linux-kernel - Re: dm-crypt performance regression due to workqueue changes

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Date: Sun, 30 Jun 2024 20:49:48 +0200 (CEST)
From: Mikulas Patocka <mpatocka@...hat.com>
To: Tejun Heo <tj@...nel.org>
cc: Lai Jiangshan <jiangshanlai@...il.com>, Waiman Long <longman@...hat.com>, 
    Mike Snitzer <snitzer@...nel.org>, Laurence Oberman <loberman@...hat.com>, 
    Jonathan Brassow <jbrassow@...hat.com>, Ming Lei <minlei@...hat.com>, 
    Ondrej Kozina <okozina@...hat.com>, Milan Broz <gmazyland@...il.com>, 
    linux-kernel@...r.kernel.org, dm-devel@...ts.linux.dev, 
    users@...ts.libvirt.org
Subject: Re: dm-crypt performance regression due to workqueue changes



On Sun, 30 Jun 2024, Tejun Heo wrote:

> Hello,
> 
> On Sat, Jun 29, 2024 at 08:15:56PM +0200, Mikulas Patocka wrote:
> 
> > With 6.5, we get 3600MiB/s; with 6.6 we get 1400MiB/s.
> > 
> > The reason is that virt-manager by default sets up a topology where we 
> > have 16 sockets, 1 core per socket, 1 thread per core. And that workqueue 
> > patch avoids moving work items across sockets, so it processes all 
> > encryption work only on one virtual CPU.
> > 
> > The performance degradation may be fixed with "echo 'system'
> > >/sys/module/workqueue/parameters/default_affinity_scope" - but it is 
> > regression anyway, as many users don't know about this option.
> > 
> > How should we fix it? There are several options:
> > 1. revert back to 'numa' affinity
> > 2. revert to 'numa' affinity only if we are in a virtual machine
> > 3. hack dm-crypt to set the 'numa' affinity for the affected workqueues
> > 4. any other solution?
> 
> Do you happen to know why libvirt is doing that? There are many other
> implications to configuring the system that way and I don't think we want to
> design kernel behaviors to suit topology information fed to VMs which can be
> arbitrary.
> 
> Thanks.

I don't know why. I added users@...ts.libvirt.org to the CC.

How should libvirt properly advertise "we have 16 threads that are 
dynamically scheduled by the host kernel, so the latencies between them 
are changing and unpredictable"?

Mikulas