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=-2.4 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS, URIBL_BLOCKED,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 8DECEECDE43 for ; Fri, 19 Oct 2018 03:58:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3228B20838 for ; Fri, 19 Oct 2018 03:58:51 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="vPqXPLiC" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3228B20838 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=joelfernandes.org 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 S1727011AbeJSMDB (ORCPT ); Fri, 19 Oct 2018 08:03:01 -0400 Received: from mail-pg1-f195.google.com ([209.85.215.195]:36543 "EHLO mail-pg1-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726663AbeJSMDB (ORCPT ); Fri, 19 Oct 2018 08:03:01 -0400 Received: by mail-pg1-f195.google.com with SMTP id f18-v6so15193815pgv.3 for ; Thu, 18 Oct 2018 20:58:48 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=64vq2/B6XPZ3Ilprbin+GaWg1J+1g9ESsyUeba3mxT8=; b=vPqXPLiCetM6+kgQ2s1oZ9nC5PBskNAaUP2A3APwQma0gzsOwCeK/jyIjkFpwU7kcd c5k+Uz016IC9Vge7cT4TwFXstNUz8pblPuHLI9OR0r0nmVlkboS5WFmcagw1KTS55+gZ MVF7O361X01MbU9Uu5HwVMonPgmLD1ihAtJyI= 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=64vq2/B6XPZ3Ilprbin+GaWg1J+1g9ESsyUeba3mxT8=; b=Nzsn3/2gN6FgwqAwB/cfjR3Wmvt+H4eKgJgGuVMMTp4c673yIhl0A0R9T1w3aEtunH j6VoVXt1bZNnjuM/Qkfm8fwxfTMEbvUvDXJZgvIPf/jTAGLMQKSlIUpH93J6codSGp8n GRQwCslZrt9B2puFBD4pFO31X27SQeOyaC+xz5NEyQjG6HH/x7DCGpQJQQ3cNqL6wUFY Lh0sPEXZWfHzczW/8Yg2Blne2hxVgfQNVbTigYC/0H84A8txPaL/ZwFRKmnWpiKmnunT /SAmP1n8aikWWVDxGzKw47aBVehlEvYeaIhwksdLkFzqZNA98Mcid8L8QSbcMPOFQSX7 MfAw== X-Gm-Message-State: ABuFfoiuUIyd8yaBIkOLzSxo84vhaO7Fd9slJaZhPOI721e6t0ncm5Hp 576dfErhdCX7JeptOLwwe7+sw4f5iZE= X-Google-Smtp-Source: ACcGV62VXaL1pPIWNSCeh22C5zdFdaZ9JZ7bBHB9w+ozT7fQXE7+pPm2Eh4H0/Q8RT9aHYNEyF8w1g== X-Received: by 2002:a63:1919:: with SMTP id z25-v6mr30333822pgl.135.1539921527526; Thu, 18 Oct 2018 20:58:47 -0700 (PDT) Received: from localhost ([2620:0:1000:1601:3aef:314f:b9ea:889f]) by smtp.gmail.com with ESMTPSA id 84-v6sm36478634pfs.108.2018.10.18.20.58.45 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Thu, 18 Oct 2018 20:58:46 -0700 (PDT) Date: Thu, 18 Oct 2018 20:58:44 -0700 From: Joel Fernandes To: Steven Rostedt Cc: "Paul E. McKenney" , Nikolay Borisov , linux-kernel@vger.kernel.org, Jonathan Corbet , Josh Triplett , Lai Jiangshan , linux-doc@vger.kernel.org, Mathieu Desnoyers Subject: Re: [PATCH RFC] doc: rcu: remove obsolete (non-)requirement about disabling preemption Message-ID: <20181019035844.GA141835@joelaf.mtv.corp.google.com> References: <20181017181505.GC107185@joelaf.mtv.corp.google.com> <20181017203324.GS2674@linux.ibm.com> <20181018020751.GB99677@joelaf.mtv.corp.google.com> <20181018144637.GD2674@linux.ibm.com> <20181019000350.GB89903@joelaf.mtv.corp.google.com> <20181019001932.GR2674@linux.ibm.com> <20181019012645.GC89903@joelaf.mtv.corp.google.com> <20181018215035.5e8ff553@vmware.local.home> <20181019022529.GA155753@joelaf.mtv.corp.google.com> <20181018225223.42641c73@vmware.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181018225223.42641c73@vmware.local.home> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Oct 18, 2018 at 10:52:23PM -0400, Steven Rostedt wrote: > On Thu, 18 Oct 2018 19:25:29 -0700 > Joel Fernandes wrote: > > > On Thu, Oct 18, 2018 at 09:50:35PM -0400, Steven Rostedt wrote: > > > On Thu, 18 Oct 2018 18:26:45 -0700 > > > Joel Fernandes wrote: > > > > > > > Yes, local_irq_restore is light weight, and does not check for reschedules. > > > > > > > > I was thinking of case where ksoftirqd is woken up, but does not run unless > > > > we set the NEED_RESCHED flag. But that should get set anyway since probably > > > > ksoftirqd is of high enough priority than the currently running task.. > > > > > > > > Roughly speaking the scenario could be something like: > > > > > > > > rcu_read_lock(); > > > > <-- IPI comes in for the expedited GP, sets exp_hint > > > > local_irq_disable(); > > > > // do a bunch of stuff > > > > rcu_read_unlock(); <-- This calls the rcu_read_unlock_special which raises > > > > the soft irq, and wakesup softirqd. > > > > > > If softirqd is of higher priority than the current running task, then > > > the try_to_wake_up() will set NEED_RESCHED of the current task here. > > > > > > > Yes, only *if*. On my system, ksoftirqd is CFS nice 0. I thought expedited > > grace periods are quite important and they should complete quickly which is > > the whole reason for interrupting rcu read sections with an IPI and stuff. > > IMO there should be no harm in setting NEED_RESCHED unconditionally anyway > > for possible benefit of systems where the ksoftirqd is not of higher priority > > than the currently running task, and we need to run it soon on the CPU. But > > I'm Ok with whatever Paul and you want to do here. > > > Setting NEED_RESCHED unconditionally wont help. Because even if we call > schedule() ksoftirqd will not be scheduled! If it's CFS nice 0, and the > current task still has quota to run, if you call schedule, you'll just > waste time calculating that the current task should still be running. > It's equivalent to calling yield() (which is why we removed all yield() > users in the kernel, because *all* of them were buggy!). This is *why* > it only calls schedule *if* softirqd is of higher priority. Yes, ok. you are right the TTWU path should handle setting the NEED_RESCHED flag or not and unconditionally setting it does not get us anything. I had to go through the code a bit since it has been a while since I explored it. So Paul, I'm Ok with your latest patch for the issue we discussed and don't think much more can be done barring raising of ksofitrqd priorities :-) So I guess the synchronize_rcu_expedited will just cope with the deal between local_irq_enable and the next scheduling point.. :-) thanks, - Joel