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

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Message-ID: <20231207014626-mutt-send-email-mst@kernel.org>
Date:   Thu, 7 Dec 2023 01:48:40 -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 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?


-- 
MST