Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.37 as permitted sender) client-ip=23.128.96.37;
Date:   Thu, 7 Dec 2023 01:48:40 -0500
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Tobias Huschle <huschle@linux.ibm.com>
Cc:     Abel Wu <wuyun.abel@bytedance.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Linux Kernel <linux-kernel@vger.kernel.org>,
        kvm@vger.kernel.org, virtualization@lists.linux.dev,
        netdev@vger.kernel.org, jasowang@redhat.com
Subject: Re: Re: Re: EEVDF/vhost regression (bisected to 86bfbb7ce4f6
 sched/fair: Add lag based placement)
Message-ID: <20231207014626-mutt-send-email-mst@kernel.org>
References: <c7b38bc27cc2c480f0c5383366416455@linux.ibm.com>
 <20231117092318.GJ8262@noisy.programming.kicks-ass.net>
 <ZVdbdSXg4qefTNtg@DESKTOP-2CCOB1S.>
 <20231117123759.GP8262@noisy.programming.kicks-ass.net>
 <46a997c2-5a38-4b60-b589-6073b1fac677@bytedance.com>
 <ZVyt4UU9+XxunIP7@DESKTOP-2CCOB1S.>
 <20231122100016.GO8262@noisy.programming.kicks-ass.net>
 <6564a012.c80a0220.adb78.f0e4SMTPIN_ADDED_BROKEN@mx.google.com>
 <d4110c79-d64f-49bd-9f69-0a94369b5e86@bytedance.com>
 <07513.123120701265800278@us-mta-474.us.mimecast.lan>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <07513.123120701265800278@us-mta-474.us.mimecast.lan>
Precedence: bulk

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