From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:55440) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cjn26-0004jV-Rl for qemu-devel@nongnu.org; Fri, 03 Mar 2017 08:14:07 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cjn23-0000BY-M2 for qemu-devel@nongnu.org; Fri, 03 Mar 2017 08:14:06 -0500 Received: from mx1.redhat.com ([209.132.183.28]:50216) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1cjn23-0000BI-Fe for qemu-devel@nongnu.org; Fri, 03 Mar 2017 08:14:03 -0500 References: <33183CC9F5247A488A2544077AF19020DA1D01C2@DGGEMA505-MBX.china.huawei.com> <20170303120054.GB2439@work-vm> <10d66d73-a269-acb4-bc3d-2250793cba8e@redhat.com> <20170303131150.GC2439@work-vm> From: Paolo Bonzini Message-ID: Date: Fri, 3 Mar 2017 14:14:00 +0100 MIME-Version: 1.0 In-Reply-To: <20170303131150.GC2439@work-vm> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [Bug?] BQL about live migration List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Dr. David Alan Gilbert" Cc: "Gonglei (Arei)" , "quintela@redhat.com" , "qemu-devel@nongnu.org" , yanghongyang , Huangzhichao On 03/03/2017 14:11, Dr. David Alan Gilbert wrote: > * 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? Actually I thought that would be the case already (in fact CANCELLING is internal only; the outside world sees it as "active" in query-migrate). Lei, what is the runstate? (That is, why did cont succeed at all)? Paolo > 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. That's not really possible, because of how condition variables work. :(