From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:44825) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Ztu0d-0000ML-Dt for qemu-devel@nongnu.org; Wed, 04 Nov 2015 04:05:36 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Ztu0Y-0004t3-Pr for qemu-devel@nongnu.org; Wed, 04 Nov 2015 04:05:35 -0500 Received: from mx1.redhat.com ([209.132.183.28]:46448) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Ztu0Y-0004sp-Hv for qemu-devel@nongnu.org; Wed, 04 Nov 2015 04:05:30 -0500 Date: Wed, 4 Nov 2015 09:05:25 +0000 From: "Dr. David Alan Gilbert" Message-ID: <20151104090525.GA2702@work-vm> References: <20151103122353.GB17670@work-vm> <874mh3z1hb.fsf@emacs.mitica> <20151103134716.GC17670@work-vm> <5639770B.4090103@cn.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5639770B.4090103@cn.fujitsu.com> Subject: Re: [Qemu-devel] safety of migration_bitmap_extend List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Wen Congyang Cc: den@openvz.org, qemu-devel@nongnu.org, lizhijian@cn.fujitsu.com, Juan Quintela * Wen Congyang (wency@cn.fujitsu.com) wrote: > On 11/03/2015 09:47 PM, Dr. David Alan Gilbert wrote: > > * Juan Quintela (quintela@redhat.com) wrote: > >> "Dr. David Alan Gilbert" wrote: > >>> Hi, > >>> I'm trying to understand why migration_bitmap_extend is correct/safe; > >>> If I understand correctly, you're arguing that: > >>> > >>> 1) the migration_bitmap_mutex around the extend, stops any sync's happening > >>> and so no new bits will be set during the extend. > >>> > >>> 2) If migration sends a page and clears a bitmap entry, it doesn't > >>> matter if we lose the 'clear' because we're copying it as > >>> we extend it, because losing the clear just means the page > >>> gets resent, and so the data is OK. > >>> > >>> However, doesn't (2) mean that migration_dirty_pages might be wrong? > >>> If a page was sent, the bit cleared, and migration_dirty_pages decremented, > >>> then if we copy over that bitmap and 'set' that bit again then migration_dirty_pages > >>> is too small; that means that either migration would finish too early, > >>> or more likely, migration_dirty_pages would wrap-around -ve and > >>> never finish. > >>> > >>> Is there a reason it's really safe? > >> > >> No. It is reasonably safe. Various values of reasonably. > >> > >> migration_dirty_pages should never arrive at values near zero. Because > >> we move to the completion stage way before it gets a value near zero. > >> (We could have very, very bad luck, as in it is not safe). > > > > That's only true if we hit the qemu_file_rate_limit() in ram_save_iterate; > > if we don't hit the rate limit (e.g. because we're CPU or network limited > > to slower than the set limit) then I think ram_save_iterate will go all the > > way to sending every page; if that happens it'll go once more > > around the main migration loop, and call the pending routine, and now get > > a -ve (very +ve) number of pending pages, so continuously do ram_save_iterate > > again. > > > > We've had that type of bug before when we messed up the dirty-pages calculation > > during hotplug. > > IIUC, migration_bitmap_extend() is called when migration is running, and we hotplug > a device. > > In this case, I think we hold the iothread mutex when migration_bitmap_extend() is called. > > ram_save_complete() is also protected by the iothread mutex. > > So if migration_bitmap_extend() is called, the migration thread may be blocked in > migration_completion() and wait it. qemu_savevm_state_complete() will be called after > migration_completion() returns. But I don't think ram_save_iterate is protected by that lock, and my concern is that the dirty-pages calculation is wrong during the iteration phase, and then the iteration phase will never exit and never try and get to ram_save_complete. Dave > > Thanks > Wen Congyang > > > > >> Now, do we really care if migration_dirty_pages is exact? Not really, > >> we just use it to calculate if we should start the throotle or not. > >> That only test that each 1 second, so if we have written a couple of > >> pages that we are not accounting for, things should be reasonably safe. > >> > >> Once told that, I don't know why we didn't catch that problem during > >> review (yes, I am guilty here). Not sure how to really fix it, > >> thought. I think that the problem is more theoretical than real, but > > > > Dave > > > >> .... > >> > >> Thanks, Juan. > >> > >>> > >>> Dave > >>> > >>> -- > >>> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK > > -- > > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK > > > > . > > > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK