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

[Michael S. Tsirkin]

On Fri, Aug 11, 2023 at 01:51:36PM -0500, Mike Christie wrote:
> On 8/10/23 1:57 PM, Michael S. Tsirkin wrote:
> > On Sat, Jul 22, 2023 at 11:03:29PM -0500, michael.christie@xxxxxxxxxx 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@xxxxxxxxxx>
> >>>> Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> >>>
> >>>
> >>> 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?
> >>
> >> 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.
> > 
> > 
> > Mike I'm unsure what to do at this point. Regressions are not nice
> > but if the kernel is released with the new userspace api we won't
> > be able to revert. So what's the plan?
> > 
> 
> I'm sort of stumped. I still can't replicate the problem out of the box. 6.3 and
> 6.4 perform the same for me. I've tried your setup and settings and with different
> combos of using things like tuned and irqbalance.
> 
> I can sort of force the issue. In 6.4, the vhost thread inherits it's settings
> from the parent thread. In 6.3, the vhost thread inherits from kthreadd and we
> would then reset the sched settings. So in 6.4 if I just tune the parent differently
> I can cause different performance. If we want the 6.3 behavior we can do the patch
> below.
> 
> However, I don't think you guys are hitting this because you are just running
> qemu from the normal shell and were not doing anything fancy with the sched
> settings.
> 
> 
> diff --git a/kernel/vhost_task.c b/kernel/vhost_task.c
> index da35e5b7f047..f2c2638d1106 100644
> --- a/kernel/vhost_task.c
> +++ b/kernel/vhost_task.c
> @@ -2,6 +2,7 @@
>  /*
>   * Copyright (C) 2021 Oracle Corporation
>   */
> +#include <uapi/linux/sched/types.h>
>  #include <linux/slab.h>
>  #include <linux/completion.h>
>  #include <linux/sched/task.h>
> @@ -22,9 +23,16 @@ struct vhost_task {
>  
>  static int vhost_task_fn(void *data)
>  {
> +	static const struct sched_param param = { .sched_priority = 0 };
>  	struct vhost_task *vtsk = data;
>  	bool dead = false;
>  
> +	/*
> +	 * Don't inherit the parent's sched info, so we maintain compat from
> +	 * when we used kthreads and it reset this info.
> +	 */
> +	sched_setscheduler_nocheck(current, SCHED_NORMAL, &param);
> +
>  	for (;;) {
>  		bool did_work;
>  
> 
> 

yes seems unlikely, still, attach this to bugzilla so it can be
tested?

and, what will help you debug? any traces to enable?

Also wasn't there another issue with a non standard config?
Maybe if we fix that it will by chance fix this one too?

> 
>