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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 67485C4167B for ; Thu, 9 Nov 2023 11:00:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233485AbjKILAy (ORCPT ); Thu, 9 Nov 2023 06:00:54 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46154 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231703AbjKILAw (ORCPT ); Thu, 9 Nov 2023 06:00:52 -0500 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 4F7072702 for ; Thu, 9 Nov 2023 03:00:49 -0800 (PST) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id E4B7512FC; Thu, 9 Nov 2023 03:01:33 -0800 (PST) Received: from FVFF77S0Q05N (unknown [10.57.37.150]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 544683F64C; Thu, 9 Nov 2023 03:00:44 -0800 (PST) Date: Thu, 9 Nov 2023 11:00:38 +0000 From: Mark Rutland To: Ankur Arora Cc: linux-kernel@vger.kernel.org, tglx@linutronix.de, peterz@infradead.org, torvalds@linux-foundation.org, paulmck@kernel.org, linux-mm@kvack.org, x86@kernel.org, akpm@linux-foundation.org, luto@kernel.org, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, willy@infradead.org, mgorman@suse.de, jon.grimm@amd.com, bharata@amd.com, raghavendra.kt@amd.com, boris.ostrovsky@oracle.com, konrad.wilk@oracle.com, jgross@suse.com, andrew.cooper3@citrix.com, mingo@kernel.org, bristot@kernel.org, mathieu.desnoyers@efficios.com, geert@linux-m68k.org, glaubitz@physik.fu-berlin.de, anton.ivanov@cambridgegreys.com, mattst88@gmail.com, krypton@ulrich-teichert.org, rostedt@goodmis.org, David.Laight@aculab.com, richard@nod.at, mjguzik@gmail.com Subject: Re: [RFC PATCH 00/86] Make the kernel preemptible Message-ID: References: <20231107215742.363031-1-ankur.a.arora@oracle.com> <87o7g3epcu.fsf@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87o7g3epcu.fsf@oracle.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Nov 08, 2023 at 04:34:41PM -0800, Ankur Arora wrote: > Mark Rutland writes: > > > On Tue, Nov 07, 2023 at 01:56:46PM -0800, Ankur Arora wrote: > >> What's broken: > >> - ARCH_NO_PREEMPT (See patch-45 "preempt: ARCH_NO_PREEMPT only preempts > >> lazily") > >> - Non-x86 architectures. It's trivial to support other archs (only need > >> to add TIF_NEED_RESCHED_LAZY) but wanted to hold off until I got some > >> comments on the series. > >> (From some testing on arm64, didn't find any surprises.) > > > > When you say "testing on arm64, didn't find any surprises", I assume you mean > > with an additional patch adding TIF_NEED_RESCHED_LAZY? > > Yeah. And, handling that in the user exit path. > > > Note that since arm64 doesn't use the generic entry code, that also requires > > changes to arm64_preempt_schedule_irq() in arch/arm64/kernel/entry-common.c, to > > handle TIF_NEED_RESCHED_LAZY. > > So, the intent (which got muddied due to this overly large series) > was to delay handling TIF_NEED_RESCHED_LAZY until we are about to > return to user. Ah, I missed that detail -- thanks for clarifying! > I think arm64_preempt_schedule_irq() should only handle TIF_NEED_RESCHED > and the _TIF_NEED_RESCHED_LAZY should be handled via _TIF_WORK_MASK > and do_notify_resume(). Digging a bit more, I think that should still work. One slight clarification: arm64_preempt_schedule_irq() doesn't look at TIF_NEED_RESCHED today, as it relies on the scheduler IPI calling preempt_fold_need_resched() to propogate TIF_NEED_RESCHED into PREEMPT_NEED_RESCHED. That should still work since this series makes preempt_fold_need_resched() check tif_need_resched(RESCHED_eager). I was a bit cnofused because in the generic entry code, irqentry_exit_cond_resched() explicitly checks for TIF_NEED_RESCHED, and I'm not sure why it does that rather than relying on the scheduler IPI as above. > (The design is much clearer in Thomas' PoC: > https://lore.kernel.org/lkml/87jzshhexi.ffs@tglx/) > > >> - ftrace support for need-resched-lazy is incomplete > > > > What exactly do we need for ftrace here? > > Only support for TIF_NEED_RESCHED_LAZY which should be complete. > That comment was based on a misreading of the code. Cool; thanks! Mark.