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 X-Spam-Level: X-Spam-Status: No, score=-2.1 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id DC64EC3279B for ; Mon, 2 Jul 2018 10:39:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9154D23FE5 for ; Mon, 2 Jul 2018 10:39:26 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="IZv1Ka9p" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9154D23FE5 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=gmail.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1030337AbeGBKjZ (ORCPT ); Mon, 2 Jul 2018 06:39:25 -0400 Received: from mail-pf0-f195.google.com ([209.85.192.195]:34305 "EHLO mail-pf0-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965277AbeGBKjX (ORCPT ); Mon, 2 Jul 2018 06:39:23 -0400 Received: by mail-pf0-f195.google.com with SMTP id e10-v6so754935pfn.1 for ; Mon, 02 Jul 2018 03:39:22 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=DJCcPLsn4nFXb59xiYAUyrQxqepA/cE4v8qhRW+10gg=; b=IZv1Ka9psp13HckE2cgqHVnfO4U9Y3ThtgWkjZWO//pwqPteNKy/SnBhuccysXhvSS y9yDWG5NH0P5Uh80BGQ81ei2tpGUZxNGi1A8RaGohbXqfww/WzRaf5J2IN8UBULpBb3M frueH/fOMIjqD0c7F2qnTM59W8Ob2TOq56nDoIh5Paqnd/7cXJAAn9JQayeDOd2dpHx1 SxS+ETGNR+xu5ak+Ch8YKyGlgc2wgm5SSBRsp69Jm872Vo+7/YVZdLpv5sMD4or3DjaD I/6voE3+xp/AA3MvlxbGCv3whHARU6DhDKBCN9VzffmG2tv7+PWQuf60KmLbtzP4LzDr xF+Q== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=DJCcPLsn4nFXb59xiYAUyrQxqepA/cE4v8qhRW+10gg=; b=Fkky9dtc6Cw5Ku1A0CERkusVuYtRRkkh7/wgVD2bFMU2h4VDtLPGHoh1HlkEh7JXjM NREFo/+vXGI84eTK1UmtdspZXllhlzUyqlkYf0eztkXUcMFax5b6Q80XRUW8sycAYrWL 6GZpCLf6R1ulPpWZaIcv5CkGcYRLUNoI75A779YtSIp/FAehuUJ+m+bfDsNxALsPlhB4 MEv1M5n53YB5jo2Ab0i4miMOz5tHxR3WGYxUmAJqPJ5kSbi9teqZWAJXzTKJhVqFXt6n Leqc6yO8vI47Y/x4vaDkICHEi6rNaBchbf8p5nLolssKejPZJHP648VM7fBE0Uy/kZlf e5Sw== X-Gm-Message-State: APt69E2mUQrW/CUEo4hjdGsN5Cy3Ms57HTHgubjHNOHYx3OKJOxyK02i b7p+4x+G2isVBg2L2jr87o8= X-Google-Smtp-Source: AAOMgpd1nwJJeDKSKL+AH0lvp9il4QMWX2iJgIKRSGHHB/S962H5EKlbfVcDQzVuIIz97eZo+EskHw== X-Received: by 2002:a63:2ac4:: with SMTP id q187-v6mr20228439pgq.333.1530527962336; Mon, 02 Jul 2018 03:39:22 -0700 (PDT) Received: from localhost ([175.223.27.171]) by smtp.gmail.com with ESMTPSA id s27-v6sm38420722pfk.184.2018.07.02.03.39.20 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Mon, 02 Jul 2018 03:39:21 -0700 (PDT) Date: Mon, 2 Jul 2018 19:39:17 +0900 From: Sergey Senozhatsky To: Tetsuo Handa Cc: Sergey Senozhatsky , Petr Mladek , Steven Rostedt , Peter Zijlstra , Sergey Senozhatsky , linux-kernel@vger.kernel.org, Dmitry Vyukov Subject: Re: printk() from NMI backtrace can delay a lot Message-ID: <20180702103917.GA3673@jagdpanzerIV> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.0 (2018-05-17) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, On (07/02/18 19:26), Tetsuo Handa wrote: > Hello. > > Today I was testing conditions when/how stall watchdog fires. I noticed that > printing NMI backtraces to consoles is delayed till IRQ is enabled or somebody > else schedules printk(). This is not a welcomed behavior when the cause of > lock up is doing nearly-infinite loop with IRQ disabled. Can we improve this? Hmm. We can't call console drivers from NMI, this can deadlock on uart/etc locks. So we always need [except for panic()] someone else to print NMI message for us. Either it's IRQ on a local CPU (we need two IRQs actually - one to flush printk_nmi buffer and the second one to do console_trylock()->console_unlock()), or printk() from another CPU that would print pending logbuf entries. We used to have a fast path for print_nmi messages (direct_nmi), which soon will be used only for NMI->ftrace_dump(). Even if we re-introduce that fast path for printk_nmi [may be we can do printk_direct_nmi type of checks for printk_nmi as well] we still can't print anything from the NMI CPU. I need to look more at the data you have provided. -ss