netdev - Re: [PATCH 0/2] vhost: improve livepatch switching for heavily loaded vhost worker kthreads

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Date:   Thu, 26 Jan 2023 15:12:35 -0600
From:   "Seth Forshee (DigitalOcean)" <sforshee@...italocean.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Jason Wang <jasowang@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        netdev@...r.kernel.org, live-patching@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] vhost: improve livepatch switching for heavily
 loaded vhost worker kthreads

On Thu, Jan 26, 2023 at 06:03:16PM +0100, Petr Mladek wrote:
> On Fri 2023-01-20 16:12:20, Seth Forshee (DigitalOcean) wrote:
> > We've fairly regularaly seen liveptches which cannot transition within kpatch's
> > timeout period due to busy vhost worker kthreads.
> 
> I have missed this detail. Miroslav told me that we have solved
> something similar some time ago, see
> https://lore.kernel.org/all/20220507174628.2086373-1-song@kernel.org/

Interesting thread. I had thought about something along the lines of the
original patch, but there are some ideas in there that I hadn't
considered.

> Honestly, kpatch's timeout 1 minute looks incredible low to me. Note
> that the transition is tried only once per minute. It means that there
> are "only" 60 attempts.
> 
> Just by chance, does it help you to increase the timeout, please?

To be honest my test setup reproduces the problem well enough to make
KLP wait significant time due to vhost threads, but it seldom causes it
to hit kpatch's timeout.

Our system management software will try to load a patch tens of times in
a day, and we've seen real-world cases where patches couldn't load
within kpatch's timeout for multiple days. But I don't have such an
environment readily accessible for my own testing. I can try to refine
my test case and see if I can get it to that point.

> This low timeout might be useful for testing. But in practice, it does
> not matter when the transition is lasting one hour or even longer.
> It takes much longer time to prepare the livepatch.

Agreed. And to be clear, we cope with the fact that patches may take
hours or even days to get applied in some cases. The patches I sent are
just about improving the only case I've identified which has lead to
kpatch failing to load a patch for a day or longer.

Thanks,
Seth