Re: [PATCH v11 8/8] vhost: use vhost_tasks for worker threads

["Michael S. Tsirkin" <mst@redhat.com>]

On Sat, Jul 22, 2023 at 11:03:29PM -0500, michael.christie@oracle.com wrote:
> On 7/20/23 8:06 AM, Michael S. Tsirkin wrote:
> > On Thu, Feb 02, 2023 at 05:25:17PM -0600, Mike Christie wrote:
> >> For vhost workers we use the kthread API which inherit's its values from
> >> and checks against the kthreadd thread. This results in the wrong RLIMITs
> >> being checked, so while tools like libvirt try to control the number of
> >> threads based on the nproc rlimit setting we can end up creating more
> >> threads than the user wanted.
> >>
> >> This patch has us use the vhost_task helpers which will inherit its
> >> values/checks from the thread that owns the device similar to if we did
> >> a clone in userspace. The vhost threads will now be counted in the nproc
> >> rlimits. And we get features like cgroups and mm sharing automatically,
> >> so we can remove those calls.
> >>
> >> Signed-off-by: Mike Christie <michael.christie@oracle.com>
> >> Acked-by: Michael S. Tsirkin <mst@redhat.com>
> > 
> > 
> > Hi Mike,
> > So this seems to have caused a measureable regression in networking
> > performance (about 30%). Take a look here, and there's a zip file
> > with detailed measuraments attached:
> > 
> > https://bugzilla.redhat.com/show_bug.cgi?id=2222603
> > 
> > 
> > Could you take a look please?
> > You can also ask reporter questions there assuming you
> > have or can create a (free) account.
> > 
> 
> Sorry for the late reply. I just got home from vacation.
> 
> The account creation link seems to be down. I keep getting a
> "unable to establish SMTP connection to bz-exim-prod port 25 " error.
> 
> Can you give me Quan's email?

Thanks for getting back!  I asked whether it's ok to share the email.
For now pasted your request in the bugzilla.

> I think I can replicate the problem. I just need some extra info from Quan:
> 
> 1. Just double check that they are using RHEL 9 on the host running the VMs.
> 2. The kernel config
> 3. Any tuning that was done. Is tuned running in guest and/or host running the
> VMs and what profile is being used in each.
> 4. Number of vCPUs and virtqueues being used.
> 5. Can they dump the contents of:
> 
> /sys/kernel/debug/sched
> 
> and
> 
> sysctl  -a
> 
> on the host running the VMs.
> 
> 6. With the 6.4 kernel, can they also run a quick test and tell me if they set
> the scheduler to batch:
> 
> ps -T -o comm,pid,tid $QEMU_THREAD
> 
> then for each vhost thread do:
> 
> chrt -b -p 0 $VHOST_THREAD
> 
> Does that end up increasing perf? When I do this I see throughput go up by
> around 50% vs 6.3 when sessions was 16 or more (16 was the number of vCPUs
> and virtqueues per net device in the VM). Note that I'm not saying that is a fix.
> It's just a difference I noticed when running some other tests.

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