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=-0.6 required=3.0 tests=DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,T_DKIM_INVALID 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 13887C433EF for ; Tue, 19 Jun 2018 13:24:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id BC9CC20652 for ; Tue, 19 Jun 2018 13:24:19 +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="QMRMagH3" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BC9CC20652 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.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 S937793AbeFSNYR (ORCPT ); Tue, 19 Jun 2018 09:24:17 -0400 Received: from merlin.infradead.org ([205.233.59.134]:46074 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S937149AbeFSNYP (ORCPT ); Tue, 19 Jun 2018 09:24:15 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.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:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=U57xOQyJi2xZKWb1RAkS8/IyrISuzfq2d4ES0Bn+lN8=; b=QMRMagH37aZQpQnPWZe+2Gfn1 53S+1TJtIacIqaxM/iKfHL1hYkIWddHBli4wisUbYwMKQo4OeQdBzirtF7Ayk9/AAjrIGTvDyQ8VA I65J0sm1r4QOji+tctOTYMSMCB2ijZTKHcM0fyRjaZhz8uNQxuWNEhY6CHSgAjR1ROO07wOJ3xxtD cvLrcGIMzNRXPb7za5v52SbESzCrQS04wmkcDF0YxCPUk1GQRbhWv//k4+v0vDz64Lzd+RDFnwBrw spFYoocL9mXtwfAy2bXxWWvAjlmeSoCVZqpPrXRK/C0+6PADR/xDk5heeOMaKQzngAZy4dtSUkxcO LDGATOgPg==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1fVGbk-0002Bb-Kh; Tue, 19 Jun 2018 13:23:40 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 010AF202A607E; Tue, 19 Jun 2018 15:23:38 +0200 (CEST) Date: Tue, 19 Jun 2018 15:23:38 +0200 From: Peter Zijlstra To: Quentin Perret Cc: rjw@rjwysocki.net, gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org, mingo@redhat.com, dietmar.eggemann@arm.com, morten.rasmussen@arm.com, chris.redpath@arm.com, patrick.bellasi@arm.com, valentin.schneider@arm.com, vincent.guittot@linaro.org, thara.gopinath@linaro.org, viresh.kumar@linaro.org, tkjos@google.com, joelaf@google.com, smuckle@google.com, adharmap@quicinc.com, skannan@quicinc.com, pkondeti@codeaurora.org, juri.lelli@redhat.com, edubezval@gmail.com, srinivas.pandruvada@linux.intel.com, currojerez@riseup.net, javi.merino@kernel.org Subject: Re: [RFC PATCH v3 03/10] PM: Introduce an Energy Model management framework Message-ID: <20180619132338.GF2476@hirez.programming.kicks-ass.net> References: <20180521142505.6522-1-quentin.perret@arm.com> <20180521142505.6522-4-quentin.perret@arm.com> <20180619113408.GQ2458@hirez.programming.kicks-ass.net> <20180619125857.GY17720@e108498-lin.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180619125857.GY17720@e108498-lin.cambridge.arm.com> User-Agent: Mutt/1.10.0 (2018-05-17) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 19, 2018 at 01:58:58PM +0100, Quentin Perret wrote: > On Tuesday 19 Jun 2018 at 13:34:08 (+0200), Peter Zijlstra wrote: > > On Mon, May 21, 2018 at 03:24:58PM +0100, Quentin Perret wrote: > > > +struct em_freq_domain *em_cpu_get(int cpu) > > > +{ > > > + struct em_freq_domain *fd; > > > + unsigned long flags; > > > + > > > + read_lock_irqsave(&em_data_lock, flags); > > > + fd = per_cpu(em_data, cpu); > > > + read_unlock_irqrestore(&em_data_lock, flags); > > > > Why can't this use RCU? This is the exact thing read_locks are terrible > > at and RCU excells at. > > So the idea was that clients (the scheduler for ex) can get a reference > to a frequency domain object once, and they're guaranteed it always > exists without asking for it again. > > For example, my proposal was to have the scheduler (patch 05) build its > own private list of frequency domains on which it can iterate efficiently > in the wake-up path. If we protect this per_cpu variable with RCU, then > this isn't possible any-more. The scheduler will have to re-ask > em_cpu_get() at every wake-up, and that makes iterating over frequency > domains a whole lot more complex. > > Does that make any sense ? None what so ever... The lock doesn't guarantee stability any more than RCU does. If you hand out the pointer and then drop the read-lock, the write-lock can proceed and change the pointer right after you. The very easiest solution is to never change the data, as I think was suggested elsewhere in the thread. Construct the thing once and then never mutate.