Arei)" , "quintela@redhat.com" , "qemu-devel@nongnu.org" , yanghongyang , Huangzhichao * Paolo Bonzini (pbonzini@redhat.com) wrote: > > > On 03/03/2017 13:00, Dr. David Alan Gilbert wrote: > > Ouch that's pretty nasty; I remember Paolo explaining to me a while ago that > > their were times when run_on_cpu would have to drop the BQL and I worried about it, > > but this is the 1st time I've seen an error due to it. > > > > Do you know what the migration state was at that point? Was it MIGRATION_STATUS_CANCELLING? > > I'm thinking perhaps we should stop 'cont' from continuing while migration is in > > MIGRATION_STATUS_CANCELLING. Do we send an event when we hit CANCELLED - so that > > perhaps libvirt could avoid sending the 'cont' until then? > > No, there's no event, though I thought libvirt would poll until > "query-migrate" returns the cancelled state. Of course that is a small > consolation, because a segfault is unacceptable. I think you might get an event if you set the new migrate capability called 'events' on! void migrate_set_state(int *state, int old_state, int new_state) { if (atomic_cmpxchg(state, old_state, new_state) == old_state) { trace_migrate_set_state(new_state); migrate_generate_event(new_state); } } static void migrate_generate_event(int new_state) { if (migrate_use_events()) { qapi_event_send_migration(new_state, &error_abort); } } That event feature went in sometime after 2.3.0. > One possibility is to suspend the monitor in qmp_migrate_cancel and > resume it (with add_migration_state_change_notifier) when we hit the > CANCELLED state. I'm not sure what the latency would be between the end > of migrate_fd_cancel and finally reaching CANCELLED. I don't like suspending monitors; it can potentially take quite a significant time to do a cancel. How about making 'cont' fail if we're in CANCELLING? I'd really love to see the 'run_on_cpu' being more careful about the BQL; we really need all of the rest of the devices to stay quiesced at times. Dave > Paolo -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK