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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS 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 76A29C43381 for ; Thu, 14 Mar 2019 18:38:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 52A8B2077B for ; Thu, 14 Mar 2019 18:38:37 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727123AbfCNSih (ORCPT ); Thu, 14 Mar 2019 14:38:37 -0400 Received: from foss.arm.com ([217.140.101.70]:48572 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727062AbfCNSig (ORCPT ); Thu, 14 Mar 2019 14:38:36 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 69F85A78; Thu, 14 Mar 2019 11:38:36 -0700 (PDT) Received: from [10.1.194.37] (e113632-lin.cambridge.arm.com [10.1.194.37]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 7D1FF3F59C; Thu, 14 Mar 2019 11:38:35 -0700 (PDT) Subject: Re: [PATCH 06/14] MIPS: entry: Remove unneeded need_resched() loop To: Paul Burton Cc: "linux-kernel@vger.kernel.org" , Ralf Baechle , James Hogan , "linux-mips@vger.kernel.org" References: <20190311224752.8337-1-valentin.schneider@arm.com> <20190311224752.8337-7-valentin.schneider@arm.com> <20190314181306.k6vxmaomyqalhi65@pburton-laptop> From: Valentin Schneider Message-ID: Date: Thu, 14 Mar 2019 18:38:33 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0 MIME-Version: 1.0 In-Reply-To: <20190314181306.k6vxmaomyqalhi65@pburton-laptop> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-mips-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-mips@vger.kernel.org Hi Paul, On 14/03/2019 18:13, Paul Burton wrote: [...] > > It looks to me like commit a18815abcdfd ("Use preempt_schedule_irq.") > forgot the branch to restore_all, so would have fallen through to > ret_from_fork() & done weird things. > > Adding the branch to restore_all as you're doing here would have been a > better fix than commit cdaed73afb61 ("Fix preemption bug."). > I didn't notice the missing branch to restore_all in that first commit - that makes (more) sense now. [...] >> @@ -66,7 +65,7 @@ need_resched: >> andi t0, 1 >> beqz t0, restore_all >> jal preempt_schedule_irq >> - b need_resched >> + j restore_all > > One nit - why change from branch to jump? No actual reason there, I most likely deleted the branch, looked around, saw the "j restore_all" in @resume_userspace and went for that (shoddy I know...) > It's not a big deal, but I'd > prefer we stick with the branch ("b") instruction for a few reasons: > > - restore_all is nearby so there's no issue with it being out of range > of a branch in any variation of the MIPS ISA. > > - It's more consistent with the future of the MIPS architecture, eg. > nanoMIPS in which branch instructions all use PC-relative immediate > offsets & jump instructions are always of the "register" variety where > the destination is specified by a register rather than an immediate > encoded in the instruction (the assembler will fix it up & emit a > branch anyway, but I generally prefer to invoke less magic in these > areas...). > > - A PC-relative branch won't generate an extra reloc in a relocatable > kernel, whereas a jump will. > Makes total sense, thanks for the detailed reasoning! > Even better would be if we take advantage of this being a tail call & do > this: > > PTR_LA ra, restore_all > j preempt_schedule_irq > > (Where I left the call to preempt_schedule_irq using a jump because it > may be further away.) > Right, that's even better, I'll send a v2 with that. > Though I don't mind if you wanna just s/j/b/ & leave the tail call > optimisation for someone else to do as a later change. > > Thanks, > Paul > >> #endif >> >> FEXPORT(ret_from_kernel_thread) >> -- >> 2.20.1 >>