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Date:   Wed, 11 May 2022 21:07:54 -0700
From:   Josh Poimboeuf <jpoimboe@kernel.org>
To:     Song Liu <songliubraving@fb.com>
Cc:     Petr Mladek <pmladek@suse.com>, Rik van Riel <riel@fb.com>,
        "song@kernel.org" <song@kernel.org>,
        "joe.lawrence@redhat.com" <joe.lawrence@redhat.com>,
        "peterz@infradead.org" <peterz@infradead.org>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "vincent.guittot@linaro.org" <vincent.guittot@linaro.org>,
        "live-patching@vger.kernel.org" <live-patching@vger.kernel.org>,
        Kernel Team <Kernel-team@fb.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "jpoimboe@redhat.com" <jpoimboe@redhat.com>
Subject: Re: [RFC] sched,livepatch: call klp_try_switch_task in __cond_resched
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On Wed, May 11, 2022 at 04:33:57PM +0000, Song Liu wrote:
> >> Ideally we'd have the ORC unwinder for all arches, that would make this
> >> much easier.  But we're not there yet.
> > 
> > The alternative solution is that the process has to migrate itself
> > on some safe location.
> > 
> > One crazy idea. It still might be possible to find the called
> > functions on the stack even when it is not reliable. Then it
> > might be possible to add another ftrace handler on
> > these found functions. This other ftrace handler might migrate
> > the task when it calls this function again.
> > 
> > It assumes that the task will call the same functions again
> > and again. Also it might require that the tasks checks its
> > own stack from the ftrace handler. I am not sure if this
> > is possible.
> > 
> > There might be other variants of this approach.
> 
> This might be the ultimate solution! As ftrace allows filtering based
> on pid (/sys/kernel/tracing/set_ftrace_pid), we can technically trigger
> klp_try_switch_task() on every function of the pending tasks. If this 
> works, we should finish most of the transition in seconds. And the only
> failure there would be threads with being patched function at the very 
> sottom of its stack. Am I too optimistic here? 

It's a crazy idea, but I kind of like it ;-)  Especially this variant of
tracing all functions for the task.  We'd have to make sure unwinding
from an ftrace handler works for all arches/unwinders.

-- 
Josh