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=-2.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,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 684EDC43387 for ; Wed, 16 Jan 2019 13:47:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 387BB206C2 for ; Wed, 16 Jan 2019 13:47:43 +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="EZkGKv7a" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2404494AbfAPNpd (ORCPT ); Wed, 16 Jan 2019 08:45:33 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:35196 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730798AbfAPNp3 (ORCPT ); Wed, 16 Jan 2019 08:45:29 -0500 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=6eRszGOnJ1la1Azov4nEHV7IY0vmubqaDui3/+NiF6U=; b=EZkGKv7aDXNdtvxPxhslpN+vY nZehewa6PyWX8aekEsbok7HQIphwutkcHVms608br0btw/Ada9mo+s8Ry1mDVApWmbLnclo2yVXEN gvinI8zt1gKF4QhvKz6Ltd9DlhdYP+X1NuFASjBYAyd02ybVw+Ni+GzvlbFfzdhDqWORAm78v3x/d A78tS7gJ0tLPPA/+FtcnUt0vO/rSFi0J+U4H0WHhz6M8LZr6+1mAWDCwqTB3AjwcyS+HrujQud50z ydaFhqzoyGRiP/8iqUUMU9j1POIoj1gEHgZrU44/n7jD181XT5oiDfd3c5uAfRFLYyamRhROQ3sDh i9GP4FctA==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1gjlVT-0001u3-3t; Wed, 16 Jan 2019 13:45:23 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 8A7082019521E; Wed, 16 Jan 2019 14:45:21 +0100 (CET) Date: Wed, 16 Jan 2019 14:45:21 +0100 From: Peter Zijlstra To: Valentin Schneider Cc: Ingo Molnar , linux-kernel , Catalin Marinas , Will Deacon , Mark Rutland , Julien Thierry , Thomas Gleixner Subject: Re: preempt_schedule_irq() loop question Message-ID: <20190116134521.GG10803@hirez.programming.kicks-ass.net> References: 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 Wed, Jan 16, 2019 at 12:50:42PM +0000, Valentin Schneider wrote: > Hi, > > I've been wandering around preempt_schedule_irq() in sched/core.c, and > got curious regarding how the arch code calls it. > > The main part of preempt_schedule_irq() is: > > do { > preempt_disable(); > local_irq_enable(); > __schedule(true); > local_irq_disable(); > sched_preempt_enable_no_resched(); > } while (need_resched()); > > Yet all the arch entry.S I looked at (I stopped after arm64, arm, x86_32, > MIPS, powerpc) wrap the call to preempt_schedule_irq() in another > > do { ... } while (need_resched()) > > For instance, this is what's done in arm64: > > 1: bl preempt_schedule_irq // irq en/disable is done inside > ldr x0, [tsk, #TSK_TI_FLAGS] // get new tasks TI_FLAGS > tbnz x0, #TIF_NEED_RESCHED, 1b // needs rescheduling? > > > I naively thought this could be attributed to something like > preempt_schedule_irq() historically not having an inner loop, but it seems > to have been there since the beginning of time (or at least up to the point > where the git history stops). > > I don't see why we need to have these nested loops - AFAICT the one in > preempt_schedule_irq() would suffice. What am I missing? I think you're quite right; but I wasn't doing kernel work back when rml added the preemptible bits. Ingo, do you have any recollections that far back?