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

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Date:	Sat, 21 Feb 2015 19:57:39 +0100 (CET)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Ingo Molnar <mingo@...nel.org>
cc:	Vojtech Pavlik <vojtech@...e.com>,
	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())

On Sat, 21 Feb 2015, Ingo Molnar wrote:

> > 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 similar to my proposal I came up with not too long time ago; a 
fake signal (analogically to, but not exactly the same, what freezer is 
using), that will just make tasks cycle through userspace/kernelspace 
boundary without other side-effects.

> > 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?

What Vojtech meant here, I believe, is that the effort you have to make to 
force all tasks to queue themselves to park them on a safe place and then 
restart their execution is exactly the same as the effort you have to make 
to make kGraft converge and succeed.

But admittedly, if we reserve a special sort-of signal for making the 
tasks pass through a safe checkpoint (and make them queue there (your 
solution) or make them just pass through it and continue (current 
kGraft)), it might reduce the time this effort needs considerably.

-- 
Jiri Kosina
SUSE Labs
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