linux-kernel - Re: [patch] add /proc/pid/stack to dump task's stack trace

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Date:	Fri, 7 Nov 2008 11:49:43 +0300
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ken Chen <kenchen@...gle.com>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [patch] add /proc/pid/stack to dump task's stack trace

On Fri, Nov 07, 2008 at 09:32:49AM +0100, Ingo Molnar wrote:
> 
> * Alexey Dobriyan <adobriyan@...il.com> wrote:
> 
> > On Fri, Nov 07, 2008 at 08:59:25AM +0100, Ingo Molnar wrote:
> > > 
> > > * Ingo Molnar <mingo@...e.hu> wrote:
> > > 
> > > > > > + * buffer size used for proc read.  See proc_info_read().
> > > > > > + * 4K page size but our output routines use some slack for overruns
> > > > > > + */
> > > > > > +#define PROC_BLOCK_SIZE	(3*1024)
> > > > 
> > > > That sounds like a proper limit - the hard limit for this particular 
> > > > printout function is 4096-170, so we are well within the 
> > > > PROC_BLOCK_SIZE range.
> > > 
> > > ok, i've added Ken's patch to tip/core/stacktrace and started testing 
> > > it.
> > > 
> > > Alexey, i've added your Acked-by because you appeared to like the 
> > > patch - let me know if i should remove it.
> > 
> > Of course, I don't like it!
> > 
> > Switch to seqfiles, add entry in TID table as well.
> >
> > The idea is good, though.
> 
> oh well - Ken, could you please switch it to seqfiles?
> 
> It should be something like this to convert the currently sweet and 
> trivial function into a much more complex seqfile iterator splitup:
> 
> - the ::start method does the kmalloc of a loop state structure like 
>   this:
> 
>   {
> 	struct stack_trace backtrace;
> 	unsigned long entries[MAX_STACK_TRACE_DEPTH];
> 
> 	int iterator;
>   }
> 
>   and saves the trace. (struct stack_trace trace can be on-stack, it's 
>   only needed for the save_stack_trace() - and we keep the entries 
>   after that.)
> 
> - the ::stop method does the kfree of the loop state.
> 
> - the ::show method prints a single line, based on ->entries[->iterator]
> 
> - the ::next method does ->iterator++, and returns NULL if iterator 
>   reaches ->backtrace.nr_entries.
> 
> it will be more source code, larger kernel image, it will be more 
> fragile and will be harder to review, and it wont actually matter in 
> practice because 99.9999% of the backtraces we care about have a size 
> smaller than 3K. (and where they get larger clipping them to the first 
> 3K is perfectly fine)

Or you can do all of this in ->show(), without start/next/stop:

	for (i = 0; i < N; i++)
		seq_printf(m, "[<%p>] %pS\n", x, x);
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