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 06D95C19F28 for ; Wed, 3 Aug 2022 17:13:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237251AbiHCRNI (ORCPT ); Wed, 3 Aug 2022 13:13:08 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52816 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235731AbiHCRNG (ORCPT ); Wed, 3 Aug 2022 13:13:06 -0400 Received: from mail-wm1-x32a.google.com (mail-wm1-x32a.google.com [IPv6:2a00:1450:4864:20::32a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8E7FD2B606 for ; Wed, 3 Aug 2022 10:13:05 -0700 (PDT) Received: by mail-wm1-x32a.google.com with SMTP id i128-20020a1c3b86000000b003a3a22178beso1240580wma.3 for ; Wed, 03 Aug 2022 10:13:05 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=sender:date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=z5OCnAW+PZcXkMuuSs1kQUCSIWwKc63YJXF5AYyiWvs=; b=jNNajHux+HCM22nbhhvKyjG4WoGztDqTWXsUT0YtHR3batWmdxWXFnqaqiSopvjHc3 /q1Evq/wAS3WzGgNLVZ2T3fi2OQwh6/pNum0yWBmfZaZSNKFGYM9U1AX+eVs0QqBLwq8 3ZkDdWfO2MviryspuE8bnKOcv3z6eB0kLiKPOj1MxnvXwsjBsZwT7j51/5prDFdehf+F DD4pFJY8PjlIIOkWD7HkUDaiQfOaYId9DA16qO+j6sCbKo1GhZvus6ebdZ7Y4kG1XSJ1 VX0Ihqkx9BwYz4pqX4Qsqd2IWKmwmzPtDBZbWBH9ooVIgPRjOdLLEbAEAU02d4AufWjM 2Oow== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:sender:date:from:to:cc:subject:message-id :references:mime-version:content-disposition:in-reply-to; bh=z5OCnAW+PZcXkMuuSs1kQUCSIWwKc63YJXF5AYyiWvs=; b=HKNpgSa1CY8bhprTF9LLnQ7lV2//dtcULWoPfkq4ktGk9n5nMlXjWqUF+zw6r79ZMX EaliMv1wyRx/W1xRyjK3hTP59hHQVemGKCbylAD2EvHtSAdAwrr/1hMKhf7eRcKtnKVW XhSX1HHhJePJuZhBjuuFek+Cx8dmCIp2MYV+U6whYIJTkK3qP+Jz+bIzziIlnqVPiucp Sx/RT6GwizduTSlTKkJb6Y2+3H1Y1pQtliFTuki/GegUMZn6BU5Nmj6TrF7vmEqBFFgG hkEcpM4CFLIyV1+eu1+KdAtKm/HPvk7wL+pOAM6pnIsW1vN6HMr5UnONMzenz8EdNJb4 AMsg== X-Gm-Message-State: ACgBeo3yPt6p2uuFOX2SOourEBGWx+ohdxlXOC+7F4p5Dgj28GV9H6rE rynty5bHMuHaB4hyVrBDr5AtgKjHRjI= X-Google-Smtp-Source: AA6agR5McKuf/+273ZPdXBk02sh4g5+7ZlL1wsyoTiTCBCQNciKm76ckF2TutvNRUsFMMu/+Ol7vAQ== X-Received: by 2002:a05:600c:3d06:b0:3a3:58f6:9d96 with SMTP id bh6-20020a05600c3d0600b003a358f69d96mr3504341wmb.195.1659546784116; Wed, 03 Aug 2022 10:13:04 -0700 (PDT) Received: from gmail.com (84-236-113-167.pool.digikabel.hu. [84.236.113.167]) by smtp.gmail.com with ESMTPSA id t4-20020adfeb84000000b0021e45afa7b0sm18918509wrn.109.2022.08.03.10.13.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 03 Aug 2022 10:13:03 -0700 (PDT) Sender: Ingo Molnar Date: Wed, 3 Aug 2022 19:13:01 +0200 From: Ingo Molnar To: Steven Rostedt Cc: Liu Song , mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, bsegall@google.com, mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com, linux-kernel@vger.kernel.org, "Paul E. McKenney" Subject: Re: [PATCH] sched/debug: avoid executing show_state and causing rcu stall warning Message-ID: References: <1659489525-82994-1-git-send-email-liusong@linux.alibaba.com> <20220803084235.5d56d1e4@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220803084235.5d56d1e4@gandalf.local.home> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Steven Rostedt wrote: > > [ Adding Paul ] > > On Wed, 3 Aug 2022 09:18:45 +0800 > Liu Song wrote: > > > From: Liu Song > > > > If the number of CPUs is large, "sysrq_sched_debug_show" will execute for > > a long time. Every time I execute "echo t > /proc/sysrq-trigger" on my > > 128-core machine, the rcu stall warning will be triggered. Moreover, > > sysrq_sched_debug_show does not need to be protected by rcu_read_lock, > > and no rcu stall warning will appear after adjustment. > > > > Signed-off-by: Liu Song > > --- > > kernel/sched/core.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > index 5555e49..82c117e 100644 > > --- a/kernel/sched/core.c > > +++ b/kernel/sched/core.c > > @@ -8879,11 +8879,11 @@ void show_state_filter(unsigned int state_filter) > > sched_show_task(p); > > } > > > > + rcu_read_unlock(); > > #ifdef CONFIG_SCHED_DEBUG > > if (!state_filter) > > sysrq_sched_debug_show(); > > If this is just because sysrq_sched_debug_show() is very slow, does RCU > have a way to "touch" it? Like the watchdogs have? That is, to tell RCU > "Yes I know I'm taking a long time, but I'm still making forward progress, > don't complain about me". Then the sysrq_sched_debug_show() could have: > > for_each_online_cpu(cpu) { > /* > * Need to reset softlockup watchdogs on all CPUs, because > * another CPU might be blocked waiting for us to process > * an IPI or stop_machine. > */ > touch_nmi_watchdog(); > touch_all_softlockup_watchdogs(); > + touch_rcu(); > print_cpu(NULL, cpu); > } I'd much rather we use the specific exclusion primitive suitable for that sequence - in that case it should be cpus_read_lock()/unlock() I suspect. But the entire code sequence should be reviewed - do we anywhere walk task lists that need RCU protection? My main complaint was that we cannot just randomly drop the RCU lock with no inspection of the underlying code. Ingo