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, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED 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 C2046C2D0A3 for ; Mon, 9 Nov 2020 14:46:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6E1E6206E3 for ; Mon, 9 Nov 2020 14:46:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731819AbgKIOqL (ORCPT ); Mon, 9 Nov 2020 09:46:11 -0500 Received: from muru.com ([72.249.23.125]:47530 "EHLO muru.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730649AbgKIOqL (ORCPT ); Mon, 9 Nov 2020 09:46:11 -0500 Received: from atomide.com (localhost [127.0.0.1]) by muru.com (Postfix) with ESMTPS id 9223E80CD; Mon, 9 Nov 2020 14:46:13 +0000 (UTC) Date: Mon, 9 Nov 2020 16:45:49 +0200 From: Tony Lindgren To: Arnd Bergmann Cc: Russell King - ARM Linux admin , v.narang@samsung.com, a.sahrawat@samsung.com, Marc Zyngier , Sebastian Andrzej Siewior , Vincent Whitchurch , Nick Desaulniers , "linux-kernel@vger.kernel.org" , Nathan Huckleberry , Jian Cai , Thomas Gleixner , Dmitry Safonov <0x7f454c46@gmail.com>, Maninder Singh , Andrew Morton , Will Deacon , Valentin Schneider , Linux ARM Subject: Re: [PATCH 2/3] arm: introduce IRQ stacks Message-ID: <20201109144549.GA26857@atomide.com> References: <1602141333-17822-1-git-send-email-maninder1.s@samsung.com> <1602141333-17822-3-git-send-email-maninder1.s@samsung.com> <20201021124542.GL1551@shell.armlinux.org.uk> <20201021125740.GM1551@shell.armlinux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Arnd Bergmann [201021 16:07]: > On Wed, Oct 21, 2020 at 2:57 PM Russell King - ARM Linux admin > wrote: > > On Wed, Oct 21, 2020 at 01:45:42PM +0100, Russell King - ARM Linux admin wrote: > > > > > - define 'current' as 'this_cpu_read_stable(current_task);' > > > > > - convert to CONFIG_THREAD_INFO_IN_TASK > > > > > > That means we need to also code that up in assembly - remember, we > > > need to access thread_info from assembly code. > > > > Note also that there is a circular dependency involved. If you make > > thread_info accessible via per-cpu, then: > > > > #ifndef __my_cpu_offset > > #define __my_cpu_offset per_cpu_offset(raw_smp_processor_id()) > > #endif > > #ifdef CONFIG_DEBUG_PREEMPT > > #define my_cpu_offset per_cpu_offset(smp_processor_id()) > > #else > > #define my_cpu_offset __my_cpu_offset > > #endif > > Right, I had missed the fallback path using asm-generic/percpu.h > that is used with CONFIG_SMP && CONFIG_CPU_V6 > Almost everything either uses fixed percpu data (on UP builds) > or TPIDRPRW when building a v7-only or v6k/v7 kernel without > v6 support. > > > smp_processor_id() ultimately ends up as raw_smp_processor_id() which > > is: > > > > #define raw_smp_processor_id() (current_thread_info()->cpu) > > > > and if current_thread_info() itself involves reading from per-cpu data, > > we end up recursing... infinitely. > > > > This is why I said in the other thread: > > > > "We don't do it because we don't have a separate register to be able > > to store the thread_info pointer, and copying that lump between the > > SVC and IRQ stack will add massively to IRQ latency, especially for > > older machines." > > As discussed on IRC, I think it can still be done in one of these > ways, though admittedly none of them are perfect: > > a) add runtime patching for __my_cpu_offset() when > CONFIG_SMP_ON_UP is set. This adds complexity but avoids the > fallback for for SMP&&CPU_V6. It possibly also speeds up > running on single-cpu systems if the TPIDRPRW access adds > any measurable runtime overhead compared to patching it out. Out of these options a) sounds best to me. > b) If irq stacks are left as a compile-time option, that could be > made conditional on "!(SMP&&CPU_V6)". Presumably very > few people still run kernels built that way any more. The only > supported platforms are i.MX3, OMAP2 and Realview-eb, all of > which are fairly uncommon these days and would usually > run v6-only non-SMP kernels. This has been working just fine for years though. In general, removing the conditional compile ifdefferey has made things quite a bit easier for us, so let's continue on that. > c) If we decide that we no longer care about that configuration > at all, we could decide to just make SMP depend on !CPU_V6, > and possibly kill off the entire SMP_ON_UP patching logic. > I suspect we still want to keep SMP_ON_UP for performance > reasons, but I don't know how significant they are to start with. And this too has been working just fine for years :) Regards, Tony