linux-kernel - Re: [RFC] sched,livepatch: call stop_one_cpu in klp_check_and_switch

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Date:   Mon, 9 May 2022 13:09:49 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Rik van Riel <riel@...riel.com>
Cc:     Song Liu <song@...nel.org>, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org, mingo@...hat.com,
        peterz@...radead.org, vincent.guittot@...aro.org,
        jpoimboe@...hat.com, joe.lawrence@...hat.com, kernel-team@...com
Subject: Re: [RFC] sched,livepatch: call stop_one_cpu in
 klp_check_and_switch_task

On Mon, May 09, 2022 at 03:49:52PM -0400, Rik van Riel wrote:
> On Mon, 2022-05-09 at 12:17 -0700, Josh Poimboeuf wrote:
> > On Mon, May 09, 2022 at 03:10:16PM -0400, Rik van Riel wrote:
> > 
> > 
> > > Should kernel threads that can use a lot of CPU have
> > > something in their outer loop to transition KLPs,
> > > just like the idle task does?
> > 
> > Maybe - I suppose this is the first time we've had an issue with
> > CPU-bound kthreads.  I didn't know that was a thing ;-)
> > 
> Kworkers have as much work as you want them to do, and with
> things like btrfs compression that can be quite a bit.

To prevent patching, it would need to be some kind of sustained CPU
activity, rather than a burst.  I guess we haven't seen that show up as
a real-world problem until now.

If you're able to identify which kthreads would be problematic, then
yeah, defining a "transition point" in their outer loops could be an
option.

We could look also at a more general approach, like stack checking from
an irq handler.  But as Petr alluded to, that would be problematic for
CONFIG_FRAME_POINTER.

We could maybe deprecate frame pointers on x86 for live patching, but I
think other arches would have a similar problem unless they were to do
something like the ORC unwinder.

-- 
Josh