linux-kernel - Re: Re: Re: EEVDF/vhost regression (bisected to 86bfbb7ce4f6 sched/fair: Add lag based placement)

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Date:   Fri, 8 Dec 2023 05:31:18 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Tobias Huschle <huschle@...ux.ibm.com>
Cc:     Abel Wu <wuyun.abel@...edance.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        kvm@...r.kernel.org, virtualization@...ts.linux.dev,
        netdev@...r.kernel.org, jasowang@...hat.com
Subject: Re: Re: Re: EEVDF/vhost regression (bisected to 86bfbb7ce4f6
 sched/fair: Add lag based placement)

On Fri, Dec 08, 2023 at 10:24:16AM +0100, Tobias Huschle wrote:
> On Thu, Dec 07, 2023 at 01:48:40AM -0500, Michael S. Tsirkin wrote:
> > On Thu, Dec 07, 2023 at 07:22:12AM +0100, Tobias Huschle wrote:
> > > 3. vhost looping endlessly, waiting for kworker to be scheduled
> > > 
> > > I dug a little deeper on what the vhost is doing. I'm not an expert on
> > > virtio whatsoever, so these are just educated guesses that maybe
> > > someone can verify/correct. Please bear with me probably messing up 
> > > the terminology.
> > > 
> > > - vhost is looping through available queues.
> > > - vhost wants to wake up a kworker to process a found queue.
> > > - kworker does something with that queue and terminates quickly.
> > > 
> > > What I found by throwing in some very noisy trace statements was that,
> > > if the kworker is not woken up, the vhost just keeps looping accross
> > > all available queues (and seems to repeat itself). So it essentially
> > > relies on the scheduler to schedule the kworker fast enough. Otherwise
> > > it will just keep on looping until it is migrated off the CPU.
> > 
> > 
> > Normally it takes the buffers off the queue and is done with it.
> > I am guessing that at the same time guest is running on some other
> > CPU and keeps adding available buffers?
> > 
> 
> It seems to do just that, there are multiple other vhost instances
> involved which might keep filling up thoses queues. 
> 

No vhost is ever only draining queues. Guest is filling them.

> Unfortunately, this makes the problematic vhost instance to stay on
> the CPU and prevents said kworker to get scheduled. The kworker is
> explicitly woken up by vhost, so it wants it to do something.
> 
> At this point it seems that there is an assumption about the scheduler
> in place which is no longer fulfilled by EEVDF. From the discussion so
> far, it seems like EEVDF does what is intended to do.
> 
> Shouldn't there be a more explicit mechanism in use that allows the
> kworker to be scheduled in favor of the vhost?
> 
> It is also concerning that the vhost seems cannot be preempted by the
> scheduler while executing that loop.


Which loop is that, exactly?

-- 
MST