From mboxrd@z Thu Jan 1 00:00:00 1970 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 88659C001DC for ; Mon, 17 Jul 2023 18:36:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230495AbjGQSgL (ORCPT ); Mon, 17 Jul 2023 14:36:11 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37270 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229946AbjGQSgG (ORCPT ); Mon, 17 Jul 2023 14:36:06 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 9983599 for ; Mon, 17 Jul 2023 11:36:05 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 2BF3D611F3 for ; Mon, 17 Jul 2023 18:36:05 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 85CB2C433C8; Mon, 17 Jul 2023 18:36:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1689618964; bh=2Tt8ICMKfzWDkMea/gUwrBxO6s1VpyH+p9cl1SdgSpw=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=Qp668aFcRBGP8zLzY6GQvJLG8x7s3Cq1ojefAVvPI8CpHQePhJ6chLqgHnKFZawlM IGnvCX9z/cIBkB4dwzII7t5oPYyvRjSncG+YciQjojoTHrMOCXf3gRfWlMGXtlpjo2 6OUW5v1SiD4VIbM9kkzVYmzl2jSNqw2A9VtmvGiofypvvDz8TQhODBB+bYd0srS+oM JLz2ckx42b0+dcoq3UXJgvBJ0s6F3L8hGSaq1trPY+KBQWbZ5wjJEJ3oc2ziou5CxJ N5nIAoMtSBvuLW1QpoNrHMBIQq3IoZ5g31+4sjL57ym8XwqsSpBG9Fl1mP7rb2XwGm tr/FfnuYD3+Ig== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 328F6CE04CD; Mon, 17 Jul 2023 11:36:04 -0700 (PDT) From: "Paul E. McKenney" To: peterz@infradead.org, jgross@suse.com, vschneid@redhat.com, yury.norov@gmail.com Cc: linux-kernel@vger.kernel.org, imran.f.khan@oracle.com, kernel-team@meta.com, "Paul E . McKenney" Subject: [PATCH csd-lock 2/2] smp: Reduce NMI traffic from CSD waiters to CSD destination Date: Mon, 17 Jul 2023 11:36:02 -0700 Message-Id: <20230717183602.1099773-2-paulmck@kernel.org> X-Mailer: git-send-email 2.40.1 In-Reply-To: <96818440-a922-4b43-8871-50358e18b523@paulmck-laptop> References: <96818440-a922-4b43-8871-50358e18b523@paulmck-laptop> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Imran Khan On systems with hundreds of CPUs, if most of the CPUs detect a CSD hang, then all of these waiting CPUs send an NMI to the destination CPU in order to dump its backtrace. Given enough NMIs, the destination CPU will spent much of its time producing backtraces, thus further delaying that CPU's response to the original CSD IPI. In the worst case, by the time destination CPU is done producing all of these backtrace NMIs, the CSD wait timeout will have elapsed so that the waiters resend their backtrace NMIs again, further delaying forward progress. Therefore, to avoid these delays, issue the backtrace NMI only from the first waiter. The destination CPU's other waiters can make use of backtrace obtained from the first waiter's NMI. Signed-off-by: Imran Khan Cc: Peter Zijlstra Cc: Juergen Gross Cc: Valentin Schneider Cc: Yury Norov Signed-off-by: Paul E. McKenney --- kernel/smp.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) diff --git a/kernel/smp.c b/kernel/smp.c index 1d41a0cb54f1..8455a53465af 100644 --- a/kernel/smp.c +++ b/kernel/smp.c @@ -46,6 +46,8 @@ static DEFINE_PER_CPU_ALIGNED(struct call_function_data, cfd_data); static DEFINE_PER_CPU_SHARED_ALIGNED(struct llist_head, call_single_queue); +static DEFINE_PER_CPU(atomic_t, trigger_backtrace) = ATOMIC_INIT(1); + static void __flush_smp_call_function_queue(bool warn_cpu_offline); int smpcfd_prepare_cpu(unsigned int cpu) @@ -253,7 +255,8 @@ static bool csd_lock_wait_toolong(struct __call_single_data *csd, u64 ts0, u64 * *bug_id, !cpu_cur_csd ? "unresponsive" : "handling this request"); } if (cpu >= 0) { - dump_cpu_task(cpu); + if (atomic_cmpxchg_acquire(&per_cpu(trigger_backtrace, cpu), 1, 0)) + dump_cpu_task(cpu); if (!cpu_cur_csd) { pr_alert("csd: Re-sending CSD lock (#%d) IPI from CPU#%02d to CPU#%02d\n", *bug_id, raw_smp_processor_id(), cpu); arch_send_call_function_single_ipi(cpu); @@ -434,9 +437,14 @@ static void __flush_smp_call_function_queue(bool warn_cpu_offline) struct llist_node *entry, *prev; struct llist_head *head; static bool warned; + atomic_t *tbt; lockdep_assert_irqs_disabled(); + /* Allow waiters to send backtrace NMI from here onwards */ + tbt = this_cpu_ptr(&trigger_backtrace); + atomic_set_release(tbt, 1); + head = this_cpu_ptr(&call_single_queue); entry = llist_del_all(head); entry = llist_reverse_order(entry); -- 2.40.1