From nobody Wed Sep 30 13:38:13 2026 Received: from gentwo.org (gentwo.org [62.72.0.81]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2F9173546D7 for ; Fri, 7 Aug 2026 20:47:41 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=62.72.0.81 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786135662; cv=none; b=miZGWqqHxrUTy18aqGCiNH/BnLXkkXY4Py6RTAe8AWsJ5LhjwbC96lnB9w38LtOVGvr1F1OXTlZEfzQvaD12Zz4xdINwB+tWy26YnNWraEG1HtmOPkvq3+UbUcmALhTCyyjIiCDSFZIpc+jyJcIltETQyfF7iIjF+iwdIr4nJsY= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786135662; c=relaxed/simple; bh=mfEsBOrO7lr0isltJPkyHJ+4eMIcGVuo9esOZeDKJ68=; h=From:Date:Subject:MIME-Version:Content-Type:Message-Id:To:Cc; b=GkZotYV9FYRQ8Ob1aLEgFtiQpnAQzPhpdTf14wPjcPXNBu2LD64Cx4BJ/Bdx0rZh/hM8qWHOpa5YBGn6ClEyXkoQUTAovZXxXwieKDuxaADpYbJtE/QaqntLK97FQEU5AolnlxVQNIbTPsI2Vfvgn1eGAZK6qVxz6S6znCXloyw= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=gentwo.org; spf=pass smtp.mailfrom=gentwo.org; dkim=pass (1024-bit key) header.d=gentwo.org header.i=@gentwo.org header.b=gExIrWYd; arc=none smtp.client-ip=62.72.0.81 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=gentwo.org Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gentwo.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=gentwo.org header.i=@gentwo.org header.b="gExIrWYd" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=gentwo.org; s=default; t=1786135144; bh=mfEsBOrO7lr0isltJPkyHJ+4eMIcGVuo9esOZeDKJ68=; h=From:Date:Subject:To:Cc:From; b=gExIrWYdhbp65Z761K2fYR3ZO0DVYrplsDOiguBAwzwWgJlcrev4oIGnjHjZvgJFZ d3z4xNff8X7NAXG4hWMnkZ9+1PiQyizLvriMNwrzzsTw6VCSXiqZRPnntsW40sZrPm VntW58g3Hrn/XCcuWiadeSUKcKRaO6Nfbf2Lk2oY= Received: from sut02sys-r112.scc-lab.amperecomputing.com (localhost [127.0.0.1]) by gentwo.org (Postfix) with ESMTPS id 2A3AE40EBA; Fri, 07 Aug 2026 13:39:04 -0700 (PDT) From: "Shubhang Kaushik (Ampere)" Date: Fri, 07 Aug 2026 13:38:52 -0700 Subject: [PATCH v3] sched/core: Skip rq->avg_idle update without a valid idle_stamp Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Message-Id: <20260807-master-v3-1-c328354efed3@gentwo.org> X-B4-Tracking: v=1; b=H4sIAFtCdmoC/2WMyw7CIBBFf6WZtRgG7ANX/odxgTBtWVgMENQ0/ XdpFxrj8tzcc2aIFBxFOFYzBMouOj8VkLsKzKingZizhUFw0fBWdOymY6LA6trY1hiUQnEo53u g3j230PlSeHQx+fDauhnX9S+RkSHrVW3VleOhteI00JQefu/DAGsji6/X8ebjieIR9lKj5kZx+ eMty/IGsoRGitUAAAA= X-Change-ID: 20260728-master-55cd7cc13290 To: Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , K Prateek Nayak , John Stultz , Zhan Xusheng , Christopher Lameter , Shubhang Kaushik Cc: linux-kernel@vger.kernel.org, "Shubhang Kaushik (Ampere)" X-Mailer: b4 0.14.2 X-Developer-Signature: v=1; a=ed25519-sha256; t=1786135144; l=3608; i=sh@gentwo.org; s=20251010; h=from:subject:message-id; bh=mfEsBOrO7lr0isltJPkyHJ+4eMIcGVuo9esOZeDKJ68=; b=lteQ5SoGUnpLVNJqyKCrSF8AIG111IH7Giuiis/+ApaNlJPBj5ihnSEX/7kBdmWkVwTTwWseF uUIdwPqeyCxC9EAH8+AiJu3P4jXLvHooprog1Tuf2QdxjmJN3Px94NA X-Developer-Key: i=sh@gentwo.org; a=ed25519; pk=jc8YIRvxPSyJaBRe5y+a4N0RXKBUEcAh8+OFhlROXPY= Commit 4b603f1551a73 ("sched: Update rq->avg_idle when a task is moved to an idle CPU") moved rq->avg_idle accounting out of the wakeup path and into put_prev_task_idle(), so that the idle interval is consumed whenever the idle task is switched out. The wakeup-side accounting that it replaced only updated rq->avg_idle when rq->idle_stamp was non-zero. The new helper lost that validity check and unconditionally computes: rq_clock(rq) - rq->idle_stamp If rq->idle_stamp is zero, this uses rq_clock(rq) as the sample. That is not a valid idle duration and can immediately drive rq->avg_idle to its clamp. This can happen when sched_balance_newidle() returns before setting rq->idle_stamp, for example when this_rq->ttwu_pending is set. In that case the rq can switch to the idle task with idle_stamp still zero and leave idle again when the pending wakeup is processed. Other paths can also switch to the idle task without setting rq->idle_stamp via newidle_balance(), for example find_proxy_task() or force-idling. Restore the idle_stamp validity check in update_rq_avg_idle() and skip the rq->avg_idle update when there is no measured idle interval. Fixes: 4b603f1551a73 ("sched: Update rq->avg_idle when a task is moved to a= n idle CPU") Reviewed-by: K Prateek Nayak Acked-by: John Stultz Signed-off-by: Shubhang Kaushik (Ampere) Reviewed-by: Vincent Guittot Reviewed-by: Zhan Xusheng --- Temporary tracing under hackbench load confirmed that update_rq_avg_idle() can be reached with rq->idle_stamp =3D=3D 0. Hackbench showed no material regression versus v7.2-rc5 mainline. Related discussion: https://lore.kernel.org/r/20260423023322.1293923-1-firelzrd@gmail.com This is a narrower variant of the earlier proposal. It keeps the rq->idle_stamp guard in update_rq_avg_idle(), but intentionally does not stamp idle entry from set_next_task_idle(), preserving the existing newidle accounting model and avoiding force-idle/proxy-exec accounting concerns. --- Changes in v3: - Describe the sched_balance_newidle()/ttwu_pending path as an example of entering idle without a valid rq->idle_stamp. - Drop unlikely() from the idle_stamp check. - Add Acked-by from John Stultz. Link to v2: https://lore.kernel.org/r/20260806-master-v2-1-e1f3a1a0c903@gen= two.org Changes in v2: - Add Reviewed-by from Prateek. - Mention find_proxy_task() and force-idling as examples of paths that can switch to the idle task without a valid rq->idle_stamp. - Cc John Stultz. Link to v1: https://lore.kernel.org/r/20260728-master-v1-1-f95d9b0147d2@gen= two.org --- kernel/sched/core.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 96226707c2f6135341aa779b8262f113e103d8ad..68fa724cd668fb6c4cead329d05= fa95e2f1ea5db 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -3732,11 +3732,17 @@ static inline void ttwu_do_wakeup(struct task_struc= t *p) =20 void update_rq_avg_idle(struct rq *rq) { - u64 delta =3D rq_clock(rq) - rq->idle_stamp; - u64 max =3D 2*rq->max_idle_balance_cost; + u64 idle_stamp =3D rq->idle_stamp; + u64 delta, max; + + if (!idle_stamp) + return; + + delta =3D rq_clock(rq) - idle_stamp; =20 update_avg(&rq->avg_idle, delta); =20 + max =3D 2 * rq->max_idle_balance_cost; if (rq->avg_idle > max) rq->avg_idle =3D max; rq->idle_stamp =3D 0; --- base-commit: 3f008280327ba5ad132965abab0c7846283cef0c change-id: 20260728-master-55cd7cc13290 Best regards, --=20 Shubhang Kaushik (Ampere)