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

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Message-ID: <20150221181852.GA8406@gmail.com>
Date:	Sat, 21 Feb 2015 19:18:52 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Vojtech Pavlik <vojtech@...e.com>
Cc:	Jiri Kosina <jkosina@...e.cz>,
	Josh Poimboeuf <jpoimboe@...hat.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())


* Vojtech Pavlik <vojtech@...e.com> wrote:

> On Fri, Feb 20, 2015 at 08:49:01PM +0100, Ingo Molnar wrote:
> 
> > > ... 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.
> 
> Can you suggest a way how they would interfere? The 
> transition happens on entering or returning from a 
> syscall, there is no influence between individual tasks.

Well, a 'new' task does not stop executing after returning 
from the syscall, right? If it's stopped (until all 
patching is totally complete) then you are right and I 
concede your point.

If it's allowed to continue its workload then my point 
stands: subsequent execution of 'new' tasks can interfere 
with, slow down, interact with 'old' tasks trying to get 
patched.

> > 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.
> 
> The 'simple' method has to catch and freeze all tasks one 
> by one in syscall entry/exit, at the kernel/userspace 
> boundary, until all are frozen and then patch the system 
> atomically.

Correct.

> This means that each and every sleeping task in the 
> system has to be woken up in some way (sending a signal 
> ...) to exit from a syscall it is sleeping in. Same for 
> CPU hogs. All kernel threads need to be parked.

Yes - although I'd not use signals for this, signals have 
side effects - but yes, something functionally equivalent.

> This is exactly what you need to do for kGraft to 
> complete patching.

My understanding of kGraft is that by default you allow 
tasks to continue 'in the new universe' after they are 
patched. Has this changed or have I misunderstood the 
concept?

Thanks,

	Ingo
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