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=-3.8 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS 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 EEF0DC433E0 for ; Mon, 13 Jul 2020 13:36:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CAE7C2072D for ; Mon, 13 Jul 2020 13:36:22 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="dTUuatlg" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729871AbgGMNgV (ORCPT ); Mon, 13 Jul 2020 09:36:21 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49664 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729747AbgGMNgU (ORCPT ); Mon, 13 Jul 2020 09:36:20 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8AE84C061755; Mon, 13 Jul 2020 06:36:20 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=9EMK+UYrL6jQjzlvQppNu91JO3Sh4jNLd4V1M4+eiGU=; b=dTUuatlg0KVwBCRqwtaZ2s9TzC 1S/HjNyZfYj0F/RACCE/EfF8yeNRRyX3JpVsCeOkBljoXS5FXUnkL/FIjBK9E3s4X1QURF42QlC7p NGr+4XNrY5lYFQCUci4QxnnwIhzXqvMwNIVSXsERBSscAex+gjl2AKhs/ioLo6wxTZZW/kpW8Fgg5 I6k9W19UFGSMyqxEGoaiRs2H4D/UEDYi79TrTIegUYGyJegtVlwPlSw4Np5fDb1jDyS++RYAiR1rm YGuXazd/Yx3LRTgX950c4nQkTEpXCb+BII+dwqc3+LOG1OMcOs5oIhIq1zBKP+yY+ZwMu2f7iuE/V c8ufUVgQ==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1juych-0002Nc-Oz; Mon, 13 Jul 2020 13:36:00 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 8AD2D300F7A; Mon, 13 Jul 2020 15:35:58 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 70EF820D27C6B; Mon, 13 Jul 2020 15:35:58 +0200 (CEST) Date: Mon, 13 Jul 2020 15:35:58 +0200 From: Peter Zijlstra To: Qais Yousef Cc: Ingo Molnar , Doug Anderson , Jonathan Corbet , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Luis Chamberlain , Kees Cook , Iurii Zaikin , Quentin Perret , Valentin Schneider , Patrick Bellasi , Pavan Kondeti , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH v6 1/2] sched/uclamp: Add a new sysctl to control RT default boost value Message-ID: <20200713133558.GK10769@hirez.programming.kicks-ass.net> References: <20200706142839.26629-1-qais.yousef@arm.com> <20200706142839.26629-2-qais.yousef@arm.com> <20200713112125.GG10769@hirez.programming.kicks-ass.net> <20200713121246.xjif3g4zpja25o5r@e107158-lin.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200713121246.xjif3g4zpja25o5r@e107158-lin.cambridge.arm.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jul 13, 2020 at 01:12:46PM +0100, Qais Yousef wrote: > On 07/13/20 13:21, Peter Zijlstra wrote: > > It's monday, and I cannot get my brain working.. I cannot decipher the > > comments you have with the smp_[rw]mb(), what actual ordering do they > > enforce? > > It was a bit of a paranoia to ensure that readers on other cpus see the new > value after this point. IIUC that's not something any barrier can provide. Barriers can only order between (at least) two memory operations: X = 1; y = Y; smp_wmb(); smp_rmb(); Y = 1; x = X; guarantees that if y == 1, then x must also be 1. Because the left hand side orders the store of Y after the store of X, while the right hand side order the load of X after the load of Y. Therefore, if the first load observes the last store, the second load must observe the first store. Without a second variable, barriers can't guarantee _anything_. Which is why any barrier comment should refer to at least two variables. > > Also, your synchronize_rcu() relies on write_lock() beeing > > non-preemptible, which isn't true on PREEMPT_RT. > > > > The below seems simpler... > Hmm maybe I am missing something obvious, but beside the race with fork; I was > worried about another race and that's what the synchronize_rcu() is trying to > handle. > > It's the classic preemption in the middle of RMW operation race. > > copy_process() sysctl_uclamp > > sched_post_fork() > __uclamp_sync_rt() > // read sysctl > // PREEMPT > for_each_process_thread() > // RESUME > // write syctl to p > > 2. sysctl_uclamp happens *during* sched_post_fork() > > There's the risk of the classic preemption in the middle of RMW where another > CPU could have changed the shared variable after the current CPU has already > read it, but before writing it back. Aah.. I see. > I protect this with rcu_read_lock() which as far as I know synchronize_rcu() > will ensure if we do the update during this section; we'll wait for it to > finish. New forkees entering the rcu_read_lock() section will be okay because > they should see the new value. > > spinlocks() and mutexes seemed inferior to this approach. Well, didn't we just write in another patch that p->uclamp_* was protected by both rq->lock and p->pi_lock?