linux-kernel - Re: [PATCH 3/3] tracing/function-return-tracer: add the overrun field

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Date:	Fri, 21 Nov 2008 20:48:19 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Frédéric Weisbecker <fweisbec@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] tracing/function-return-tracer: add the overrun
	field


* Frédéric Weisbecker <fweisbec@...il.com> wrote:

> When the tracer will be launched, I will hold the tasklist_lock to 
> allocate/insert the dynamic arrays. So in this atomic context, I 
> will not be able to call kmalloc with GFP_KERNEL. And I fear that 
> using GFP_ATOMIC for possible hundreds of tasks would be clearly 
> unacceptable.
> 
> What do you think of this way:
> 
> _tracer activates
> _a function enters the tracer entry-hooker. If the array is allocated
> for the current task, that's well. If not I launch a kernel thread
> that will later allocate an array for the current task (I will pass
> the pid as a parameter). So the current task will be soon be traced.
> _ when a process forks, I can allocate a dynamic array for the new
> task without problem (I hope).
> 
> So some tasks will not be traced at the early beggining of tracing 
> but they will soon all be traced.... There is perhaps a problem with 
> tasks that are sleeping for long times... There will be some losses 
> once they will be awaken...

i'd suggest a different approach that is simpler:

- step0: set flag that "all newly created tasks need the array 
  allocated from now on".

- step1: allocate N arrays outside tasklist_lock

- step2: take tasklist_lock, loop over all tasks that exist and pass 
  in the N arrays to all tasks that still need it.

  If tasks were 'refilled', drop tasklist_lock and go back to step 1.

- step3: free N (superfluously allocated) arrays

Make N something like 32 to not get into a bad quadratic nr_tasks 
double loop in practice. (Possibly allocate arrays[32] dynamically as 
well at step0 and not have it on the kernel stack - so 32 can be 
changed to 128 or so.)

	Ingo
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