From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S966668AbbBDQLB (ORCPT ); Wed, 4 Feb 2015 11:11:01 -0500 Received: from mailout2.w1.samsung.com ([210.118.77.12]:36432 "EHLO mailout2.w1.samsung.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965489AbbBDQLA (ORCPT ); Wed, 4 Feb 2015 11:11:00 -0500 MIME-version: 1.0 Content-type: text/plain; charset=UTF-8 X-AuditID: cbfec7f5-b7fc86d0000066b7-c6-54d243ffebe8 Content-transfer-encoding: 8BIT Message-id: <1423066256.24415.13.camel@AMDC1943> Subject: Re: [rcu] [ INFO: suspicious RCU usage. ] From: Krzysztof Kozlowski To: paulmck@linux.vnet.ibm.com Cc: Russell King - ARM Linux , Fengguang Wu , LKP , linux-kernel@vger.kernel.org, Bartlomiej Zolnierkiewicz , linux-arm-kernel@lists.infradead.org, Arnd Bergmann , MarkRutland Date: Wed, 04 Feb 2015 17:10:56 +0100 In-reply-to: <20150204155615.GF5370@linux.vnet.ibm.com> References: <20150201025922.GA16820@wfg-t540p.sh.intel.com> <1422957702.17540.1.camel@AMDC1943> <20150203162704.GR19109@linux.vnet.ibm.com> <1423049947.19547.6.camel@AMDC1943> <20150204130018.GG8656@n2100.arm.linux.org.uk> <20150204131420.GC5370@linux.vnet.ibm.com> <1423059387.24415.2.camel@AMDC1943> <20150204151028.GD5370@linux.vnet.ibm.com> <1423063348.24415.10.camel@AMDC1943> <20150204155615.GF5370@linux.vnet.ibm.com> X-Mailer: Evolution 3.10.4-0ubuntu2 X-Brightmail-Tracker: H4sIAAAAAAAAA+NgFrrGLMWRmVeSWpSXmKPExsVy+t/xK7r/nS+FGMx6wW/xd9IxdouNM9az Wrx/vp7ZYtPja6wWl3fNYbO4fZnXYuXxdlaLpdcvMlm83fyd1YHT4/5edo8189YwerQ097B5 /P41idFj8Z6XTB4PDm1m8di8pN6jb8sqRo/Pm+QCOKO4bFJSczLLUov07RK4Mj4d+c9asFuw ouX+beYGxou8XYycHBICJhJd3ycyQdhiEhfurWcDsYUEljJKTDsYBGLzCghK/Jh8j6WLkYOD WUBe4silbJAws4C6xKR5i5i7GLmAyj8zStw52ccEUW8g8W3bRGYQW1jASGLlu6lgcTYBY4nN y5ewgcwREZCTWDMxCaSXWWArk8SG67vA9rIIqErMOrERrJdTwFxi8t/9rBALtjFLLN3yAqxZ QkBZorHfbQKjwCwk581COG8WkvMWMDKvYhRNLU0uKE5KzzXSK07MLS7NS9dLzs/dxAiJi687 GJceszrEKMDBqMTD29F7MUSINbGsuDL3EKMEB7OSCK+o3aUQId6UxMqq1KL8+KLSnNTiQ4xM HJxSDYzuxxn8p2d8evlN6M/6R0Lii96fD4zYttn1izBr35TK80LZx7u+dzCvf8ipnK9vFeb/ Me1Z8NO+fJZK7Ul1/7Y76skv7Lj3Prf26qGtV8un3zXeyPNxU0FKcpvxMq1fS1T3Nkw7Pn9i rFyY68HAG+u+r3deo3An6UvetIt+01+ZJxbdFp13YKmmEktxRqKhFnNRcSIAqamte2kCAAA= Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On śro, 2015-02-04 at 07:56 -0800, Paul E. McKenney wrote: > On Wed, Feb 04, 2015 at 04:22:28PM +0100, Krzysztof Kozlowski wrote: > > > > Actually the timeout versions but I think that doesn't matter. > > The wait_on_bit will busy-loop with testing for the bit. Inside the loop > > it calls the 'action' which in my case will be bit_wait_io_timeout(). > > This calls schedule_timeout(). > > Ah, good point. > > > See proof of concept in attachment. One observed issue: hot unplug from > > commandline takes a lot more time. About 7 seconds instead of ~0.5. > > Probably I did something wrong. > > Well, you do set the timeout to five seconds, and so if the condition > does not get set before the surviving CPU finds its way to the > out_of_line_wait_on_bit_timeout(), you are guaranteed to wait for at > least five seconds. > > One alternative approach would be to have a loop around a series of > shorter waits. Other thoughts? Right! That was the issue. It seems it works. I'll think also on self-adapting interval as you said below. I'll test it more and send a patch. Best regards, Krzysztof > > > > You know, this situation is giving me a bad case of nostalgia for the > > > old Sequent Symmetry and NUMA-Q hardware. On those platforms, the > > > outgoing CPU could turn itself off, and thus didn't need to tell some > > > other CPU when it was ready to be turned off. Seems to me that this > > > self-turn-off capability would be a great feature for future systems! > > > > There are a lot more issues with hotplug on ARM... > > Just trying to clean up this particular corner at the moment. ;-) > > > Patch/RFC attached. > > Again, I believe that you will need to loop over a shorter timeout > in order to get reasonable latencies. If waiting a millisecond at > a time is an energy-efficiency concern (don't know why it would be > in this rare case, but...), then one approach would be to start > with very short waits, then increase the wait time, for example, > doubling the wait time on each pass through the loop would result > in a smallish number of wakeups, but would mean that you waited > no more than twice as long as necessary. > > Thoughts?