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=-5.3 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 23F16C3A589 for ; Sun, 18 Aug 2019 22:35:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E1F8E21852 for ; Sun, 18 Aug 2019 22:35:30 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="MaYJInsP" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726083AbfHRWfa (ORCPT ); Sun, 18 Aug 2019 18:35:30 -0400 Received: from mail-pg1-f196.google.com ([209.85.215.196]:36822 "EHLO mail-pg1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726075AbfHRWfa (ORCPT ); Sun, 18 Aug 2019 18:35:30 -0400 Received: by mail-pg1-f196.google.com with SMTP id l21so5767238pgm.3 for ; Sun, 18 Aug 2019 15:35:29 -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=aoEwN5YCvSYNBJpZR2EckQTs3ZmIOpZAgC3IGRqX8bg=; b=MaYJInsP/PMTPaWj1vZ2QIk47Qsm6UpOUGUQPV0exg6tLDgFI32aRaJXGKRVxqk2tN A3jwhNK2SX655ihw0ziplM3eDB0G5dWO/x4m7xXHrNmYHRm3ADqdGNZEdor2ieOJfL6y MImy42ZCcO4JOp5DP1ldbWzQzHv2x0WTbTGzA= 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=aoEwN5YCvSYNBJpZR2EckQTs3ZmIOpZAgC3IGRqX8bg=; b=Mly9BPfiisaykdhympCHTN3/m4Gku4tK+oAAYyzCMRUUcCZBoK3b5MNRlEv4Pb2XPL QAFQmcxdfs9BNCl7A4cS9bR9U1ffL2Lfuopu7+5yhcdsISFpjV+y97HPPxyFT9Vf/62b nyEmfmMBLEwTnad4FFWTnzSgBlcDpJya6uvPSOdmzrD4dnazoCr6bx8U2dl0KmH5c9pE Xt9z2kpL0iQT5MWMp85RTmQ9JVH3547YBGYeEYEXv6mBV7g0IQ94UHeUexSYDPYyCSxC grG8c2zeFRfXjJHCtthnNxf54iGGxxNjjKDojoS7TBn4CD7NnyBLPHpXNjFClxgU8muW 2hBw== X-Gm-Message-State: APjAAAVmZbRY8SUxH5CoSa6vQur9D2A1ztBffjBi6x6ViTuVJ6PqHQBw 691Hh3iAwOfNfi2honMSzlScqw== X-Google-Smtp-Source: APXvYqzTJLti63L6/J3jxFQJ0mwzyshYe5nTokmI7xIhgpWFZGifLBAxGooTyQs80jPUrHF7K39+Cw== X-Received: by 2002:a17:90a:aa98:: with SMTP id l24mr17657063pjq.64.1566167729049; Sun, 18 Aug 2019 15:35:29 -0700 (PDT) Received: from localhost ([172.19.216.18]) by smtp.gmail.com with ESMTPSA id ck8sm11142189pjb.25.2019.08.18.15.35.27 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 18 Aug 2019 15:35:28 -0700 (PDT) Date: Sun, 18 Aug 2019 18:35:11 -0400 From: Joel Fernandes To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, Josh Triplett , Lai Jiangshan , Mathieu Desnoyers , rcu@vger.kernel.org, Steven Rostedt Subject: Re: [RFC v2] rcu/tree: Try to invoke_rcu_core() if in_irq() during unlock Message-ID: <20190818223511.GB143857@google.com> References: <20190818214948.GA134430@google.com> <20190818221210.GP28441@linux.ibm.com> <20190818223230.GA143857@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190818223230.GA143857@google.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Sun, Aug 18, 2019 at 06:32:30PM -0400, Joel Fernandes wrote: > On Sun, Aug 18, 2019 at 03:12:10PM -0700, Paul E. McKenney wrote: > > On Sun, Aug 18, 2019 at 05:49:48PM -0400, Joel Fernandes (Google) wrote: > > > When we're in hard interrupt context in rcu_read_unlock_special(), we > > > can still benefit from invoke_rcu_core() doing wake ups of rcuc > > > threads when the !use_softirq parameter is passed. This is safe > > > to do so because: > > > > > > 1. We avoid the scheduler deadlock issues thanks to the deferred_qs bit > > > introduced in commit 23634ebc1d94 ("rcu: Check for wakeup-safe > > > conditions in rcu_read_unlock_special()") by checking for the same in > > > this patch. > > > > > > 2. in_irq() implies in_interrupt() which implies raising softirq will > > > not do any wake ups. > > > > > > The rcuc thread which is awakened will run when the interrupt returns. > > > > > > We also honor 25102de ("rcu: Only do rcu_read_unlock_special() wakeups > > > if expedited") thus doing the rcuc awakening only when none of the > > > following are true: > > > 1. Critical section is blocking an expedited GP. > > > 2. A nohz_full CPU. > > > If neither of these cases are true (exp == false), then the "else" block > > > will run to do the irq_work stuff. > > > > > > This commit is based on a partial revert of d143b3d1cd89 ("rcu: Simplify > > > rcu_read_unlock_special() deferred wakeups") with an additional in_irq() > > > check added. > > > > > > Signed-off-by: Joel Fernandes (Google) > > > > OK, I will bite... If it is safe to wake up an rcuc kthread, why > > is it not safe to do raise_softirq()? > > Because raise_softirq should not be done and/or doesn't do anything > if use_softirq == false. In fact, RCU_SOFTIRQ doesn't even existing if > use_softirq == false. The "else if" condition of this patch uses for > use_softirq. > > Or, did I miss your point? > > > And from the nit department, looks like some whitespace damage on the > > comments. > > I will fix all of these in the change log, it was just a quick RFC I sent > with the idea, tagged as RFC and not yet for merging. I should also remove > the comment about " in_irq() implies in_interrupt() which implies raising > softirq" from the changelog since this patch is only concerned with the rcuc > kthread. Ah, I see you mean the comments on the code. Perhaps something went wrong when I did 'git revert' on the original patch, or some such. Anyway, please consider this as RFC-grade only. And hopefully I have been writing better change logs (really trying!!). thanks, - Joel