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Tue, 29 Dec 2020 17:14:12 +0000 From: Song Liu To: Arnaldo Carvalho de Melo CC: Namhyung Kim , linux-kernel , Peter Zijlstra , "Ingo Molnar" , Alexander Shishkin , Mark Rutland , Jiri Olsa , Kernel Team Subject: Re: [PATCH v6 2/4] perf: support build BPF skeletons with perf Thread-Topic: [PATCH v6 2/4] perf: support build BPF skeletons with perf Thread-Index: AQHW3UCte2IZwxEAGUuEynMQFyV2oaoNpmmAgABQIQCAAFsBgA== Date: Tue, 29 Dec 2020 17:14:12 +0000 Message-ID: <7219C2C5-48F1-44A3-9787-FBD39CC58514@fb.com> References: <20201228174054.907740-1-songliubraving@fb.com> <20201228174054.907740-3-songliubraving@fb.com> <20201229114828.GG521329@kernel.org> In-Reply-To: <20201229114828.GG521329@kernel.org> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-mailer: Apple Mail (2.3608.120.23.2.4) authentication-results: kernel.org; dkim=none (message not signed) header.d=none;kernel.org; dmarc=none action=none header.from=fb.com; x-originating-ip: [2620:10d:c090:400::5:7414] x-ms-publictraffictype: Email x-ms-office365-filtering-correlation-id: 9f8c7110-d63f-444c-ec22-08d8ac1d29c4 x-ms-traffictypediagnostic: BYAPR15MB2455: x-ms-exchange-transport-forked: True x-microsoft-antispam-prvs: x-fb-source: Internal x-ms-oob-tlc-oobclassifiers: OLM:8273; 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charset="us-ascii" Content-ID: <01BCDEC001CCB1428290699B1D4AE548@namprd15.prod.outlook.com> Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-AuthSource: BYAPR15MB2999.namprd15.prod.outlook.com X-MS-Exchange-CrossTenant-Network-Message-Id: 9f8c7110-d63f-444c-ec22-08d8ac1d29c4 X-MS-Exchange-CrossTenant-originalarrivaltime: 29 Dec 2020 17:14:12.6076 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 8ae927fe-1255-47a7-a2af-5f3a069daaa2 X-MS-Exchange-CrossTenant-mailboxtype: HOSTED X-MS-Exchange-CrossTenant-userprincipalname: 5IiVS7S/xuJleEOCbXmyIEnqbY4pYVQBj30L6bhKn4bEDerPEAoqxtRP8xneQRCBNccXPneP7Ixw1PFdbffI0A== X-MS-Exchange-Transport-CrossTenantHeadersStamped: BYAPR15MB2455 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.343,18.0.737 definitions=2020-12-29_13:2020-12-28,2020-12-29 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 priorityscore=1501 malwarescore=0 mlxlogscore=999 bulkscore=0 phishscore=0 lowpriorityscore=0 mlxscore=0 spamscore=0 adultscore=0 clxscore=1015 impostorscore=0 suspectscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2012290109 X-FB-Internal: deliver Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > On Dec 29, 2020, at 3:48 AM, Arnaldo Carvalho de Melo w= rote: >=20 > Em Tue, Dec 29, 2020 at 04:01:41PM +0900, Namhyung Kim escreveu: >> On Tue, Dec 29, 2020 at 2:41 AM Song Liu wrote: >>> BPF programs are useful in perf to profile BPF programs. BPF skeleton i= s >=20 >> I'm having difficulties understanding the first sentence - looks like a >> recursion. :) So do you want to use two (or more) BPF programs? >=20 > Yeah, we use perf to perf perf, so we need to use bpf with perf to perf > bpf :-) >=20 > Look at tools/perf/util/bpf_skel/bpf_prog_profiler.bpf.c, the BPF > skeleton used to create the in-kernel scaffold to profile BPF programs. >=20 > It uses two BPF programs (fentry/XXX and fexit/XXX) and some a > PERF_EVENT_ARRAY map and an array to diff counters read at exit from > counters read at exit of the profiled BPF programs and then accumulate > those diffs in another PERCPU_ARRAY. >=20 > This all ends up composing a "BPF PMU" that is what the userspace perf > tooling will read (from "accum_readings" BPF map) and 'perf stat' will > consume as if reading from an "old style perf counter" :-) >=20 > Song, did I get it right? :-) Thanks Arnaldo! I don't think anyone can explain it better. :-) Song [...]