linux-kernel - Re: [RFC] sched,livepatch: call klp_try_switch_task in __cond

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Message-ID: <20220510165244.ikfh64ertnvodxb4@treble>
Date:   Tue, 10 May 2022 09:52:44 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Rik van Riel <riel@...com>
Cc:     "pmladek@...e.com" <pmladek@...e.com>,
        "song@...nel.org" <song@...nel.org>,
        "joe.lawrence@...hat.com" <joe.lawrence@...hat.com>,
        Song Liu <songliubraving@...com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
        "live-patching@...r.kernel.org" <live-patching@...r.kernel.org>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        Kernel Team <Kernel-team@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] sched,livepatch: call klp_try_switch_task in __cond_resched

On Tue, May 10, 2022 at 04:07:42PM +0000, Rik van Riel wrote:
> On Tue, 2022-05-10 at 17:44 +0200, Petr Mladek wrote:
> > On Tue 2022-05-10 13:33:13, Rik van Riel wrote:
> > > On Tue, 2022-05-10 at 09:56 +0200, Petr Mladek wrote:
> > > 
> > 
> > > I think the best approach for us might be to just track what
> > > is causing the transition failures, and send in trivial patches
> > > to make the outer loop in such kernel threads do the same KLP
> > > transition the idle task already does.
> > 
> > I am afraid that is a way to hell. We might end up in doing
> > really crazy things if we want to complete the transition
> > in one minute.
> > 
> Now I wonder if we could just hook up a preempt notifier
> for kernel live patches. All the distro kernels already
> need the preempt notifier for KVM, anyway...
> 
> Is it crazy? Maybe a little.
> 
> Is it self contained, and something that could be done
> without inserting any extra code into a hot path while
> not in the middle of a KLP transition? Yes.
> 
> I'd be happy to come up with a patch that does that,
> unless anybody has good reasons I should not :)

I wouldn't be opposed to that, but how does it solve this problem?  If
as Peter said cond_resched() can be a NOP, then preemption would have to
be from an interrupt, in which case frame pointers aren't reliable.

-- 
Josh