From nobody Wed Dec 31 18:57:58 2025 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 143E8C4332F for ; Mon, 30 Oct 2023 14:52:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233550AbjJ3Owe (ORCPT ); Mon, 30 Oct 2023 10:52:34 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48772 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232542AbjJ3Owb (ORCPT ); Mon, 30 Oct 2023 10:52:31 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 83826C2 for ; Mon, 30 Oct 2023 07:51:43 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1698677502; 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=iZM5Ir1agMVPmsp/0h/z8SE5MvN1kGhnLqtdaxL5v4U=; b=UrNZkCfc99nrG6JWraMNbIgCnD2ZOfrDTgdal9GkGb7lbEqCs/qGmARn9osFqTfNcCfoaz W8OINYUvt20JW9JlkuuMmUnGPOwn5ZL6BH8+A9jRrGKdgkMV7YEfmqmqJVMlQdNnSiZ6eU DKPUcJZYgtY9iHojQzHSGLXYs80cuhg= Received: from mimecast-mx02.redhat.com (mx-ext.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-42-ZvuMTU3_O-aBrCNMZjNOXQ-1; Mon, 30 Oct 2023 10:51:32 -0400 X-MC-Unique: ZvuMTU3_O-aBrCNMZjNOXQ-1 Received: from smtp.corp.redhat.com (int-mx09.intmail.prod.int.rdu2.redhat.com [10.11.54.9]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 61BD61C01B2C; Mon, 30 Oct 2023 14:51:31 +0000 (UTC) Received: from vschneid-thinkpadt14sgen2i.remote.csb (unknown [10.39.194.198]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 0EA81492BE0; Mon, 30 Oct 2023 14:51:28 +0000 (UTC) From: Valentin Schneider To: linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org Cc: Daniel Bristot de Oliveira , Dietmar Eggemann , Ingo Molnar , Juri Lelli , Peter Zijlstra , Steven Rostedt , Vincent Guittot , Thomas Gleixner , Sebastian Andrzej Siewior , Tomas Glozar Subject: [PATCH] sched/fair: Make the BW replenish timer expire in hardirq context for PREEMPT_RT Date: Mon, 30 Oct 2023 15:51:04 +0100 Message-ID: <20231030145104.4107573-1-vschneid@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.9 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" Consider the following scenario under PREEMPT_RT: o A CFS task p0 gets throttled while holding read_lock(&lock) o A task p1 blocks on write_lock(&lock), making further readers enter the slowpath o A ktimers or ksoftirqd task blocks on read_lock(&lock) If the cfs_bandwidth.period_timer to replenish p0's runtime is enqueued on the same CPU as one where ktimers/ksoftirqd is blocked on read_lock(&lock), this creates a circular dependency. This has been observed to happen with: o fs/eventpoll.c::ep->lock o net/netlink/af_netlink.c::nl_table_lock (after hand-fixing the above) but can trigger with any rwlock that can be acquired in both process and softirq contexts. The linux-rt tree has had 1ea50f9636f0 ("softirq: Use a dedicated thread for timer wakeups.") which helped this scenario for non-rwlock locks by ensuring the throttled task would get PI'd to FIFO1 (ktimers' default priority). Unfortunately, rwlocks cannot sanely do PI as they allow multiple readers. Make the period_timer expire in hardirq context under PREEMPT_RT. The callback for this timer can end up doing a lot of work, but this is mitigated somewhat when using nohz_full / CPU isolation: the timers *are* pinned, but on the CPUs the taskgroups are created on, which is usually going to be HK CPUs. Link: https://lore.kernel.org/all/xhsmhttqvnall.mognet@vschneid.remote.csb/ Signed-off-by: Valentin Schneider --- kernel/sched/fair.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 8767988242ee3..15cf7de865a97 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6236,7 +6236,7 @@ void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b, = struct cfs_bandwidth *paren cfs_b->hierarchical_quota =3D parent ? parent->hierarchical_quota : RUNTI= ME_INF; =20 INIT_LIST_HEAD(&cfs_b->throttled_cfs_rq); - hrtimer_init(&cfs_b->period_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_PINN= ED); + hrtimer_init(&cfs_b->period_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_PINN= ED_HARD); cfs_b->period_timer.function =3D sched_cfs_period_timer; =20 /* Add a random offset so that timers interleave */ @@ -6263,7 +6263,7 @@ void start_cfs_bandwidth(struct cfs_bandwidth *cfs_b) =20 cfs_b->period_active =3D 1; hrtimer_forward_now(&cfs_b->period_timer, cfs_b->period); - hrtimer_start_expires(&cfs_b->period_timer, HRTIMER_MODE_ABS_PINNED); + hrtimer_start_expires(&cfs_b->period_timer, HRTIMER_MODE_ABS_PINNED_HARD); } =20 static void destroy_cfs_bandwidth(struct cfs_bandwidth *cfs_b) --=20 2.41.0