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

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Date:   Wed, 11 May 2022 18:09:28 +0000
From:   Rik van Riel <riel@...com>
To:     "jpoimboe@...nel.org" <jpoimboe@...nel.org>
CC:     "song@...nel.org" <song@...nel.org>,
        "joe.lawrence@...hat.com" <joe.lawrence@...hat.com>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.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>,
        Song Liu <songliubraving@...com>,
        Kernel Team <Kernel-team@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "pmladek@...e.com" <pmladek@...e.com>
Subject: Re: [RFC] sched,livepatch: call klp_try_switch_task in __cond_resched

On Tue, 2022-05-10 at 18:12 -0700, Josh Poimboeuf wrote:
> On Wed, May 11, 2022 at 12:46:32AM +0000, Rik van Riel wrote:
> > On Tue, 2022-05-10 at 17:37 -0700, Josh Poimboeuf wrote:
> > > On Wed, May 11, 2022 at 12:35:11AM +0000, Rik van Riel wrote:
> > > > On Tue, 2022-05-10 at 23:57 +0000, Song Liu wrote:
> > > > > 
> > > > > So, if we come back to the same question: is this a bug (or a
> > > > > suboptimal
> > > > > behavior that worth fixing)? If so, we are open to any
> > > > > solution
> > > > > that 
> > > > > would also help PREEMPT and/or non-x86 arches. 
> > > > > 
> > > > Using the preempt notifiers during KLP transition should
> > > > work equally well for PREEMPT and !PREEMPT. It also does
> > > > not insert any additional code into the scheduler while
> > > > there is no KLP transition going on.
> > > 
> > > As I've been saying, this is not going to work for PREEMPT
> > > because,
> > > without ORC, we can't reliably unwind from an IRQ handler, so the
> > > kthread won't get patched.
> > > 
> > Isn't the sched_out preempt notifier always run in
> > process context?
> > 
> > What am I missing?
> 
> Maybe it's technically process context at that point.  But the
> important
> point is that the call to the scheduler via preempt_schedule_irq()
> originates from the "return from interrupt" path.

Ahhhh, I think I understand.

Does that mean if the scheduling of the kernel thread originated
from an IRQ, the KLP transition will fail probably?

However, if the call to schedule came from a voluntary preemption,
for example through a cond_resched() or due to the thread going
to sleep a little bit, the stack walk will be reliable, and the
KLP transition may succeed?