From nobody Thu Nov 28 12:50:02 2024 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 535328F40; Wed, 2 Oct 2024 14:57:50 +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=1727881070; cv=none; b=T8WY5LmtG2Uxpjue1jhemzIMygQZwR+whCEdH3yLiellB2Q3T7JkxJl9QLVF6ejDZ4X/j0ltDV0bvwqBDdk1P1y0GHAy3Hhj7gtqIgdWnjy+izDOyni6n3D1SYK41AronOob83IwZGmdzh6qRJUvet3IJN20yMuKVcB+cuNBlt0= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1727881070; c=relaxed/simple; bh=SYJkoM5PGYwL725ftsKaTc8bvBhYpRQfbSnmPZHax3c=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=jxI3jcGKmzVdFsLMq3PMVeA/gbr+o/kiHHk2Jxxnb4AyJFx104wYTWzGdE5WROGC3uUHPoN0rIO1CGfgM7lvQ0AgOyMiQ1xtBdEH/yr6cWfDIoE7vkK493AVPaIucDd326JVQAHHOsPr+4fnS3r2ZT7swb/YCwNxuZ9aJkooC1A= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=awdZS7b1; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="awdZS7b1" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0BF31C4CECE; Wed, 2 Oct 2024 14:57:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1727881070; bh=SYJkoM5PGYwL725ftsKaTc8bvBhYpRQfbSnmPZHax3c=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=awdZS7b1nsHiDnErnZfcDNy/1vYhTsJaUk57GzK/w+4XLqRQLgV8tY9KoOHRitF2l EQ3x6edRBguRS8DZqE6jh0vwI6bI391QZZlQ7WocZptxfGhQcnguyErkNt0jLADrSG zqXTN54x9n+WgGyuiWjfN1EWRy/FrOSGAQhOE2mjRLk2kJSzzyqQQLWKpbGaeciv+S jwTbRR/l4DCelLOWk+xRazfYpv1Gxw4O/PLUk9MnTIRGERNZz3I5ZCPdYhNLo+G8/Q tZoYesd75TzH2EDUoHZaqjqPEEE1niDla8mRXUoAV//0iKeg84lmfja/33SHtiEBgN gylgITOO4nJHQ== From: Frederic Weisbecker To: LKML Cc: Frederic Weisbecker , Boqun Feng , Joel Fernandes , Neeraj Upadhyay , "Paul E . McKenney" , Uladzislau Rezki , Zqiang , rcu , kernel test robot Subject: [PATCH 2/3] rcu/nocb: Fix rcuog wake-up from offline softirq Date: Wed, 2 Oct 2024 16:57:37 +0200 Message-ID: <20241002145738.38226-3-frederic@kernel.org> X-Mailer: git-send-email 2.46.0 In-Reply-To: <20241002145738.38226-1-frederic@kernel.org> References: <20241002145738.38226-1-frederic@kernel.org> 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 Content-Type: text/plain; charset="utf-8" After a CPU has set itself offline and before it eventually calls rcutree_report_cpu_dead(), there are still opportunities for callbacks to be enqueued, for example from an IRQ. When that happens on NOCB, the rcuog wake-up is deferred through an IPI to an online CPU in order not to call into the scheduler and risk arming the RT-bandwidth after hrtimers have been migrated out and disabled. But performing a synchronized IPI from an IRQ is buggy as reported in the following scenario: WARNING: CPU: 1 PID: 26 at kernel/smp.c:633 smp_call_function_single Modules linked in: rcutorture torture CPU: 1 UID: 0 PID: 26 Comm: migration/1 Not tainted 6.11.0-rc1-00012-g9139= f93209d1 #1 Stopper: multi_cpu_stop+0x0/0x320 <- __stop_cpus+0xd0/0x120 RIP: 0010:smp_call_function_single swake_up_one_online __call_rcu_nocb_wake __call_rcu_common ? rcu_torture_one_read call_timer_fn __run_timers run_timer_softirq handle_softirqs irq_exit_rcu ? tick_handle_periodic sysvec_apic_timer_interrupt The periodic tick must be shutdown when the CPU is offline, just like is done for oneshot tick. This must be fixed but this is not enough: softirqs can happen on any hardirq tail and reproduce the above scenario. Fix this with introducing a special deferred rcuog wake up mode when the CPU is offline. This deferred wake up doesn't arm any timer and simply wait for rcu_report_cpu_dead() to be called in order to flush any pending rcuog wake up. Reported-by: kernel test robot Closes: https://lore.kernel.org/oe-lkp/202409231644.4c55582d-lkp@intel.com Fixes: 9139f93209d1 ("rcu/nocb: Fix RT throttling hrtimer armed from offlin= e CPU") Signed-off-by: Frederic Weisbecker Reviewed-by: Joel Fernandes (Google) --- kernel/rcu/tree.h | 1 + kernel/rcu/tree_nocb.h | 14 ++++++++++++-- 2 files changed, 13 insertions(+), 2 deletions(-) diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h index a9a811d9d7a3..7ed060edd12b 100644 --- a/kernel/rcu/tree.h +++ b/kernel/rcu/tree.h @@ -290,6 +290,7 @@ struct rcu_data { #define RCU_NOCB_WAKE_LAZY 2 #define RCU_NOCB_WAKE 3 #define RCU_NOCB_WAKE_FORCE 4 +#define RCU_NOCB_WAKE_OFFLINE 5 =20 #define RCU_JIFFIES_TILL_FORCE_QS (1 + (HZ > 250) + (HZ > 500)) /* For jiffies_till_first_fqs and */ diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h index 2fb803f863da..8648233e1717 100644 --- a/kernel/rcu/tree_nocb.h +++ b/kernel/rcu/tree_nocb.h @@ -295,6 +295,8 @@ static void wake_nocb_gp_defer(struct rcu_data *rdp, in= t waketype, case RCU_NOCB_WAKE_FORCE: if (rdp_gp->nocb_defer_wakeup < RCU_NOCB_WAKE) mod_timer(&rdp_gp->nocb_timer, jiffies + 1); + fallthrough; + case RCU_NOCB_WAKE_OFFLINE: if (rdp_gp->nocb_defer_wakeup < waketype) WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype); break; @@ -562,8 +564,16 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp,= bool was_alldone, lazy_len =3D READ_ONCE(rdp->lazy_len); if (was_alldone) { rdp->qlen_last_fqs_check =3D len; - // Only lazy CBs in bypass list - if (lazy_len && bypass_len =3D=3D lazy_len) { + if (cpu_is_offline(rdp->cpu)) { + /* + * Offline CPUs can't call swake_up_one_online() from IRQs. Rely + * on the final deferred wake-up rcutree_report_cpu_dead() + */ + rcu_nocb_unlock(rdp); + wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_OFFLINE, + TPS("WakeEmptyIsDeferredOffline")); + } else if (lazy_len && bypass_len =3D=3D lazy_len) { + // Only lazy CBs in bypass list rcu_nocb_unlock(rdp); wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_LAZY, TPS("WakeLazy")); --=20 2.46.0