linux-kernel - Re: live patching design (was: Re: [PATCH 1/3] sched: add sched_task

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Message-ID: <20150220194901.GB3603@gmail.com>
Date:	Fri, 20 Feb 2015 20:49:01 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	Josh Poimboeuf <jpoimboe@...hat.com>,
	Vojtech Pavlik <vojtech@...e.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Seth Jennings <sjenning@...hat.com>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: live patching design (was: Re: [PATCH 1/3] sched: add
 sched_task_call())


* Jiri Kosina <jkosina@...e.cz> wrote:

> > All fundamental pieces of the simple method are 
> > necessary to get guaranteed time transition from the 
> > complicated method: task tracking and transparent 
> > catching of them, handling kthreads, etc.
> > 
> > My argument is that the simple method should be 
> > implemented first and foremost.
> > 
> > Then people can do add-on features to possibly spread 
> > out the new function versions in a more complicated way 
> > if they want to avoid the stop-all-tasks transition - 
> > although I'm not convinced about it: I'm sure sure many 
> > sysadmins would like the bug patching to be over with 
> > quickly and not have their systems in an intermediate 
> > state like kgraft does it.
> 
> ... the choice the sysadmins have here is either have the 
> system running in an intermediate state, or have the 
> system completely dead for the *same time*. Because to 
> finish the transition successfully, all the tasks have to 
> be woken up in any case.

That statement is false: an 'intermediate state' system 
where 'new' tasks are still running is still running and 
will interfere with the resolution of 'old' tasks.

> But I do get your point; what you are basically saying is 
> that your preference is what kgraft is doing, and option 
> to allow for a global synchronization point should be 
> added in parallel to the gradual lazy migration.

I think you misunderstood: the 'simple' method I outlined 
does not just 'synchronize', it actually executes the live 
patching atomically, once all tasks are gathered and we 
know they are _all_ in a safe state.

I.e. it's in essence the strong stop-all atomic patching 
model of 'kpatch', combined with the reliable avoidance of 
kernel stacks that 'kgraft' uses.

That should be the starting point, because it's the most 
reliable method.

Thanks,

	Ingo
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