From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:60431) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dq03m-00028w-0I for qemu-devel@nongnu.org; Thu, 07 Sep 2017 12:53:50 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dq03h-0007zD-36 for qemu-devel@nongnu.org; Thu, 07 Sep 2017 12:53:46 -0400 Received: from mail-wr0-x243.google.com ([2a00:1450:400c:c0c::243]:37663) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_128_CBC_SHA1:16) (Exim 4.71) (envelope-from ) id 1dq03g-0007ym-Oq for qemu-devel@nongnu.org; Thu, 07 Sep 2017 12:53:41 -0400 Received: by mail-wr0-x243.google.com with SMTP id u48so122714wrf.4 for ; Thu, 07 Sep 2017 09:53:39 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <20170907120227.GE23040@pxdev.xzpeter.org> References: <1503471071-2233-1-git-send-email-peterx@redhat.com> <20170906145043.GG15535@stefanha-x1.localdomain> <20170906151436.GF2215@work-vm> <20170907093546.GE2098@work-vm> <20170907120227.GE23040@pxdev.xzpeter.org> From: Stefan Hajnoczi Date: Thu, 7 Sep 2017 17:53:37 +0100 Message-ID: Content-Type: text/plain; charset="UTF-8" Subject: Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Peter Xu Cc: "Dr. David Alan Gilbert" , qemu-devel , Laurent Vivier , Fam Zheng , Juan Quintela , Markus Armbruster , Michael Roth , Paolo Bonzini On Thu, Sep 7, 2017 at 1:02 PM, Peter Xu wrote: > On Thu, Sep 07, 2017 at 11:09:29AM +0100, Stefan Hajnoczi wrote: >> On Thu, Sep 7, 2017 at 10:35 AM, Dr. David Alan Gilbert >> wrote: >> > * Stefan Hajnoczi (stefanha@gmail.com) wrote: >> >> On Wed, Sep 6, 2017 at 4:14 PM, Dr. David Alan Gilbert >> >> wrote: >> >> > * Stefan Hajnoczi (stefanha@gmail.com) wrote: >> >> >> On Wed, Aug 23, 2017 at 02:51:03PM +0800, Peter Xu wrote: >> >> >> > The root problem is that, monitor commands are all handled in main >> >> >> > loop thread now, no matter how many monitors we specify. And, if main >> >> >> > loop thread hangs due to some reason, all monitors will be stuck. >> >> >> >> >> >> I see a larger issue with postcopy: existing QEMU code assumes that >> >> >> guest memory access is instantaneous. >> >> >> >> >> >> Postcopy breaks this assumption and introduces blocking points that can >> >> >> now take unbounded time. >> >> >> >> >> >> This problem isn't specific to the monitor. It can also happen to other >> >> >> components in QEMU like the gdbstub. >> >> >> >> >> >> Do we need an asynchronous memory API? Synchronous memory access should >> >> >> only be allowed in vcpu threads. >> >> > >> >> > It would probably be useful for gdbstub where the overhead of async >> >> > doesn't matter; but doing that for all IO emulation is hard. >> >> >> >> Why is it hard? >> >> >> >> Memory access can be synchronous in the vcpu thread. That eliminates >> >> a lot of code straight away. >> >> >> >> Anything using dma-helpers.c is already async. They just don't know >> >> that the memory access part is being made async too :). >> > >> > Can you point me to some info on that ? >> >> IDE and SCSI use dma-helpers.c to perform I/O: >> hw/ide/core.c:892: s->bus->dma->aiocb = >> dma_blk_io(blk_get_aio_context(s->blk), >> hw/ide/macio.c:189: s->bus->dma->aiocb = >> dma_blk_io(blk_get_aio_context(s->blk), &s->sg, >> hw/scsi/scsi-disk.c:348: r->req.aiocb = >> dma_blk_io(blk_get_aio_context(s->qdev.conf.blk), >> hw/scsi/scsi-disk.c:551: r->req.aiocb = >> dma_blk_io(blk_get_aio_context(s->qdev.conf.blk), >> >> They pass a scatter-gather list of guest RAM addresses to >> dma-helpers.c. They receive a callback when I/O has finished. >> >> Try following the code path. Request submission may be from a vcpu >> thread or IOThread. Completion occurs in the main loop or an >> IOThread. >> >> The main point is that this API is already asynchronous. If any >> changes are needed for async guest memory access (not sure, I haven't >> checked), then at least the dma-helpers.c users do not need to be >> modified. >> >> >> The remaining cases are virtio and some other devices. >> >> >> >> If you are worried about performance, the first rule is that async >> >> memory access is only needed on the destination side when post-copy is >> >> active. Maybe use setjmp to return from the signal handler and queue >> >> a callback for when the page has been loaded. >> > >> > I'm not sure it's worth trying to be too clever at avoiding this; >> > I see the fact that we're doing IO with the bql held as a more >> > fundamental problem. >> >> QEMU should be doing I/O syscalls in async fashion or threadpool >> workers (no BQL) so the BQL is not an issue. Anything else could >> cause unbounded waits even without postcopy. > > E.g. when vcpu got page faulted with BQL taken, while the main thread > needs the BQL to dispatch anything, including monitor commands. > > So I think it's a multiplex problem - we need to solve both (1) main > thread accessing guest memories which is still missing, and (2) BQL > deadlocks between vcpu threads and main thread. I think we need a single solution and cannot treat these as separate. This is because the same virtio device emulation code may run in 3 contexts: 1. vcpu thread (ioeventfd=off) 2. main loop thread (ioeventfd=on) 3. IOThread (ioeventfd=on, iothread=) If you try to solve them separately then the code won't work in all 3 contexts anymore. Stefan