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

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Date:	Sun, 23 Nov 2008 14:18:50 +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:

> 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).

the retry loop should solve that aspect - and the chunking solves the 
"dont run too long with a lock held" problem.

> I will try this. Thanks.

looks good, applied :)

	Ingo
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