From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4CB774A23; Fri, 31 Oct 2025 19:00:48 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761937249; cv=none; b=gdCxy06mR1RE7wA0FFuj5DFYU4rQBHqTTD6ZCHltNUrNjnZ3YwjTV+eqZq2/UpV6VnRTesnFIrVHNl0lBQWivTc567+3KfZWuEq8rbL6q1Gz15Wnnwp3LdaqpBrzPawyFH+rl/n7SMvl9mFZZolRp7UmA4U1vYL5oFYDVc7XUyc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761937249; c=relaxed/simple; bh=RWs1yrQBByMB0Ajp5Oy5HX3k99V4RMS+MxY+pO58YRc=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=nCPlSqrfOJw1AWHZamca9CiYcgC5UC236PWN+0bNglMj7n19e477Iqn95kRkJ5QV/9go5Aq748dtqYuV9aMxcpX+zS7ZGnJoSeOgCUNJv6rSaIM5w29yrBEXv+PLbE1d4LOeB9FoLUAuCj53y6E2gVmCPSgavxRMwr95PWuGXds= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ZjWL7R6n; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="ZjWL7R6n" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4D61AC4CEF1; Fri, 31 Oct 2025 19:00:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1761937248; bh=RWs1yrQBByMB0Ajp5Oy5HX3k99V4RMS+MxY+pO58YRc=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=ZjWL7R6nMRbcFhuRMZY/Ent5SVWUFtoPmvQIfT0UbK0SoGxwsDDsS5n9b+3C4z6kN NgQM3onpX4PgrWnAITohe5u4PNYaUuSY3viBNS5NWxm7UfKJyPskti2QJhGUrMFteK RtW9mCgnUMTOcnuz5TMVbCMcuEi7E52dTGcMZhtG0rvmGcT7+4JJ9sdvc90M6VFswZ 8ikuSvaMQCF3WF47ZbJRqGNHidrfDW3SKmuopMhgmTTkHJyCWmUGKdWY7hAF+qINzh 9BOPi46J60BdJt/hOuXoijQGzBJlm4LPOeCpjV+9gYrMXPDOqxPlU6y/xxCloL5Pay 6SmBy92pZUv6g== Date: Fri, 31 Oct 2025 12:00:46 -0700 From: Namhyung Kim To: Ian Rogers Cc: Shuai Xue , alexander.shishkin@linux.intel.com, peterz@infradead.org, james.clark@arm.com, leo.yan@linaro.org, mingo@redhat.com, baolin.wang@linux.alibaba.com, acme@kernel.org, mark.rutland@arm.com, jolsa@kernel.org, adrian.hunter@intel.com, linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, nathan@kernel.org, bpf@vger.kernel.org Subject: Re: [PATCH] perf record: skip synthesize event when open evsel failed Message-ID: References: <20251023015043.38868-1-xueshuai@linux.alibaba.com> <5a06462a-697d-47b6-b51e-6438005b6130@linux.alibaba.com> Precedence: bulk X-Mailing-List: linux-perf-users@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Hello, On Fri, Oct 31, 2025 at 09:04:38AM -0700, Ian Rogers wrote: > On Thu, Oct 30, 2025 at 7:36 PM Shuai Xue wrote: > > > > 在 2025/10/31 01:32, Ian Rogers 写道: > > > On Wed, Oct 29, 2025 at 5:55 AM Shuai Xue wrote: > > >> > > >> > > >> > > >> 在 2025/10/24 10:45, Shuai Xue 写道: > > >>> > > >>> > > >>> 在 2025/10/24 00:08, Ian Rogers 写道: > > >>>> On Wed, Oct 22, 2025 at 6:50 PM Shuai Xue wrote: > > >>>>> > > >>>>> When using perf record with the `--overwrite` option, a segmentation fault > > >>>>> occurs if an event fails to open. For example: > > >>>>> > > >>>>> perf record -e cycles-ct -F 1000 -a --overwrite > > >>>>> Error: > > >>>>> cycles-ct:H: PMU Hardware doesn't support sampling/overflow-interrupts. Try 'perf stat' > > >>>>> perf: Segmentation fault > > >>>>> #0 0x6466b6 in dump_stack debug.c:366 > > >>>>> #1 0x646729 in sighandler_dump_stack debug.c:378 > > >>>>> #2 0x453fd1 in sigsegv_handler builtin-record.c:722 > > >>>>> #3 0x7f8454e65090 in __restore_rt libc-2.32.so[54090] > > >>>>> #4 0x6c5671 in __perf_event__synthesize_id_index synthetic-events.c:1862 > > >>>>> #5 0x6c5ac0 in perf_event__synthesize_id_index synthetic-events.c:1943 > > >>>>> #6 0x458090 in record__synthesize builtin-record.c:2075 > > >>>>> #7 0x45a85a in __cmd_record builtin-record.c:2888 > > >>>>> #8 0x45deb6 in cmd_record builtin-record.c:4374 > > >>>>> #9 0x4e5e33 in run_builtin perf.c:349 > > >>>>> #10 0x4e60bf in handle_internal_command perf.c:401 > > >>>>> #11 0x4e6215 in run_argv perf.c:448 > > >>>>> #12 0x4e653a in main perf.c:555 > > >>>>> #13 0x7f8454e4fa72 in __libc_start_main libc-2.32.so[3ea72] > > >>>>> #14 0x43a3ee in _start ??:0 > > >>>>> > > >>>>> The --overwrite option implies --tail-synthesize, which collects non-sample > > >>>>> events reflecting the system status when recording finishes. However, when > > >>>>> evsel opening fails (e.g., unsupported event 'cycles-ct'), session->evlist > > >>>>> is not initialized and remains NULL. The code unconditionally calls > > >>>>> record__synthesize() in the error path, which iterates through the NULL > > >>>>> evlist pointer and causes a segfault. > > >>>>> > > >>>>> To fix it, move the record__synthesize() call inside the error check block, so > > >>>>> it's only called when there was no error during recording, ensuring that evlist > > >>>>> is properly initialized. > > >>>>> > > >>>>> Fixes: 4ea648aec019 ("perf record: Add --tail-synthesize option") > > >>>>> Signed-off-by: Shuai Xue > > >>>> > > >>>> This looks great! I wonder if we can add a test, perhaps here: > > >>>> https://web.git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/tests/shell/record.sh?h=perf-tools-next#n435 > > >>>> something like: > > >>>> ``` > > >>>> $ perf record -e foobar -F 1000 -a --overwrite -o /dev/null -- sleep 0.1 > > >>>> ``` > > >>>> in a new test subsection for test_overwrite? foobar would be an event > > >>>> that we could assume isn't present. Could you help with a test > > >>>> covering the problems you've uncovered and perhaps related flags? > > >>>> > > >>> > > >>> Hi, Ian, > > >>> > > >>> Good suggestion, I'd like to add a test. But foobar may not a good case. > > >>> > > >>> Regarding your example: > > >>> > > >>> perf record -e foobar -a --overwrite -o /dev/null -- sleep 0.1 > > >>> event syntax error: 'foobar' > > >>> \___ Bad event name > > >>> > > >>> Unable to find event on a PMU of 'foobar' > > >>> Run 'perf list' for a list of valid events > > >>> > > >>> Usage: perf record [] [] > > >>> or: perf record [] -- [] > > >>> > > >>> -e, --event event selector. use 'perf list' to list available events > > >>> > > >>> > > >>> The issue with using foobar is that it's an invalid event name, and the > > >>> perf parser will reject it much earlier. This means the test would exit > > >>> before reaching the part of the code path we want to verify (where > > >>> record__synthesize() could be called). > > >>> > > >>> A potential alternative could be testing an error case such as EACCES: > > >>> > > >>> perf record -e cycles -C 0 --overwrite -o /dev/null -- sleep 0.1 > > >>> > > >>> This could reproduce the scenario of a failure when attempting to access > > >>> a valid event, such as due to permission restrictions. However, the > > >>> limitation here is that users may override > > >>> /proc/sys/kernel/perf_event_paranoid, which affects whether or not this > > >>> test would succeed in triggering an EACCES error. > > >>> > > >>> > > >>> If you have any other suggestions or ideas for a better way to simulate > > >>> this situation, I'd love to hear them. > > >>> > > >>> Thanks. > > >>> Shuai > > >> > > >> Hi, Ian, > > >> > > >> Gentle ping. > > > > > > Sorry, for the delay. I was trying to think of a better way given the > > > problems you mention and then got distracted. I wonder if a legacy > > > event that core PMUs never implement would be a good candidate to > > > test. For example, the event "node-prefetch-misses" is for "Local > > > memory prefetch misses" but the memory controller tends to be a > > > separate PMU and this event is never implemented to my knowledge. > > > Running this locally I see: > > > > > > ``` > > > $ perf record -e node-prefetch-misses -a --overwrite -o /dev/null -- sleep 0.1 > > > Lowering default frequency rate from 4000 to 1750. > > > Please consider tweaking /proc/sys/kernel/perf_event_max_sample_rate. > > > Error: > > > Failure to open event 'cpu_atom/node-prefetch-misses/' on PMU > > > 'cpu_atom' which will be removed. > > > No fallback found for 'cpu_atom/node-prefetch-misses/' for error 2 > > > Error: > > > Failure to open event 'cpu_core/node-prefetch-misses/' on PMU > > > 'cpu_core' which will be removed. > > > No fallback found for 'cpu_core/node-prefetch-misses/' for error 2 > > > Error: > > > Failure to open any events for recording. > > > perf: Segmentation fault > > > #0 0x55a487ad8b87 in dump_stack debug.c:366 > > > #1 0x55a487ad8bfd in sighandler_dump_stack debug.c:378 > > > #2 0x55a4878c6f94 in sigsegv_handler builtin-record.c:722 > > > #3 0x7f72aae49df0 in __restore_rt libc_sigaction.c:0 > > > #4 0x55a487b57ef8 in __perf_event__synthesize_id_index > > > synthetic-events.c:1862 > > > #5 0x55a487b58346 in perf_event__synthesize_id_index synthetic-events.c:1943 > > > #6 0x55a4878cb2a3 in record__synthesize builtin-record.c:2150 > > > #7 0x55a4878cdada in __cmd_record builtin-record.c:2963 > > > #8 0x55a4878d11ca in cmd_record builtin-record.c:4453 > > > #9 0x55a48795b3cc in run_builtin perf.c:349 > > > #10 0x55a48795b664 in handle_internal_command perf.c:401 > > > #11 0x55a48795b7bd in run_argv perf.c:448 > > > #12 0x55a48795bb06 in main perf.c:555 > > > #13 0x7f72aae33ca8 in __libc_start_call_main libc_start_call_main.h:74 > > > #14 0x7f72aae33d65 in __libc_start_main_alias_2 libc-start.c:128 > > > #15 0x55a4878acf41 in _start perf[52f41] > > > Segmentation fault > > > ``` > > > > > > Hi, Ian， > > > > Is node-prefetch-misses a platform specific event? Running it on ARM Yitian 710 > > and Intel SPR platform, I see: > > > > $sudo perf record -e node-prefetch-misses > > Error: > > The node-prefetch-misses event is not supported. > > Hi Shuai, > > So node-prefetch-misses is a legacy event. Perf has a notion of events > that are inbuilt to the kernel/PMU driver and get special fixed > encodings. That said, the PMU driver in the kernel can just fail to > support the events and I think that's uniformly the case for > node-prefetch-misses. As shown by my reproduction of the crash, which > I hope this suffices for a test - i.e. it is an event that parses but > one that is never supported. Maybe it's platform dependent. I have no idea what's the best for this test. Any uncore event would work as well but it's not standardized. I'll merge this fix first. Thanks, Namhyung