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

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Message-ID: <ZfLjrgn5rlPOD5Xa@DESKTOP-2CCOB1S.>
Date: Thu, 14 Mar 2024 12:46:54 +0100
From: Tobias Huschle <huschle@...ux.ibm.com>
To: Luis Machado <luis.machado@....com>
Cc: "Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>,
        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,
        nd <nd@....com>
Subject: Re: EEVDF/vhost regression (bisected to 86bfbb7ce4f6 sched/fair: Add
 lag based placement)

On Tue, Mar 12, 2024 at 09:45:57AM +0000, Luis Machado wrote:
> On 3/11/24 17:05, Michael S. Tsirkin wrote:
> > 
> > Are we going anywhere with this btw?
> > 
> >
> 
> I think Tobias had a couple other threads related to this, with other potential fixes:
> 
> https://lore.kernel.org/lkml/20240228161018.14253-1-huschle@linux.ibm.com/
> 
> https://lore.kernel.org/lkml/20240228161023.14310-1-huschle@linux.ibm.com/
> 

Sorry, Michael, should have provided those threads here as well.

The more I look into this issue, the more things to ponder upon I find.
It seems like this issue can (maybe) be fixed on the scheduler side after all.

The root cause of this regression remains that the mentioned kworker gets
a negative lag value and is therefore not elligible to run on wake up.
This negative lag is potentially assigned incorrectly. But I'm not sure yet.

Anytime I find something that can address the symptom, there is a potential
root cause on another level, and I would like to avoid to just address a
symptom to fix the issue, wheras it would be better to find the actual
root cause.

I would nevertheless still argue, that vhost relies rather heavily on the fact
that the kworker gets scheduled on wake up everytime. But I don't have a 
proposal at hand that accounts for potential side effects if opting for
explicitly initiating a schedule.
Maybe the assumption, that said kworker should always be selected on wake 
up is valid. In that case the explicit schedule would merely be a safety 
net.

I will let you know if something comes up on the scheduler side. There are
some more ideas on my side how this could be approached.