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=-2.1 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 BA825C3A5A2 for ; Tue, 3 Sep 2019 20:06:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8AD33207E0 for ; Tue, 3 Sep 2019 20:06:19 +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="ilKCQ7Ou" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727192AbfICUGS (ORCPT ); Tue, 3 Sep 2019 16:06:18 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:53716 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726005AbfICUGS (ORCPT ); Tue, 3 Sep 2019 16:06:18 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.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=s/mxBv481g4gknPtfmgtkEvU8Nr3mAfkiqeLHsTDIQ0=; b=ilKCQ7OunKsvHtouU1msoWwVh 4G5Ha3b/T1/I/Sah2EY9NQ/wKA6JfImRCwX1VWlUMrt8TsPKtvjnSq3xyUGhpoEO5D2XYl2Jh28qj KoACNHg2EvRbMxy15aJnQ6zj2iZ7pk61sHtakaNpZ2eH8/vqxkM8KPyS8gAy9Vj1gCPK0kW+aXF52 t3tSd35yTSdOGZYEnzG4vrkGqrmC7UvmFEvFPkTYZ5TPtdnoHi5TLozGxg95lY9U3bIA0fJs71bI8 fV015J+2lXOo+TBuja8TMrlMTBKqCib/+xMCROmKKDPi4ChNIZfQAJ7IEU2NSFOgKF1Hhtyo9LAgg TXHv1XeYg==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.92 #3 (Red Hat Linux)) id 1i5F41-0006sN-ME; Tue, 03 Sep 2019 20:06:07 +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 39697301A76; Tue, 3 Sep 2019 22:05:27 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 7A1A12097777E; Tue, 3 Sep 2019 22:06:03 +0200 (CEST) Date: Tue, 3 Sep 2019 22:06:03 +0200 From: Peter Zijlstra To: Linus Torvalds Cc: "Eric W. Biederman" , Oleg Nesterov , Russell King - ARM Linux admin , Chris Metcalf , Christoph Lameter , Kirill Tkhai , Mike Galbraith , Thomas Gleixner , Ingo Molnar , Linux List Kernel Mailing , Davidlohr Bueso , "Paul E. McKenney" Subject: Re: [PATCH 2/3] task: RCU protect tasks on the runqueue Message-ID: <20190903200603.GW2349@hirez.programming.kicks-ass.net> References: <87tv9uiq9r.fsf@x220.int.ebiederm.org> <87k1aqt23r.fsf_-_@x220.int.ebiederm.org> <878sr6t21a.fsf_-_@x220.int.ebiederm.org> <20190903074117.GX2369@hirez.programming.kicks-ass.net> <20190903074718.GT2386@hirez.programming.kicks-ass.net> <87k1apqqgk.fsf@x220.int.ebiederm.org> <874l1tp7st.fsf@x220.int.ebiederm.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 Tue, Sep 03, 2019 at 12:18:47PM -0700, Linus Torvalds wrote: > Now, if you can point to some particular field where that ordering > makes sense for the particular case of "make it active on the > runqueue" vs "look up the task from the runqueue using RCU", then I do > think that the whole release->acquire consistency makes sense. > > But it's not clear that such a field exists, particularly when this is > in no way the *common* way to even get a task pointer, and other paths > do *not* use the runqueue as the serialization point. Even if we could find a case (and I'm not seeing one in a hurry), I would try really hard to avoid adding extra barriers here and instead make the consumer a little more complicated if at all possible. The Power folks got rid of a SYNC (yes, more expensive than LWSYNC) from their __switch_to() implementation and that had a measurable impact. 9145effd626d ("powerpc/64: Drop explicit hwsync in context switch")