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

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Date:	Fri, 21 Nov 2008 21:07:33 +0100
From:	"Frédéric Weisbecker" <fweisbec@...il.com>
To:	"Ingo Molnar" <mingo@...e.hu>
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

2008/11/21 Ingo Molnar <mingo@...e.hu>:
>
> * 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
>

Ok. I thought about this method but wondered about the fact that
kmalloc can schedule and then I could run in an infinite loop (or a
too long one).
I will try this. Thanks.
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