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From:   Song Liu <songliubraving@fb.com>
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
CC:     bpf <bpf@vger.kernel.org>,
        Network Development <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Alexei Starovoitov <ast@kernel.org>,
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        Kernel Team <Kernel-team@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>,
        Andrii Nakryiko <andriin@fb.com>
Subject: Re: [PATCH v4 bpf-next 2/4] bpf: introduce helper
 bpf_get_task_stack()
Thread-Topic: [PATCH v4 bpf-next 2/4] bpf: introduce helper
 bpf_get_task_stack()
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Date:   Tue, 30 Jun 2020 06:12:52 +0000
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> On Jun 29, 2020, at 9:18 PM, Alexei Starovoitov <alexei.starovoitov@gmail=
.com> wrote:
>=20
> On Sun, Jun 28, 2020 at 10:58 PM Song Liu <songliubraving@fb.com> wrote:
>>=20
>> Introduce helper bpf_get_task_stack(), which dumps stack trace of given
>> task. This is different to bpf_get_stack(), which gets stack track of
>> current task. One potential use case of bpf_get_task_stack() is to call
>> it from bpf_iter__task and dump all /proc/<pid>/stack to a seq_file.
>>=20
>> bpf_get_task_stack() uses stack_trace_save_tsk() instead of
>> get_perf_callchain() for kernel stack. The benefit of this choice is tha=
t
>> stack_trace_save_tsk() doesn't require changes in arch/. The downside of
>> using stack_trace_save_tsk() is that stack_trace_save_tsk() dumps the
>> stack trace to unsigned long array. For 32-bit systems, we need to
>> translate it to u64 array.
>>=20
>> Acked-by: Andrii Nakryiko <andriin@fb.com>
>> Signed-off-by: Song Liu <songliubraving@fb.com>
>=20
> It doesn't apply:
> Applying: bpf: Introduce helper bpf_get_task_stack()
> Using index info to reconstruct a base tree...
> error: patch failed: kernel/bpf/stackmap.c:471
> error: kernel/bpf/stackmap.c: patch does not apply
> error: Did you hand edit your patch?
> It does not apply to blobs recorded in its index.
> Patch failed at 0002 bpf: Introduce helper bpf_get_task_stack()

Hmm.. seems "git format-patch -b" (--ignore-space-change) breaks it:

# without -b, works fine

$ git format-patch HEAD~1
0001-bpf-introduce-helper-bpf_get_task_stack.patch
$ git reset --hard HEAD~1
HEAD is now at c385fe4fbd7bc perf: expose get/put_callchain_entry()
$ git am ./0001-bpf-introduce-helper-bpf_get_task_stack.patch
Applying: bpf: introduce helper bpf_get_task_stack()


# with -b, doesn't apply :(

$ git format-patch -b HEAD~1
0001-bpf-introduce-helper-bpf_get_task_stack.patch
$ git reset --hard HEAD~1
HEAD is now at c385fe4fbd7bc perf: expose get/put_callchain_entry()
$ git am ./0001-bpf-introduce-helper-bpf_get_task_stack.patch
Applying: bpf: introduce helper bpf_get_task_stack()
error: patch failed: kernel/bpf/stackmap.c:471
error: kernel/bpf/stackmap.c: patch does not apply
Patch failed at 0001 bpf: introduce helper bpf_get_task_stack()
hint: Use 'git am --show-current-patch' to see the failed patch
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

Let me see how to fix it...