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charset="us-ascii" Content-ID: Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Exchange-CrossTenant-Network-Message-Id: b260ee35-5329-45a0-e5e1-08d77a193921 X-MS-Exchange-CrossTenant-originalarrivaltime: 06 Dec 2019 06:55:05.1016 (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: tLEnHYq4Wf9DoND6x6Hh7yyPv5Uk8RS6razaX0EDReNSQn+WKyrTDmUNt8cA6cx+ X-MS-Exchange-Transport-CrossTenantHeadersStamped: MN2PR15MB2605 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.95,18.0.572 definitions=2019-12-06_01:2019-12-04,2019-12-06 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 adultscore=0 mlxlogscore=900 spamscore=0 mlxscore=0 phishscore=0 clxscore=1011 bulkscore=0 impostorscore=0 priorityscore=1501 suspectscore=0 lowpriorityscore=0 malwarescore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-1910280000 definitions=main-1912060059 X-FB-Internal: deliver Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Dec 05, 2019 at 04:12:26PM -0800, Daniel Xu wrote: > Last-branch-record is an intel CPU feature that can be configured to > record certain branches that are taken during code execution. This data > is particularly interesting for profile guided optimizations. perf has > had LBR support for a while but the data collection can be a bit coarse > grained. >=20 > We (Facebook) have recently run a lot of experiments with feeding > filtered LBR data to various PGO pipelines. We've seen really good > results (+2.5% throughput with lower cpu util and lower latency) by > feeding high request latency LBR branches to the compiler on a > request-oriented service. We used bpf to read a special request context > ID (which is how we associate branches with latency) from a fixed > userspace address. Reading from the fixed address is why bpf support is > useful. >=20 > Aside from this particular use case, having LBR data available to bpf > progs can be useful to get stack traces out of userspace applications > that omit frame pointers. >=20 > This patch adds support for LBR data to bpf perf progs. Please re-submit when bpf-next opens.