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Sun, 2 Nov 2025 07:12:27 GMT Received: from smtpav03.dal12v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 70A4958060; Sun, 2 Nov 2025 07:12:27 +0000 (GMT) Received: from smtpav03.dal12v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 62F2B58056; Sun, 2 Nov 2025 07:12:20 +0000 (GMT) Received: from [9.43.108.135] (unknown [9.43.108.135]) by smtpav03.dal12v.mail.ibm.com (Postfix) with ESMTP; Sun, 2 Nov 2025 07:12:19 +0000 (GMT) Message-ID: <42541486-45ef-484a-a409-779dcb8f7b29@linux.ibm.com> Date: Sun, 2 Nov 2025 12:42:18 +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 v2] sched/fair: Prefer cache locality for EAS wakeup To: Shubhang Kaushik Cc: Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , Shubhang Kaushik , Shijie Huang , Frank Wang , Christopher Lameter , Adam Li , linux-kernel@vger.kernel.org, Madadi Vineeth Reddy References: <20251030-b4-follow-up-v2-1-19a23c83b837@os.amperecomputing.com> Content-Language: en-US From: Madadi Vineeth Reddy In-Reply-To: <20251030-b4-follow-up-v2-1-19a23c83b837@os.amperecomputing.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: 7aXy-A3cuKJQdRxI9C0qPCE1-lE-ORs3 X-Proofpoint-GUID: 9DWFgudUm5O2ZYNNMk8Yf52OTnPtVIgz X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUxMTAxMDAyMSBTYWx0ZWRfX4VR2BEmxzx8+ BdHbg721VejtgbU/3zoJCv2Vlj3JY/bFbmSDloh6ikxwEBY9pqtoeX+1Q+o683UmKOtICMjcivX rf9q+o+/kaut/XoeIM38rszRIxhxDY9DXkw6uhn4NarTT0//Qbekwb9+08+AhXcXmfC4VfkJrNW 9m8ZJJTvRYZFJHo0xr1efvjF9h8TytPEFRVtag8GtIU3OKMVbS7xRr5qrupI+cgKfflgVxUZsKn /m510/4zIvAjMcf+2tutO3MoHHsK61x5BDPQr0OPxuFTjuYDh1ip+KkJvHPyTCtgsLbv8wzJI/Z Q3IZlUNzAEkjahLMzhnCOP+jy1tM7GHqexJ106Gc6NZqBQFOZashxc462mPp5fAPq/nxRWH21GG 1lsi97aHswKxMzYuDbibkw8A9pvtew== X-Authority-Analysis: v=2.4 cv=U6qfzOru c=1 sm=1 tr=0 ts=6907045e cx=c_pps a=5BHTudwdYE3Te8bg5FgnPg==:117 a=5BHTudwdYE3Te8bg5FgnPg==:17 a=IkcTkHD0fZMA:10 a=6UeiqGixMTsA:10 a=VkNPw1HP01LnGYTKEx00:22 a=VwQbUJbxAAAA:8 a=vzhER2c_AAAA:8 a=GMmcQ8BEhZfpoHoJ1HUA:9 a=QEXdDO2ut3YA:10 a=0YTRHmU2iG2pZC6F1fw2:22 a=cPQSjfK2_nFv0Q5t_7PE:22 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=2025-11-02_02,2025-10-29_03,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 bulkscore=0 adultscore=0 impostorscore=0 spamscore=0 phishscore=0 clxscore=1011 malwarescore=0 lowpriorityscore=0 suspectscore=0 priorityscore=1501 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.19.0-2510240000 definitions=main-2511010021 Hi Shubhang, On 31/10/25 00:49, Shubhang Kaushik wrote: > When Energy Aware Scheduling (EAS) is enabled, a task waking up on a > sibling CPU might migrate away from its previous CPU even if that CPU > is not overutilized. This sacrifices cache locality and introduces > unnecessary migration overhead. > > This patch refines the wakeup heuristic in `select_idle_sibling()`. If > EAS is active and the task's previous CPU (`prev`) is not overutilized, > the scheduler will prioritize waking the task on `prev`, avoiding an > unneeded migration and preserving cache-hotness. > > --- > v2: > - Addressed reviewer comments to handle this special condition > within the selection logic, prioritizing the > previous CPU if not overutilized for EAS. > - Link to v1: https://lore.kernel.org/all/20251017-b4-sched-cfs-refactor-propagate-v1-1-1eb0dc5b19b3@os.amperecomputing.com/ > > Signed-off-by: Shubhang Kaushik > --- > kernel/sched/fair.c | 12 +++++++++--- > 1 file changed, 9 insertions(+), 3 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 25970dbbb27959bc130d288d5f80677f75f8db8b..ac94463627778f09522fb5420f67b903a694ad4d 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -7847,9 +7847,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target) > asym_fits_cpu(task_util, util_min, util_max, target)) > return target; > > - /* > - * If the previous CPU is cache affine and idle, don't be stupid: > - */ > + /* Reschedule on an idle, cache-sharing sibling to preserve affinity: */ > if (prev != target && cpus_share_cache(prev, target) && > (available_idle_cpu(prev) || sched_idle_cpu(prev)) && > asym_fits_cpu(task_util, util_min, util_max, prev)) { > @@ -7861,6 +7859,14 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target) > prev_aff = prev; > } > > + /* > + * If the previous CPU is not overutilized, prefer it for cache locality. > + * This prevents migration away from a cache-hot CPU that can still > + * handle the task without causing an overload. > + */ > + if (sched_energy_enabled() && !cpu_overutilized(prev)) > + return prev; > + The above !cpu_overutilized(prev) is placed before recent_used_cpu idle check. This means if prev is busy (but not overutilized) and recent_used_cpu is completely idle, the task returns to prev and misses the idle opportunity. Is cache locality prioritized even over idle CPU availability? Are there measurements showing this trade-off is worthwhile for real workloads? Thank you, Madadi Vineeth Reddy > /* > * Allow a per-cpu kthread to stack with the wakee if the > * kworker thread and the tasks previous CPUs are the same. > > --- > base-commit: e53642b87a4f4b03a8d7e5f8507fc3cd0c595ea6 > change-id: 20251030-b4-follow-up-ff03b4533a2d > > Best regards,