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X-CSE-ConnectionGUID: xK6WSdPRSNaXivZjiJhObw== X-CSE-MsgGUID: m8zWjG6wT462U0LGrX8dZA== X-IronPort-AV: E=McAfee;i="6800,10657,11605"; a="52195631" X-IronPort-AV: E=Sophos;i="6.19,285,1754982000"; d="scan'208";a="52195631" Received: from orviesa010.jf.intel.com ([10.64.159.150]) by fmvoesa110.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 06 Nov 2025 17:23:10 -0800 X-CSE-ConnectionGUID: LxrbqWnfSc+pzUnvTjOYqA== X-CSE-MsgGUID: pNJIK9weTKqRu9hGayn3Hg== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.19,285,1754982000"; d="scan'208";a="187212574" Received: from unknown (HELO [10.241.241.201]) ([10.241.241.201]) by orviesa010-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 06 Nov 2025 17:23:10 -0800 Message-ID: <5f84fe4f-90ef-42d6-8a3a-c1f515a7832a@intel.com> Date: Thu, 6 Nov 2025 17:23:09 -0800 Precedence: bulk X-Mailing-List: linux-kernel@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> <652bf158-ba9e-4a97-b4c3-3a7f7e39fe85@intel.com> Content-Language: en-US From: "Chen, Zide" In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 11/6/2025 10:52 AM, Namhyung Kim wrote: > On Tue, Nov 04, 2025 at 11:10:44AM -0800, Chen, Zide wrote: >> >> >> On 11/3/2025 7:48 PM, Namhyung Kim wrote: >>> Hello, >>> >>> Sorry for the delay. >>> >>> On Mon, Oct 27, 2025 at 11:56:52AM -0700, Chen, Zide wrote: >>>> >>>> >>>> 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. >>> >>> Why does the first succeed and the second fail? Don't they have the >>> same SAMPLE_READ and SAMPLE_TID + inherit flags? >> >> Sorry, my previous reply wasn’t entirely accurate. The first event >> (cpu/mem-loads-aux/S) succeeds because it’s not a precise event >> (precise_ip == 0). > > I'm not sure how it matters. I've tested the same command line on SPR > and got this message. It says it failed to open because of inherit and > SAMPE_READ. It didn't have precise_ip too. > > $ perf record -e cpu/mem-loads-aux/S -vv true |& less > ... > ------------------------------------------------------------ > perf_event_attr: > type 4 (cpu) > size 136 > config 0x8203 (mem-loads-aux) > { sample_period, sample_freq } 4000 > sample_type IP|TID|TIME|READ|ID|PERIOD > read_format ID|LOST > disabled 1 > inherit 1 > mmap 1 > comm 1 > freq 1 > enable_on_exec 1 > task 1 > sample_id_all 1 > mmap2 1 > comm_exec 1 > ksymbol 1 > bpf_event 1 > ------------------------------------------------------------ > sys_perf_event_open: pid 1161023 cpu 0 group_fd -1 flags 0x8 > sys_perf_event_open failed, error -22 > Using PERF_SAMPLE_READ / :S modifier is not compatible with inherit, falling back to no-inherit. > ... > > And it fell back to no-inherit and succeeded. On my SPR, with either kernel 6.18.0-rc4 or the older 6.17.0-rc6, my test results are different from yours — I didn’t see any EINVAL, and there was no fallback. :) It’s strange, but even so, since there’s no group leader in this case, I assume that when it falls back to non-inherit, it should pass the following check. if (task && group_leader && group_leader->attr.inherit != attr.inherit) { err = -EINVAL; goto err_task; } > I've also found that it > worked even with precise_ip = 3. > > $ perf record -e cpu/mem-loads-aux/PS -vv true |& less > ... > sys_perf_event_open: pid 1172834 cpu 0 group_fd -1 flags 0x8 > sys_perf_event_open failed, error -22 > Using PERF_SAMPLE_READ / :S modifier is not compatible with inherit, falling back to no-inherit. > ------------------------------------------------------------ > perf_event_attr: > type 4 (cpu) > size 136 > config 0x8203 (mem-loads-aux) > { sample_period, sample_freq } 4000 > sample_type IP|TID|TIME|READ|ID|PERIOD > read_format ID|LOST > disabled 1 > mmap 1 > comm 1 > freq 1 > enable_on_exec 1 > task 1 > precise_ip 3 <<<---- here > sample_id_all 1 > mmap2 1 > comm_exec 1 > ksymbol 1 > bpf_event 1 > ------------------------------------------------------------ > sys_perf_event_open: pid 1172834 cpu 0 group_fd -1 flags 0x8 = 4 > ... Again, on my machine, I didn’t see EINVAL, and no fallback to non-inherit. In my test, glc_get_event_constraints() successfully forces this event (config == 0x8203) to fixed counter 0, so there’s no issue here. > And it works fine on my machine. > > $ perf record -e '{cpu/mem-loads-aux/S,cpu/mem-loads/PS}' ls > ... > [ perf record: Woken up 1 times to write data ] > [ perf record: Captured and wrote 0.033 MB perf.data (6 samples) ] I don't know why it works for you, but in my tests, this event: Opening: cpu/mem-loads/PS ------------------------------------------------------------ perf_event_attr: type 4 (cpu) size 248 config 0x1cd (mem_trans_retired.load_latency_gt_1024) { sample_period, sample_freq } 4000 sample_type IP|TID|TIME|READ|ID|PERIOD read_format ID|GROUP|LOST inherit 1 freq 1 precise_ip 3 sample_id_all 1 { bp_addr, config1 } 0x3 ------------------------------------------------------------ It gets emptyconstraint, then it can't schedule the event on any counter and x86_schedule_events() returns -EINVAL. glc_get_event_constraints() { struct event_constraint *c; // It gets the constraint INTEL_PLD_CONSTRAINT(0x1cd, 0xfe) // from intel_pebs_constraints(), c = icl_get_event_constraints(cpuc, idx, event); // When it tries to force :ppp event to fixed counter 0 if ((event->attr.precise_ip == 3) && !constraint_match(&fixed0_constraint, event->hw.config)) { // It happens the constrain doesn't mask fixed counter 0 if (c->idxmsk64 & BIT_ULL(0)) { return &counter0_constraint; // It gets here. return &emptyconstraint; } return c; } After that, it falls back to non-inherit, and it fails again because the inherit attribute differs from the group leader’s. This carries over to the precise_ip fallback path in the current code. >> >> The second event fails with -EINVAL because, on some platforms, events >> with precise_ip = 3 must be scheduled on fixed counter 0, and it fails >> if it happens that this counter is unavailable. >> >> In the current code, the first fallback attempt (inherit = 0) also fails >> because the inherit attribute differs from that of the group leader >> (first event). > > So I don't understand this. Either the first event failed due to > inherit set or the second event should succeed with inherit. Maybe > there's an unknown bug or something. > > Thanks, > namhyung >