Re: using skip>0 with bpf_get_stack()

From: Namhyung Kim <namhyung@kernel.org>
To: Eugene Loh <eugene.loh@oracle.com>
Cc: Yonghong Song <yhs@fb.com>, bpf@vger.kernel.org
Subject: Re: using skip>0 with bpf_get_stack()
Date: Fri, 4 Mar 2022 12:37:33 -0800	[thread overview]
Message-ID: <YiJ4jTB8siLwxAEN@google.com> (raw)
In-Reply-To: <4e2e5738-b103-d340-753e-7e37e06304c4@fb.com>

Hello,

On Mon, Jun 28, 2021 at 08:33:11PM -0700, Yonghong Song wrote:
> 
> 
> On 6/25/21 6:22 PM, Eugene Loh wrote:
> > 
> > On 6/1/21 5:48 PM, Yonghong Song wrote:
> > > Could you submit a patch for this? Thanks!
> > 
> > Sure.  Thanks for looking at this and sorry about the long delay getting
> > back to you.
> > 
> > Could you take a look at the attached, proposed patch?  As you see in
> > the commit message, I'm unclear about the bpf_get_stack*_pe() variants.
> > They might use an earlier construct callchain, and I do not know ho
> > init_nr was set for them.
> 
> I think bpf_get_stackid() and __bpf_get_stackid() implementation is correct.
> Did you find any issues?
> 
> For bpf_get_stack_pe, see:
> 
> https://lore.kernel.org/bpf/20200723180648.1429892-2-songliubraving@fb.com/
> I think you should not change bpf_get_stack() function.
> __bpf_get_stack() is used by bpf_get_stack() and bpf_get_stack_pe().
> In bpf_get_stack_pe(), callchain is fetched by perf event infrastructure
> if event->attr.sample_type & __PERF_SAMPLE_CALLCHAIN_EARLY is true.
> 
> Just focus on __bpf_get_stack(). We could factor __bpf_get_stackid(),
> but unless we have a bug, I didn't see it is necessary.
> 
> It will be good if you can add a test for the change, there is a stacktrace
> test prog_tests/stacktrace_map.c, you can take a look,
> and you can add a subtest there.
> 
> Next time, you can submit a formal patch with `git send-email ...` to
> this alias. This way it is easier to review compared to attachment.

Any updates on this?  I'm hitting the same issue and found this before
sending a fix.

Thanks,
Namhyung