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Marchesi" , Beau Belgrave , Linus Torvalds , Andrew Morton , Jens Axboe , Florian Weimer , Sam James Subject: Re: [RFC PATCH v1 12/16] unwind_user/backchain: Introduce back chain user space unwinding Message-ID: <20250801123647.9905A43-hca@linux.ibm.com> References: <20250710163522.3195293-1-jremus@linux.ibm.com> <20250710163522.3195293-13-jremus@linux.ibm.com> <63665c54-db44-452f-b321-1162ff6c3fe4@linux.ibm.com> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: wmg_jJYi_bAxFEdtwgpPXeefh0YLfbdi X-Proofpoint-GUID: wmg_jJYi_bAxFEdtwgpPXeefh0YLfbdi X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUwODAxMDA5MiBTYWx0ZWRfX1Kf3ofBZeGu1 QofOzzqthUplNvFwtwQYixbImxKlfN1xZT8EfXqd13DTYS1bwEpz1jejGIxtcSTmlMvLsiH8HGm B1SKosiCxTz39sR0NomiFtBWyML7ROuf0dzeJWMwefF4q3cHWmacBR/2+HC4bOf76TZx9uKndSY 0tjnQHBo7JWgpLOCT9F38jAI6VGAeRZ8fhGyZYaS580zJ9Di/ocxBByG2aFM8o7QZ8xO7+kdFpJ RA65SxfwkzkA1F4Kmlox97bj5+uboCOGWe80/JbgaCzJZAAD+pvbkNK4XSB3fd72vDuKZAV8sIB ypbhdM/g/EUxFjaPY/8sBtt8zkuRq6PUPjwPwA9GfwZ4QU1m6sYQbScEuAfWWHEhvauieThPVMH EtjHZdzD3QkdQuTePaYv7iwrINOljCk+lXi3dqAeZ8F7yL7cZwDlsc+4O+D8H2sA3CG3OqwF X-Authority-Analysis: v=2.4 cv=BJOzrEQG c=1 sm=1 tr=0 ts=688cb4e7 cx=c_pps a=3Bg1Hr4SwmMryq2xdFQyZA==:117 a=3Bg1Hr4SwmMryq2xdFQyZA==:17 a=kj9zAlcOel0A:10 a=2OwXVqhp2XgA:10 a=1JbVbOoES49UGEVtLQsA:9 a=CjuIK1q_8ugA:10 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1099,Hydra:6.1.9,FMLib:17.12.80.40 definitions=2025-08-01_04,2025-07-31_03,2025-03-28_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 impostorscore=0 phishscore=0 suspectscore=0 spamscore=0 lowpriorityscore=0 mlxlogscore=585 priorityscore=1501 malwarescore=0 mlxscore=0 bulkscore=0 adultscore=0 clxscore=1011 classifier=spam authscore=0 authtc=n/a authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.19.0-2505280000 definitions=main-2508010092 On Thu, Jul 17, 2025 at 10:19:54PM -0700, Josh Poimboeuf wrote: > On Thu, Jul 17, 2025 at 02:20:12PM +0200, Jens Remus wrote: > > > Also, if distros aren't even compiling with -mbackchain, I wonder if we > > > can just not do this altogether :-) > > > > My original intent was to use unwind user's for_each_user_frame() to > > replace the exiting stack tracing logic in arch_stack_walk_user_common() > > in arch/s390/kernel/stacktrace.c, which currently supports backchain. > > Given that for_each_user_frame() was made private in the latest unwind > > user series version hinders me. The use was also low, because the > > currentl arch_stack_walk_user_common() implementation does not support > > page faults, so that the attempt to use unwind user sframe would always > > fail and fallback to unwind user backchain. My hope was that somebody > > with more Kernel skills could give me a few hints at how it could be > > made to support deferred unwind. :-) > > I believe stack_trace_save_user() is only used by ftrace, and that will > no longer be needed once ftrace starts using unwind_user. > > Maybe Heiko knows if that backchain user stacktrace code has any users? > > If distros aren't building with -mbackchain, maybe backchain support can > be considered obsoleted by sframe, and we can get away with not > implementing it. I guess that's a valid option. I know only of some special cases where users compile everything on their own with -mbackchain to make this work on a per-case basis. It shouldn't cause to much pain for them to switch to sframe, as soon as that is available.