From nobody Wed Nov 27 00:42:50 2024 Received: from out-182.mta0.migadu.com (out-182.mta0.migadu.com [91.218.175.182]) (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 22D171D0140 for ; Mon, 14 Oct 2024 22:06:09 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.182 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1728943572; cv=none; b=SWG4JUH1ABQl3Yfzuit7xpnYumKYiGBx0BCrNsuS9vq1ObnBi4LL3J/R93m4C6yavIm6+2YNTSxzHLQxKCK92PZjyGraoXXm4JFSc/yFttezS0apV5TMzANQGzp8humYQNmhDu/jFpyX+tOv/7smZn4gkXNbmb/Xb4mllNY1tFk= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1728943572; c=relaxed/simple; bh=5qTbAz9HdW7boogj3+5WSD1OI0ME1rdWeb8QLX1Yb74=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=lpwRJWDiyWus9eR3dTrAUmEEp8OVDZM/Ye8Dpv/6+K6R5ayP0uTvYGuicj+sDa8mysLv6aOBkiQAWWM4JWUoejtfq8tqtLPcsu3F3Bz8suWWm7HmSul/UKM4Z9f8PXc7fMTymo7z5aALZYLE2pL0r/rHL0tDfczUfFsp7OtMGDg= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=aa6rcK+U; arc=none smtp.client-ip=91.218.175.182 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="aa6rcK+U" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1728943568; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=6Eg2qUencFOdUUjQJy4th10S6VBboP5vdfVxJHTHFD0=; b=aa6rcK+U1YP9YjIK5G4PEjvZs7EnQMeWXMiS4sqxEEPElCLAwmxVnRT2J5DfqsrnpBGILb DuPHa6/zzLIFJga72ibL5HIdDlE4qOqM4JvYN10f70G25eVVw6mX5oOhUD9ipR+N+yeDpB 9l4pbldTC+kBRHc7Of7R7jZ2W1ChPoQ= From: Andrea Righi To: Tejun Heo , David Vernet , Peter Zijlstra , Ingo Molnar , Juri Lelli , Vincent Guittot Cc: Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , linux-kernel@vger.kernel.org, bpf@vger.kernel.org Subject: [PATCH v2] sched_ext: Trigger ops.update_idle() from pick_task_idle() Date: Tue, 15 Oct 2024 00:06:03 +0200 Message-ID: <20241014220603.35280-1-andrea.righi@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Migadu-Flow: FLOW_OUT Content-Type: text/plain; charset="utf-8" With the consolidation of put_prev_task/set_next_task(), see commit 436f3eed5c69 ("sched: Combine the last put_prev_task() and the first set_next_task()"), we are now skipping the transition between these two functions when the previous and the next tasks are the same. As a result, ops.update_idle() is now called only once when the CPU transitions to the idle class. If the CPU stays active (e.g., through a call to scx_bpf_kick_cpu()), ops.update_idle() will not be triggered again since the task remains unchanged (rq->idle). While this behavior seems generally correct, it can cause issues in certain sched_ext scenarios. For example, a BPF scheduler might use logic like the following to keep the CPU active under specific conditions: void BPF_STRUCT_OPS(sched_update_idle, s32 cpu, bool idle) { if (!idle) return; if (condition) scx_bpf_kick_cpu(cpu, 0); } A call to scx_bpf_kick_cpu() wakes up the CPU, so in theory, ops.update_idle() should be triggered again until the condition becomes false. However, this doesn't happen, and scx_bpf_kick_cpu() doesn't produce the expected effect. In practice, this change badly impacts performance in user-space schedulers that rely on ops.update_idle() to activate user-space components. For instance, in the case of scx_rustland, performance drops significantly (e.g., gaming benchmarks fall from ~60fps to ~10fps). To address this, trigger ops.update_idle() from pick_task_idle() rather than set_next_task_idle(). This restores the correct behavior of ops.update_idle() and it allows to fix the performance regression in scx_rustland. Fixes: 7c65ae81ea86 ("sched_ext: Don't call put_prev_task_scx() before pick= ing the next task") Signed-off-by: Andrea Righi --- kernel/sched/idle.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) ChangeLog v1 -> v2: - move the logic from put_prev_set_next_task() to scx_update_idle() diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c index d2f096bb274c..5a10cbc7e9df 100644 --- a/kernel/sched/idle.c +++ b/kernel/sched/idle.c @@ -459,13 +459,13 @@ static void put_prev_task_idle(struct rq *rq, struct = task_struct *prev, struct t static void set_next_task_idle(struct rq *rq, struct task_struct *next, bo= ol first) { update_idle_core(rq); - scx_update_idle(rq, true); schedstat_inc(rq->sched_goidle); next->se.exec_start =3D rq_clock_task(rq); } =20 struct task_struct *pick_task_idle(struct rq *rq) { + scx_update_idle(rq, true); return rq->idle; } =20 --=20 2.47.0