From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 0CC352F5B for ; Mon, 16 Dec 2024 19:04:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1734375893; cv=none; b=ehJEvf5KBXlBjf1R2fh9guaAy8bigxFex89zmXwKk6naZVqHcZThrKpU5Zs1RJ1s1CNSCG6iE+Fz9LGO/V/A5E3Y5QbRgQCz4JlGufP3WYI2VnvyXiZOxnFnPnXs5EJndm2JpAjtIDnXUtPQKHuxr3KGPA0C7ObLfs+nnqDGcuY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1734375893; c=relaxed/simple; bh=pryFWyOFOEDHk615hWgPzFh8BmqBeR4WY5a979bgRnM=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=PYmesMEKhnKZS9dfRLUkJfi3SPwKG/+79Rder/LErUTL7rBlPVg6khvGskGg3SlHzubIQCXmtG3AxqcB+KFOqLvE5gXuMoS7mQtcNeLNcTyrXQYuiroVYXbOhxk7PIy4QR6ZANM3zq4xEwqvh/2TUGqWXMSpvXdHjCxt9AG06Hk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com 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 1A06D11FB; Mon, 16 Dec 2024 11:05:17 -0800 (PST) Received: from J2N7QTR9R3.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 7663B3F58B; Mon, 16 Dec 2024 11:04:48 -0800 (PST) Date: Mon, 16 Dec 2024 19:04:43 +0000 From: Mark Rutland To: Petr Tesarik Cc: linux-rt-users@vger.kernel.org Subject: Re: Lazy preemption on arm64 Message-ID: References: <20241216190451.1c61977c@mordecai.tesarici.cz> Precedence: bulk X-Mailing-List: linux-rt-users@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20241216190451.1c61977c@mordecai.tesarici.cz> On Mon, Dec 16, 2024 at 07:04:51PM +0100, Petr Tesarik wrote: > Hi all, > > what is the plan for implementing PREEMPT_LAZY on arm64? > > There used to be RT patch series which enabled lazy preemption on > arm64, but this architecture was "sacrificed" in v6.6-rc6-rt10, as > collateral damage of switching to PREEMPT_AUTO. > > IIUC lazy preemption is currently implemented only for architectures > with CONFIG_GENERIC_ENTRY, but there is no inherent dependency on it. > So, is the plan to convert arm64 to GENERIC_ENTRY (and then get > PREEMPT_LAZY for free), or is somebody working on CONFIG_PREEMPT_LAZY > for arm64 without that conversion? I don't think there's an agreed upon plan either way. Jinjie Ruan has been looking to move arm64 over to GENERIC_ENTRY: https://lore.kernel.org/all/20241206101744.4161990-1-ruanjinjie@huawei.com/ AFAICT, the only bits that we get "for free" from GENERIC_ENTRY would be the logic in raw_irqentry_exit_cond_resched() and exit_to_user_mode_loop(), and all we'd need to enable this on arm64 as-is would be as below. ... so how important is this? Mark. ---->8---- >From 16ae3c84d8e1691fe670dbfb4c643cab4fa2065f Mon Sep 17 00:00:00 2001 From: Mark Rutland Date: Mon, 16 Dec 2024 18:32:44 +0000 Subject: [PATCH] HACK: arm64: enable PREEMPT_LAZY Signed-off-by: Mark Rutland --- arch/arm64/Kconfig | 1 + arch/arm64/include/asm/thread_info.h | 12 +++++++----- arch/arm64/kernel/entry-common.c | 2 +- 3 files changed, 9 insertions(+), 6 deletions(-) diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig index 100570a048c5e..7926bc78a1c46 100644 --- a/arch/arm64/Kconfig +++ b/arch/arm64/Kconfig @@ -39,6 +39,7 @@ config ARM64 select ARCH_HAS_NMI_SAFE_THIS_CPU_OPS select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE select ARCH_HAS_NONLEAF_PMD_YOUNG if ARM64_HAFT + select ARCH_HAS_PREEMPT_LAZY select ARCH_HAS_PTE_DEVMAP select ARCH_HAS_PTE_SPECIAL select ARCH_HAS_HW_PTE_YOUNG diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h index 1114c1c3300a1..a2125407901ed 100644 --- a/arch/arm64/include/asm/thread_info.h +++ b/arch/arm64/include/asm/thread_info.h @@ -59,11 +59,12 @@ void arch_setup_new_exec(void); #define TIF_SIGPENDING 0 /* signal pending */ #define TIF_NEED_RESCHED 1 /* rescheduling necessary */ -#define TIF_NOTIFY_RESUME 2 /* callback before returning to user */ -#define TIF_FOREIGN_FPSTATE 3 /* CPU's FP state is not current's */ -#define TIF_UPROBE 4 /* uprobe breakpoint or singlestep */ -#define TIF_MTE_ASYNC_FAULT 5 /* MTE Asynchronous Tag Check Fault */ -#define TIF_NOTIFY_SIGNAL 6 /* signal notifications exist */ +#define TIF_NEED_RESCHED_LAZY 2 /* Lazy rescheduling needed */ +#define TIF_NOTIFY_RESUME 3 /* callback before returning to user */ +#define TIF_FOREIGN_FPSTATE 4 /* CPU's FP state is not current's */ +#define TIF_UPROBE 5 /* uprobe breakpoint or singlestep */ +#define TIF_MTE_ASYNC_FAULT 6 /* MTE Asynchronous Tag Check Fault */ +#define TIF_NOTIFY_SIGNAL 7 /* signal notifications exist */ #define TIF_SYSCALL_TRACE 8 /* syscall trace active */ #define TIF_SYSCALL_AUDIT 9 /* syscall auditing */ #define TIF_SYSCALL_TRACEPOINT 10 /* syscall tracepoint for ftrace */ @@ -85,6 +86,7 @@ void arch_setup_new_exec(void); #define _TIF_SIGPENDING (1 << TIF_SIGPENDING) #define _TIF_NEED_RESCHED (1 << TIF_NEED_RESCHED) +#define _TIF_NEED_RESCHED_LAZY (1 << TIF_NEED_RESCHED_LAZY) #define _TIF_NOTIFY_RESUME (1 << TIF_NOTIFY_RESUME) #define _TIF_FOREIGN_FPSTATE (1 << TIF_FOREIGN_FPSTATE) #define _TIF_SYSCALL_TRACE (1 << TIF_SYSCALL_TRACE) diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c index b260ddc4d3e9a..7993fab0cab4c 100644 --- a/arch/arm64/kernel/entry-common.c +++ b/arch/arm64/kernel/entry-common.c @@ -132,7 +132,7 @@ static void do_notify_resume(struct pt_regs *regs, unsigned long thread_flags) do { local_irq_enable(); - if (thread_flags & _TIF_NEED_RESCHED) + if (thread_flags & (_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY)) schedule(); if (thread_flags & _TIF_UPROBE) -- 2.30.2