linux-kernel - Re: [PATCH 04/16] perf: Add ability to attach user stack dump to sample

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Message-ID: <20120607140719.GC1672@m.brq.redhat.com>
Date:	Thu, 7 Jun 2012 16:07:19 +0200
From:	Jiri Olsa <jolsa@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>, acme@...hat.com,
	mingo@...e.hu, paulus@...ba.org, cjashfor@...ux.vnet.ibm.com,
	eranian@...gle.com, gorcunov@...nvz.org, tzanussi@...il.com,
	mhiramat@...hat.com, robert.richter@....com, fche@...hat.com,
	linux-kernel@...r.kernel.org, masami.hiramatsu.pt@...achi.com,
	drepper@...il.com, asharma@...com, benjamin.redelings@...cent.org
Subject: Re: [PATCH 04/16] perf: Add ability to attach user stack dump to
 sample

On Thu, Jun 07, 2012 at 12:07:56PM +0200, Frederic Weisbecker wrote:
> On Thu, May 24, 2012 at 12:51:20PM +0200, Peter Zijlstra wrote:
> > On Wed, 2012-05-23 at 21:32 +0200, Jiri Olsa wrote:
> > > +static void
> > > +perf_output_sample_ustack(struct perf_output_handle *handle, u64 dump_size,
> > > +                         struct pt_regs *regs)
> > > +{
> > > +       u64 size;
> > > +
> > > +       /* Case of a kernel thread, nothing to dump */
> > > +       if (!regs) {
> > > +               size = 0;
> > > +               perf_output_put(handle, size);
> > > +       } else {
> > > +               unsigned long sp;
> > > +               unsigned int rem;
> > > +               u64 dyn_size;
> > > +
> > > +               /*
> > > +                * Static size: we always dump the size
> > > +                * requested by the user because most of the
> > > +                * time, the top of the user stack is not
> > > +                * paged out.
> > > +                */
> > > +               size = round_up(dump_size, sizeof(u64));
> > 
> > You also do this in the prepare thing..
> > 
> > > +               perf_output_put(handle, size);
> > > +
> > > +               sp = user_stack_pointer(regs);
> > > +               rem = __output_copy_user(handle, (void *)sp, size);
> > > +               dyn_size = size - rem;
> > > +
> > > +               /* What couldn't be dumped is zero padded */
> > > +               while (rem--) {
> > > +                       char zero = 0;
> > > +                       perf_output_put(handle, zero);
> > > +               }
> > 
> > Does this matter? If we don't write it the worst that can happen is that
> > we leave previous ring-bugger content around, but since we already are
> > privileged to read that (and very likely already have) there's no
> > problem with that..
> > 
> > I know not zero-ing is ugly, but its also faster.. and do we care about
> > them silly zeros?
> 
> The problem there is that the unwinder may then deal with random values
> from previous records in the buffer and think it is some real stack values.
> This may give strange results. At least zeroing was limiting this effect.
> 
> I think the best would be to do like you say: if we can't dump everything
> then just leave the state of the buffer as is. But also output in the end
> the effective size of the stack that has been dumped. So that we know what
> we can give to the unwinder and what we can not.

yep, thats the case right now.. just putting away the zeros

jirka
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