From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:40163) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WQzPF-0005J5-GI for qemu-devel@nongnu.org; Fri, 21 Mar 2014 09:22:47 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1WQzP9-0006Fv-G4 for qemu-devel@nongnu.org; Fri, 21 Mar 2014 09:22:41 -0400 Received: from mx1.redhat.com ([209.132.183.28]:6160) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WQzP9-0006FN-8E for qemu-devel@nongnu.org; Fri, 21 Mar 2014 09:22:35 -0400 Received: from int-mx01.intmail.prod.int.phx2.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) by mx1.redhat.com (8.14.4/8.14.4) with ESMTP id s2LDMYDN000684 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK) for ; Fri, 21 Mar 2014 09:22:34 -0400 Date: Fri, 21 Mar 2014 13:22:27 +0000 From: "Dr. David Alan Gilbert" Message-ID: <20140321132224.GA22211@work-vm> References: <1395399490-13295-1-git-send-email-dgilbert@redhat.com> <8761n7itlx.fsf@elfo.mitica> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <8761n7itlx.fsf@elfo.mitica> Subject: Re: [Qemu-devel] [PATCH 1/1] Count used RAMBlock pages for migration_dirty_pages List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Juan Quintela Cc: qemu-devel@nongnu.org * Juan Quintela (quintela@redhat.com) wrote: > "Dr. David Alan Gilbert (git)" wrote: > > From: "Dr. David Alan Gilbert" > > > > This is a fix for a bug* triggered by a migration after hot unplugging > > a few virtio-net NICs, that caused migration never to converge, because > > 'migration_dirty_pages' is incorrectly initialised. > > Good catch. > > > 'migration_dirty_pages' is used as a tally of the number of outstanding > > dirty pages, to give the migration code an idea of how much more data > > will need to be transferred, and thus whether it can end the iterative > > phase. > > > > It was initialised to the total size of the RAMBlock address space, > > however hotunplug can leave this space sparse, and hence > > migration_dirty_pages ended up too large. > > > > Note that the code tries to be careful when counting to deal with > > RAMBlocks that share the same end/start page - I don't know > > if this is actually possible and it does complicate the code, > > but since there was other code that dealt with unaligned RAMBlocks > > it seemed possible. > > Couldn't we just check at block addition that it dont' overlap? > > What code do you mean? > > My understanding is that the "normal" way of creating new RAMBlocks is > with qemu_ram_alloc_from_ptr(), and my reading is that block never > overlap. (Important words of the sentence: "my reading"). I don't think they overlap, but I worry that the end of one block and the start of the next might be on the same page. The code that got me worried was migration_bitmap_sync_range that seemd to be general; but actually that's worrying about 64bit words not pages. What happens with things like '/rom@etc/table-loader' which is only 4k on x86 when they are on boxes with bigger target_page. > > Signed-off-by: Dr. David Alan Gilbert > > > > (* https://bugzilla.redhat.com/show_bug.cgi?id=1074913 ) > > --- > > arch_init.c | 41 +++++++++++++++++++++++++++++++++++++---- > > 1 file changed, 37 insertions(+), 4 deletions(-) > > > > diff --git a/arch_init.c b/arch_init.c > > index f18f42e..ef0e98d 100644 > > --- a/arch_init.c > > +++ b/arch_init.c > > @@ -727,11 +727,8 @@ static void reset_ram_globals(void) > > static int ram_save_setup(QEMUFile *f, void *opaque) > > { > > RAMBlock *block; > > - int64_t ram_pages = last_ram_offset() >> TARGET_PAGE_BITS; > > + int64_t ram_bitmap_pages; > > > > - migration_bitmap = bitmap_new(ram_pages); > > - bitmap_set(migration_bitmap, 0, ram_pages); > > - migration_dirty_pages = ram_pages; > > mig_throttle_on = false; > > dirty_rate_high_cnt = 0; > > > > @@ -770,6 +767,42 @@ static int ram_save_setup(QEMUFile *f, void *opaque) > > bytes_transferred = 0; > > reset_ram_globals(); > > > > + ram_bitmap_pages = last_ram_offset() >> TARGET_PAGE_BITS; > > + migration_bitmap = bitmap_new(ram_bitmap_pages); > > + bitmap_set(migration_bitmap, 0, ram_bitmap_pages); > > + /* > > + * Count the total number of pages used by ram blocks. We clear the dirty > > + * bit for the start/end of each ramblock as we go so that we don't double > > + * count ramblocks that have overlapping pages - at entry the whole dirty > > + * bitmap is set. > > + */ > > + migration_dirty_pages = 0; > > + QTAILQ_FOREACH(block, &ram_list.blocks, next) { > > + uint64_t block_pages = 0; > > + ram_addr_t saddr, eaddr; > > + > > + saddr = block->mr->ram_addr; > > + eaddr = saddr + block->length - 1; > > If my assumtpion is true: block->lenght-1 / TARGET_PAGE_SIZE (rounded > up) should be enough, no? > > Reason for this is that migration bitmap handling is already slow, and > we are adding a whole two passes here? Oh yes, if we are sure the ramblocks never meet on the same page all this becomes a lot simpler (indeed it was a lot simpler yesterday before I got worried about them touching). Dave -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK