linux-kernel - Re: [RFC PATCH 0/9] livepatch: consistency model

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Message-ID: <alpine.LNX.2.00.1502131532020.14133@pobox.suse.cz>
Date:	Fri, 13 Feb 2015 15:40:14 +0100 (CET)
From:	Miroslav Benes <mbenes@...e.cz>
To:	Jiri Kosina <jkosina@...e.cz>
cc:	Josh Poimboeuf <jpoimboe@...hat.com>,
	Seth Jennings <sjenning@...hat.com>,
	Vojtech Pavlik <vojtech@...e.cz>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/9] livepatch: consistency model

On Fri, 13 Feb 2015, Jiri Kosina wrote:

> On Fri, 13 Feb 2015, Josh Poimboeuf wrote:
> 
> > > How about we take a slightly different aproach -- put a probe (or ftrace) 
> > > on __switch_to() during a klp transition period, and examine stacktraces 
> > > for tasks that are just about to start running from there?
> > > 
> > > The only tasks that would not be covered by this would be purely CPU-bound 
> > > tasks that never schedule. But we are likely in trouble with those anyway, 
> > > because odds are that non-rescheduling CPU-bound tasks are also 
> > > RT-priority tasks running on isolated CPUs, which we will fail to handle 
> > > anyway.
> > > 
> > > I think Masami used similar trick in his kpatch-without-stopmachine 
> > > aproach.
> > 
> > Yeah, that's definitely an option, though I'm really not too crazy about
> > it.  Hooking into the scheduler is kind of scary and disruptive.  
> 
> This is basically about running a stack checking for ->next before 
> switching to it, i.e. read-only operation (admittedly inducing some 
> latency, but that's the same with locking the runqueue). And only when in 
> transition phase.
> 
> > We'd also have to wake up all the sleeping processes.
> 
> Yes, I don't think there is a way around that.

I think there are two options how to do it if I understand you correctly.

1. we would put a probe on __switch_to and afterwards wake up all the 
   sleeping processes.

2. we would do it in an asynchronous manner. We would put a probe and let 
   the processes to wake themselves. The transition delayed workqueue 
   would only check if there is some non-migrated process. Of course if 
   some process sleeps for a long time it would take a long time to 
   complete the patching. It would be up to the user to send a signal to 
   the process to wake up.

Does it make sense? If yes, I cannot decide which approach is better.

Miroslav
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