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Thu, 13 Nov 2025 08:27:06 +0000 (GMT) Message-ID: <2841a37f-a6d2-44dc-9578-b40804e960d0@linux.ibm.com> Date: Thu, 13 Nov 2025 13:56:13 +0530 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 2/2] sched/fair: Reimplement NEXT_BUDDY to align with EEVDF goals To: Peter Zijlstra Cc: Mel Gorman , Ingo Molnar , Juri Lelli , Dietmar Eggemann , Valentin Schneider , Chris Mason , linux-kernel@vger.kernel.org, Madadi Vineeth Reddy References: <20251112122521.1331238-1-mgorman@techsingularity.net> <20251112122521.1331238-3-mgorman@techsingularity.net> <20251112144823.GE3245006@noisy.programming.kicks-ass.net> Content-Language: en-US From: Madadi Vineeth Reddy In-Reply-To: <20251112144823.GE3245006@noisy.programming.kicks-ass.net> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: DhursG3foKX_uf7bIRw6YdPsaEK98Gwd X-Proofpoint-ORIG-GUID: DhursG3foKX_uf7bIRw6YdPsaEK98Gwd X-Authority-Analysis: v=2.4 cv=VMPQXtPX c=1 sm=1 tr=0 ts=6915965e cx=c_pps a=aDMHemPKRhS1OARIsFnwRA==:117 a=aDMHemPKRhS1OARIsFnwRA==:17 a=IkcTkHD0fZMA:10 a=6UeiqGixMTsA:10 a=VkNPw1HP01LnGYTKEx00:22 a=12gGZJXDivS0f4jQg_wA:9 a=QEXdDO2ut3YA:10 a=cPQSjfK2_nFv0Q5t_7PE:22 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUxMTA4MDAyMiBTYWx0ZWRfX6Ov48NrvoCe9 jC1pppHDrxSrUGnG9DPwQkiIIU2qVKkrn+I96rd3bB5koQWErxdXV9RbFNAIB9GmPuBOve/6pDx Xha9O6UMfMFCjgm0KVrSB19cKGUZ51moN/lDoALTBupN/nh3oOprZax5PJb5FL5k3bm9dXqzojI xe9jINGYsXhHf1HCIh87VxOrVsbJRHPbv8g7ciQT1372jW1TGcl0pkG3jvzX37C0eZ/ftfHTVBm Ly8lHY0I5Be3CrVsQwkiRP6G5ZilpvsnRV83aWGVUwS5VFldCy0N84E0jbUwecvAvNw9CDQ588L poxn0/OiTShEmAwi8n9GLApOLcVjgTE6BZcpBR8KvNx0Vz310S+VEIU8L9fTVqBvgnwk5HJJ1o1 n2gmNhJLx2cufQKpiBCzspyKFPScMw== X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1121,Hydra:6.1.9,FMLib:17.12.100.49 definitions=2025-11-12_06,2025-11-12_01,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 impostorscore=0 priorityscore=1501 lowpriorityscore=0 bulkscore=0 clxscore=1015 phishscore=0 spamscore=0 malwarescore=0 adultscore=0 suspectscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.19.0-2510240000 definitions=main-2511080022 Hi Peter, On 12/11/25 20:18, Peter Zijlstra wrote: > On Wed, Nov 12, 2025 at 12:25:21PM +0000, Mel Gorman wrote: > >> + /* Prefer picking wakee soon if appropriate. */ >> + if (sched_feat(NEXT_BUDDY) && >> + set_preempt_buddy(cfs_rq, wake_flags, pse, se)) { >> + >> + /* >> + * Decide whether to obey WF_SYNC hint for a new buddy. Old >> + * buddies are ignored as they may not be relevant to the >> + * waker and less likely to be cache hot. >> + */ >> + if (wake_flags & WF_SYNC) >> + preempt_action = preempt_sync(rq, wake_flags, pse, se); >> + } > > Why only do preempt_sync() when NEXT_BUDDY? Nothing there seems to > depend on buddies. IIUC, calling preempt_sync() Without NEXT_BUDDY would force a reschedule after the threshold, but no buddy would be set. This means pick_eevdf() would run with normal EEVDF deadline selection, potentially picking a different task instead of the wakee.The forced reschedule would accomplish nothing for the wakees. That said, I see your point that the WF_SYNC threshold check could still reduce context switch overhead even without guaranteeing wakee selection. Thanks, Madadi Vineeth Reddy