From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mgamail.intel.com (mgamail.intel.com [198.175.65.16]) (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 E2D7332AACD; Mon, 27 Oct 2025 18:56:52 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=198.175.65.16 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761591414; cv=none; b=LvRoM6dpoOp82rhIns5wGL+ixpl/wgMxwSKc+6P33QIbrgNBItKzwx2KkVfzzje/6dF3OPRYG2vzC1rvE9XcqadMMdJz+v8vE0FXeysoPhCVtn/AzOHZfIgc/Z+N6CuLLy61XzlSO6r7GQ3L9rMqhgjTq1z/d1D0FF9NT2nz1t8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761591414; c=relaxed/simple; bh=ey78BkZ5aYqNf94WaChAbrxxD/aeaOc0WHaEt32jLpI=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=b/UdzKucaW7yLIoYMZrUirYI2PG7YFs5wT/llt+VUtld5nHF10vkl8d+nVwpDSu8MTV5/FWfNJ4NtlBTz5PsHj0rOwTsjSqIvWh4u4gQ+ZHj6x2LbTQC259yU91rjfS+XbT7bJ7Bcjejup2EqS/wFqKhzCE9k4NyXJuu39GjOdw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=intel.com; spf=pass smtp.mailfrom=intel.com; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.b=NBdQ9fMP; arc=none smtp.client-ip=198.175.65.16 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=intel.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=intel.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.b="NBdQ9fMP" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1761591413; x=1793127413; h=message-id:date:mime-version:subject:to:cc:references: from:in-reply-to:content-transfer-encoding; bh=ey78BkZ5aYqNf94WaChAbrxxD/aeaOc0WHaEt32jLpI=; b=NBdQ9fMP1fbUWSwIVgj8ukLpeoLOnQO0ld17Qc7sDFbOJZ2rTWl8BZYA 1YN5VQoNHkYtRLWH/allIzpTpa0BRdpNGKWSw89ewYPRsPCQlJUphU2C9 ThfqRTUcknC3VVUhK7Wv/p5enikC/Zqlp5uqeu0IKv6pQ4dIKVihrcuOl PtmtKbUgMxyUFbxkHE/If6b6KcsCW++t+hCoR1TNV+CVC+nxGfl6Rc/x7 ly9mSB+kGmJaNgGJX+hhOAOaFWh8CG2/qTCTPNQ83tN4i9+GX9Ub/qfxC 03F34Vn9u4iaDQ6bygR5gMKAcKiqlShsRM/1mjE2L8Lq0SXVk+4lRVfMf Q==; X-CSE-ConnectionGUID: 1VVr0etfTniBsG7kx2et/Q== X-CSE-MsgGUID: yEbKIzloSrmeVPPNeD/fdA== X-IronPort-AV: E=McAfee;i="6800,10657,11586"; a="63833611" X-IronPort-AV: E=Sophos;i="6.19,259,1754982000"; d="scan'208";a="63833611" Received: from orviesa003.jf.intel.com ([10.64.159.143]) by orvoesa108.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 27 Oct 2025 11:56:52 -0700 X-CSE-ConnectionGUID: z096U8MZQMW5Yxmhweqjow== X-CSE-MsgGUID: Av9LkBWzTxO9V+b8pJ+xjQ== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.19,259,1754982000"; d="scan'208";a="189160163" Received: from unknown (HELO [10.241.242.66]) ([10.241.242.66]) by ORVIESA003-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 27 Oct 2025 11:56:53 -0700 Message-ID: Date: Mon, 27 Oct 2025 11:56:52 -0700 Precedence: bulk X-Mailing-List: linux-perf-users@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] perf tools: Refactor precise_ip fallback logic To: Namhyung Kim Cc: linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, Peter Zijlstra , Adrian Hunter , Ingo Molnar , Jiri Olsa , Mark Rutland , Arnaldo Carvalho de Melo , Ian Rogers , Alexander Shishkin , thomas.falcon@intel.com, dapeng1.mi@linux.intel.com, xudong.hao@intel.com References: <20251022220802.1335131-1-zide.chen@intel.com> <576a7d2b-0a82-4738-8b86-507e4d841524@intel.com> Content-Language: en-US From: "Chen, Zide" In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 10/25/2025 5:42 PM, Namhyung Kim wrote: > On Fri, Oct 24, 2025 at 11:03:17AM -0700, Chen, Zide wrote: >> >> >> On 10/23/2025 7:30 PM, Namhyung Kim wrote: >>> Hello, >>> >>> On Wed, Oct 22, 2025 at 03:08:02PM -0700, Zide Chen wrote: >>>> Commit c33aea446bf555ab ("perf tools: Fix precise_ip fallback logic") >>>> unconditionally called the precise_ip fallback and moved it after the >>>> missing-feature checks so that it could handle EINVAL as well. >>>> >>>> However, this introduced an issue: after disabling missing features, >>>> the event could fail to open, which makes the subsequent precise_ip >>>> fallback useless since it will always fail. >>>> >>>> For example, run the following command on Intel SPR: >>>> >>>> $ perf record -e '{cpu/mem-loads-aux/S,cpu/mem-loads,ldlat=3/PS}' -- ls >>>> >>>> Opening the event "cpu/mem-loads,ldlat=3/PS" returns EINVAL when >>>> precise_ip == 3. It then sets attr.inherit = false, which triggers a >>> >>> I'm curious about this part. Why the kernel set 'inherit = false'? IOW >>> how did the leader event (mem-loads-aux) succeed with inherit = true >>> then? >> >> Initially, the inherit = true for both the group leader >> (cpu/mem-loads-aux/S) and the event in question (cpu/mem-loads,ldlat=3/PS). >> >> When the second event fails with EINVAL, the current logic calls >> evsel__detect_missing_features() first. Since this is a PERF_SAMPLE_READ >> event, the inherit attribute falls back to false, according to the >> fallback order implemented in evsel__detect_missing_features(). > > Right, that means the kernel doesn't support PERF_SAMPLE_READ with > inherit = true. How did the first event succeed to open then? The perf tool sets PERF_SAMPLE_TID for Inherit + PERF_SAMPLE_READ events, as implemented in commit 90035d3cd876 ("tools/perf: Allow inherit + PERF_SAMPLE_READ when opening event"). Meanwhile, commit 7e8b255650fc ("perf: Support PERF_SAMPLE_READ with inherit") rejects a perf event if has_inherit_and_sample_read(attr) is true and PERF_SAMPLE_TID is not set in attr->sample_type. Therefore, the first event succeeded, while the one opened in evsel__detect_missing_features() which doesn't have PERF_SAMPLE_TID failed. >> >>> >>>> kernel check failure since it doesn't match the group leader's inherit >>>> attribute. As a result, it continues to fail even after precise_ip is >>>> reduced. >>>> >>>> By moving the precise_ip fallback earlier, this issue is resolved, as >>>> well as the kernel test robot report mentioned in commit >>>> c33aea446bf555ab. >>>> >>>> No negative side effects are expected, because the precise_ip level is >>>> restored by evsel__precise_ip_fallback() if the fallback does not help. >>> >>> I'm not sure.. some events may need a different (i.e. lower) precise >>> level than the max. I think checking the missing feature later will >>> use the max precise level always. >> >> Yes, but seems the basic idea of the event open fallback logic is to >> check whether it's lucky enough to open the event by falling back one >> single attribute, not multiple attributes. >> >> evsel__precise_ip_fallback() can restore the precise_ip level after a >> failed attempt, while evsel__detect_missing_features() cannot recover >> the event attributes from its failed try. > > I think precise_ip_fallback() is just a trial and error for each possible > value. While detect_missing_features() checks what the current kernel > supports. Trying different precise_ip values with unsupported attributes > doesn't make sense. When it returns -EINVAL, the cause could be an unsupported precise_ip or something else. We could either end up with "trying different precise_ip values with unsupported attributes", or "trying attributes with unsupported precise_ip". The perf tool’s fallback logic is a “best effort” mechanism to fix only one issue, not multiple ones. So, IMO, we should place evsel__detect_missing_features() as the last fallback attempt, since it does not restore the event attributes after a failed try. > Thanks, > Namhyung > >> >> Therefore, falling back precise_ip first maintains the intended >> “one-by-one” fallback logic. If it’s placed later, it may combine two >> fallbacks, which can cause failures like the example above. Of course, >> in theory, there might be cases where an event can be opened if both >> precise_ip and another feature are relaxed together, but I haven’t >> exhaustively checked whether such cases actually exist. >> >>> Thanks, >>> Namhyung >>> >>>> >>>> This also aligns with commit 2b70702917337a8d ("perf tools: Remove >>>> evsel__handle_error_quirks()"). >>>> >>>> Fixes: af954f76eea56453 ("perf tools: Check fallback error and order") >>>> Fixes: c33aea446bf555ab ("perf tools: Fix precise_ip fallback logic") >>>> Reviewed-by: Dapeng Mi >>>> Signed-off-by: Zide Chen >>>> --- >>>> tools/perf/util/evsel.c | 6 +++--- >>>> 1 file changed, 3 insertions(+), 3 deletions(-) >>>> >>>> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c >>>> index ca74514c8707..6ce32533a213 100644 >>>> --- a/tools/perf/util/evsel.c >>>> +++ b/tools/perf/util/evsel.c >>>> @@ -2714,12 +2714,12 @@ static int evsel__open_cpu(struct evsel *evsel, struct perf_cpu_map *cpus, >>>> if (err == -EMFILE && rlimit__increase_nofile(&set_rlimit)) >>>> goto retry_open; >>>> >>>> + if (evsel__precise_ip_fallback(evsel)) >>>> + goto retry_open; >>>> + >>>> if (err == -EINVAL && evsel__detect_missing_features(evsel, cpu)) >>>> goto fallback_missing_features; >>>> >>>> - if (evsel__precise_ip_fallback(evsel)) >>>> - goto retry_open; >>>> - >>>> out_close: >>>> if (err) >>>> threads->err_thread = thread; >>>> -- >>>> 2.51.0 >>>> >>