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

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Message-ID: <alpine.LNX.2.00.1502201151390.28769@pobox.suse.cz>
Date:	Fri, 20 Feb 2015 11:58:17 +0100 (CET)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Ingo Molnar <mingo@...nel.org>
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())

On Fri, 20 Feb 2015, Ingo Molnar wrote:

>   - the complicated method spread out over time: uses the 
>     same essential mechanism plus the ftrace patching 
>     machinery to detect whether all tasks have transitioned 
>     through a version flip. [this is what kgraft does in 
>     part.]

The only difference of this to what kgraft does is that alive-enough tasks 
are not put in this kind of "out of kernel 'freezer'", but keep running. 
Modifying kgraft to (optionally) add the synchronization barrier and then 
flip the switch should be a rather trivial task, and can indeed be added 
as a simple option to the patch author / sysadmin. However ...

> 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.

(please note that this is different to suspend/resume task freezing, 
because we don't care about sleeping tasks there).

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.

> In any case, as per my arguments above, examining the kernel stack is 
> superfluous (so we won't be exposed to the fragility of it either): 
> there's no need to examine it and writing such patches is misguided...
> 
> Thanks,
> 
> 	Ingo
> 

-- 
Jiri Kosina
SUSE Labs
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