linux-kernel - Re: [PATCH 1/3] sched: add sched_task

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Date:	Fri, 20 Feb 2015 08:46:52 +0100 (CET)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
cc:	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
Subject: Re: [PATCH 1/3] sched: add sched_task_call()

On Thu, 19 Feb 2015, Josh Poimboeuf wrote:

> So I've looked at kgr_needs_lazy_migration(), but I still have no idea
> how it works.
> 
> First of all, I think reading the stack while its being written to could
> give you some garbage values, and a completely wrong nr_entries value
> from save_stack_trace_tsk().

I believe we've already been discussing this some time ago ...

I agree that this is a very crude optimization that should probably be 
either removed (which would only cause slower convergence in the presence 
of CPU-bound tasks), or rewritten to perform IPI-based stack dumping 
(probably on a voluntarily-configurable basis).

Reading garbage values could only happen if the task would be running in 
kernelspace. nr_entries would then be at least 2.

But I agree that relying on this very specific behavior is not really 
safe in general in case someone changes the stack dumping implementation 
in the future in an unpredictable way.

> But also, how would you walk a stack without knowing its stack pointer? 
> That function relies on the saved stack pointer in 
> task_struct.thread.sp, which, AFAICT, was last saved during the last 
> call to schedule().  Since then, the stack could have been completely 
> rewritten, with different size stack frames, before the task exited the 
> kernel.

Same argument holds here as well, I believe.

Thanks,

-- 
Jiri Kosina
SUSE Labs
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