Re: [Qemu-devel] [PATCH] Tap: fix vcpu long time io blocking on tap

[Wangkai (Kevin,C)]

> -----Original Message-----
> From: Stefan Hajnoczi [mailto:address@hidden
> Sent: Friday, July 11, 2014 9:04 PM
> To: Wangkai (Kevin,C)
> Cc: address@hidden; address@hidden; Lee yang
> Subject: Re: [PATCH] Tap: fix vcpu long time io blocking on tap
> 
> On Fri, Jul 11, 2014 at 01:05:30AM +0000, Wangkai (Kevin,C) wrote:
> > When used a tap as net driver for vm, if too many packets was
> > delivered to the guest os via tap interface, the guest os will be
> > blocked on io events for a long time, while tap driver was busying
> process packets.
> >
> > kvm vcpu thread block on io lock call trace:
> >   __lll_lock_wait
> >   _L_lock_1004
> >   __pthread_mutex_lock
> >   qemu_mutex_lock
> >   kvm_cpu_exec
> >   qemu_kvm_cpu_thread_fn
> >   start_thread
> >
> > qemu io thread call trace:
> >   ...
> >   qemu_net_queue_send
> >   tap_send
> >   qemu_iohandler_poll
> >   main_loop_wait
> >   main_loop
> >
> >
> > I think the qemu io lock time should be as small as possible, and the
> > io work slice should be limited at a particular ration or time.
> >
> > ---
> > Signed-off-by: Wangkai <address@hidden>
> 
> How many packets are you seeing in a single tap_send() call?
> 
> Have you profiled the tap_send() code path?  Maybe it is performing
> some operation that is very slow.
> 
> By the way, if you want good performance you should use vhost_net
> instead of userspace vhost_net.  Userspace virtio-net is not very
> optimized.
> 
> Stefan


Hi Stefan,

I am not use profile, just debug with gdb and code review.

When packets delivered, I found the VM was hung, and I check qemu run

State by gdb, I see the call trace for IO thread and vcpu thread, and

I add debug info to check how many packets within tap_send, the info below:

total recv 393520 time 1539821 us 
total recv 1270 time 4931 us
total recv 257872 time 995828 us 
total recv 10745 time 41438 us
total recv 505387 time 2000925 us
total recv 535 time 1891 us
total recv 168934 time 646008 us


regards,
Wangkai