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=-4.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no 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 931F4C4727F for ; Mon, 28 Sep 2020 12:50:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 41CDB21D7F for ; Mon, 28 Sep 2020 12:50:32 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1601297432; bh=vSqcP0mrbe2L8WpJdc6NrNWXS4Vn2KXk5nRMn8lrgno=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=ent8PmJP+XG8enesfckIY64eVihZIgOkUJ9NpseSL8r2TiZ3UqEpAATqjn2oNuhFr aaXjpF0hH5IOinTMq4v8mrMDLf8voxqbDSBVzt2e2UlGdZlMelzZqJSgqOMGH83mS8 21lCJaklxNLuYpgORaPPp9vg90UU95ATtT/32F7c= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726578AbgI1Mub (ORCPT ); Mon, 28 Sep 2020 08:50:31 -0400 Received: from mail.kernel.org ([198.145.29.99]:40182 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726327AbgI1Mua (ORCPT ); Mon, 28 Sep 2020 08:50:30 -0400 Received: from localhost (83-86-74-64.cable.dynamic.v4.ziggo.nl [83.86.74.64]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 5A02721974; Mon, 28 Sep 2020 12:50:29 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1601297429; bh=vSqcP0mrbe2L8WpJdc6NrNWXS4Vn2KXk5nRMn8lrgno=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=ITBZ71/NgZ1h+xb0un8m22k/zxMtWnd/2/U/Se+o4QHj2dYSBb36uZH0QhgIRPKhu A8K/urinr3oiq6DXrYZe5VZRnUpdPdhL0zQsmVj6P02/FxOqWBEayS3AR8UwRNS8jR a0POInjIWcV3H5lDsIvmAjkve/MHFnZ+Uhy8f0XE= Date: Mon, 28 Sep 2020 14:50:37 +0200 From: Greg KH To: psodagud@codeaurora.org Cc: Thomas Gleixner , rostedt@goodmis.org, pmladek@suse.com, sergey.senozhatsky@gmail.com, linux-kernel@vger.kernel.org, tkjos@google.com, Mohammed Khajapasha Subject: Re: [PATCH 2/2] printk: Make the console flush configurable in hotplug path Message-ID: <20200928125037.GA1661095@kroah.com> References: <1600906112-126722-1-git-send-email-psodagud@codeaurora.org> <1600906112-126722-2-git-send-email-psodagud@codeaurora.org> <20200924063352.GB592892@kroah.com> <87wo0j6nos.fsf@nanos.tec.linutronix.de> <4c4a2534824eb69d41753d2e3b2773de@codeaurora.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4c4a2534824eb69d41753d2e3b2773de@codeaurora.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Sep 27, 2020 at 07:05:34PM -0700, psodagud@codeaurora.org wrote: > On 2020-09-24 11:21, Thomas Gleixner wrote: > > On Thu, Sep 24 2020 at 08:33, Greg KH wrote: > > > On Wed, Sep 23, 2020 at 05:08:32PM -0700, Prasad Sodagudi wrote: > > > > +config CONSOLE_FLUSH_ON_HOTPLUG > > > > + bool "Enable console flush configurable in hot plug code path" > > > > + depends on HOTPLUG_CPU > > > > + def_bool n > > > > > > n is the default, no need to list it. > > > > > > > + help > > > > + In cpu hot plug path console lock acquire and release causes the > > > > + console to flush. If console lock is not free hot plug latency > > > > + increases. So make console flush configurable in hot plug path > > > > + and default disabled to help in cpu hot plug latencies. > > > > > > Why would you not want this option? > > > > > > Why isn't this just a bugfix? > > > > Because it's the normal behaviour of console lock and there are > > gazillion other ways to delay stuff in the hotplug path. > > > > CPU hotplug is not meant to be a high speed operation and if people > > think they need it to be fast then its pretty much guaranteed that they > > want it for the completely wrong reasons. > > > > This #ifdef tinkering is just digusting especially as it just tackles an > > obvious way how to delay timer migration, but does not address the > > underlying root cause. > > > > Hi tglx, > > Yes. I agree with you that there are other conditions, which could delay the > hotplug operation. But this console > flushing is not needed in the hotplug path. In the hotplug path, a core is > trying printing messages > from other core(by design of printk), delays the whole hotplug operation and > timers migration. As timers > migration gets delayed, it would impact the systems stability in device > stability testing. > To avoid timers delay in the timer migration in debug builds has to choose > this option. > > I thought of changing the timers and irq migration as priority callbacks in > the hotplug out operation > but I observed some comments like shown below. I was under impression that, > it is hard to find all this > type of conditions, so started tinkering hotplug path by changing the log > levels. > These changes helped on Qualcomm platforms testing. > /* > * On the tear-down path, timers_dead_cpu() must be invoked > * before blk_mq_queue_reinit_notify() from notify_dead(), > * otherwise a RCU stall occurs. > */ > [CPUHP_TIMERS_PREPARE] = { > .name = "timers:prepare", > .startup.single = timers_prepare_cpu, > .teardown.single = timers_dead_cpu, > }, > > Another reason for adding #ifdef is that, I was not clear why console flush > is need cpuhp callback and thought > there might be some use cases and console flush use case might not be valid > for all the users of cpu hotplug. > I will try to explore the changing the callback order to complete the timers > and irq migration early in the hotplug operation. > > Let me put some use cases of hotplug and why hotplug and hotplug latency is > important from testing point of view. > 1) Secondary cpus are hotplug out during the device suspend and hotplug in > during the resume. So cpu hotplug operation is important production devices > point of view as user presses the power key many times. But what does suspend/resume have to do with this? Why not do just an offline operation instead of unplugging the whole cpu? > 2) sysfs nodes (/sys/devices/ststem/cpu/cpu4/oneline) are present from linux > kernel, so test team wants to test cpu hotplug. There could be issues with > in generic kernel, device drivers or firmware(psci calls handling from > firmware). There could be issues with device drivers or firmware and test > teams can not leave the hotplug untested in builds. Your change isn't for testing things, speed doesn't matter when writing to sysfs nodes, right? > 3) Linux kernel also gave provision to register call backs with cpu hotplug > framework(CPUHP_AP_ONLINE_DYN) dynamic callbacks. > 3002 ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN, > "printk:online", > 3003 console_cpu_notify, NULL); > So test team wants to test if any in tree or out of tree modules have any > issues with registered call backs or not. Again, how is this a speed issue? > 4) Tracing of the cpuhp operation is important to find whether upstream > changes or out of tree modules(or firmware changes) caused latency > regression or not. But cpu hotplug is not deterministic, so how does latency matter here? confused as to the real problem here... greg k-h