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Date:   Tue, 10 May 2022 16:04:02 -0700
From:   Josh Poimboeuf <jpoimboe@kernel.org>
To:     Song Liu <songliubraving@fb.com>
Cc:     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>,
        "pmladek@suse.com" <pmladek@suse.com>,
        "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 Tue, May 10, 2022 at 07:45:49PM +0000, Song Liu wrote:
> >> A KLP transition preempt notifier would help those
> >> kernel threads transition to the new KLP version at
> >> any time they reschedule.
> > 
> > ... unless cond_resched() is a no-op due to CONFIG_PREEMPT?
> 
> Based on my understanding (and a few other folks we chatted with),
> a kernel thread can legally run for extended time, as long as it 
> calls cond_resched() at a reasonable frequency. Therefore, I 
> think we should be able to patch such thread easily, unless it 
> calls cond_resched() with being-patched function in the stack, 
> of course.

But again, with CONFIG_PREEMPT, that doesn't work.

> OTOH, Petr's mindset of allowing many minutes for the patch 
> transition is new to me. I need to think more about it. 
> Josh, what’s you opinion on this? IIUC, kpatch is designed to 
> only wait up to 60 seconds (no option to overwrite the time). 

I wouldn't be necessarily opposed to changing the kpatch timeout to
something bigger, or eliminating it altogether in favor of a WARN()
after x minutes.

> >> How much it will help is hard to predict, but I should
> >> be able to get results from a fairly large sample size
> >> of systems within a few weeks :)
> > 
> > As Peter said, keep in mind that we will need to fix other cases beyond
> > Facebook, i.e., CONFIG_PREEMPT combined with non-x86 arches which don't
> > have ORC so they can't reliably unwind from an IRQ.
> 
> I think livepatch transition may fail in different cases, and we
> don't need to address all of them in one shoot. Fixing some cases
> is an improvement as long as we don't slow down other cases. I 
> understand that adding tiny overhead to __cond_resched() may end 
> up as a visible regression. But maybe adding it to 
> preempt_schedule_common() is light enough?
> 
> Did I miss/misunderstand something?

If it's a real bug, we should fix it everywhere, not just for Facebook.
Otherwise CONFIG_PREEMPT and/or non-x86 arches become second-class
citizens.

-- 
Josh