From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 9246E2517BA; Wed, 5 Mar 2025 18:11:31 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1741198291; cv=none; b=pMvBdN2EO+26Aal7gJiUTYB64aqvKMxp5zkC9dSL+ngM6d5Tx69XTTOhpEwDhQJr2c/s52pO73kInr6qzcwDWFHeUgaJM4a3iTFJXyypJCSsYIwzSMmRiM5+tFpptb0hTCRpgw+Okt+ygEQFrfHq/R/gmyOVyxuApAGFHFQW01k= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1741198291; c=relaxed/simple; bh=TT16lU57wRpHr9XwyhNKJhktQrPMOiIF2qrXaYW3NLo=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=MaXV1jwNykkzIm+PFOGKmr37/cSfxR4ciKTd8cK/RfiGT8W8cSWY1SZ8Bg5iFfONgzfu/YClch3VhTXGQyAvkvhun/8POwEY8h+1PxJZE6fXC8H/DQnCl52BBw2PCMc/gVyo/rj2ocr4jvJeCQVSwJPBF6ghBu8SXPNjC0sqw4k= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=KIgAsEr8; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="KIgAsEr8" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1984EC4CED1; Wed, 5 Mar 2025 18:11:30 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1741198291; bh=TT16lU57wRpHr9XwyhNKJhktQrPMOiIF2qrXaYW3NLo=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=KIgAsEr8wdoD6Lz97Avt+VztbMuC/+f5czEvTTXi34CD8xzqLlZbmWNip4ESa1A7Q nd5jP1OBMjlJSVYaQHfrUuqYXq/jL6NAGPnUmAQ6U2fN3O6zJYsrWIwgNU2YhBsjZi yL5NheVAjWWURgsFXkYdsvWJdECAe6LxKOh5Fm7g= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Tejun Heo , Pat Cody , Andrea Righi Subject: [PATCH 6.12 129/150] sched_ext: Fix pick_task_scx() picking non-queued tasks when its called without balance() Date: Wed, 5 Mar 2025 18:49:18 +0100 Message-ID: <20250305174508.998040345@linuxfoundation.org> X-Mailer: git-send-email 2.48.1 In-Reply-To: <20250305174503.801402104@linuxfoundation.org> References: <20250305174503.801402104@linuxfoundation.org> User-Agent: quilt/0.68 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.12-stable review patch. If anyone has any objections, please let me know. ------------------ From: Tejun Heo commit 8fef0a3b17bb258130a4fcbcb5addf94b25e9ec5 upstream. a6250aa251ea ("sched_ext: Handle cases where pick_task_scx() is called without preceding balance_scx()") added a workaround to handle the cases where pick_task_scx() is called without prececing balance_scx() which is due to a fair class bug where pick_taks_fair() may return NULL after a true return from balance_fair(). The workaround detects when pick_task_scx() is called without preceding balance_scx() and emulates SCX_RQ_BAL_KEEP and triggers kicking to avoid stalling. Unfortunately, the workaround code was testing whether @prev was on SCX to decide whether to keep the task running. This is incorrect as the task may be on SCX but no longer runnable. This could lead to a non-runnable task to be returned from pick_task_scx() which cause interesting confusions and failures. e.g. A common failure mode is the task ending up with (!on_rq && on_cpu) state which can cause potential wakers to busy loop, which can easily lead to deadlocks. Fix it by testing whether @prev has SCX_TASK_QUEUED set. This makes @prev_on_scx only used in one place. Open code the usage and improve the comment while at it. Signed-off-by: Tejun Heo Reported-by: Pat Cody Fixes: a6250aa251ea ("sched_ext: Handle cases where pick_task_scx() is called without preceding balance_scx()") Cc: stable@vger.kernel.org # v6.12+ Acked-by: Andrea Righi Signed-off-by: Greg Kroah-Hartman --- kernel/sched/ext.c | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-) --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -3047,7 +3047,6 @@ static struct task_struct *pick_task_scx { struct task_struct *prev = rq->curr; struct task_struct *p; - bool prev_on_scx = prev->sched_class == &ext_sched_class; bool keep_prev = rq->scx.flags & SCX_RQ_BAL_KEEP; bool kick_idle = false; @@ -3067,14 +3066,18 @@ static struct task_struct *pick_task_scx * if pick_task_scx() is called without preceding balance_scx(). */ if (unlikely(rq->scx.flags & SCX_RQ_BAL_PENDING)) { - if (prev_on_scx) { + if (prev->scx.flags & SCX_TASK_QUEUED) { keep_prev = true; } else { keep_prev = false; kick_idle = true; } - } else if (unlikely(keep_prev && !prev_on_scx)) { - /* only allowed during transitions */ + } else if (unlikely(keep_prev && + prev->sched_class != &ext_sched_class)) { + /* + * Can happen while enabling as SCX_RQ_BAL_PENDING assertion is + * conditional on scx_enabled() and may have been skipped. + */ WARN_ON_ONCE(scx_ops_enable_state() == SCX_OPS_ENABLED); keep_prev = false; }