linux-kernel - Re: [PATCH 1/1] arm64: enable PREEMPT

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Message-ID: <87cyev63tt.ffs@tglx>
Date: Wed, 05 Mar 2025 13:48:30 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Valentin Schneider <vschneid@...hat.com>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-rt-users@...r.kernel.org
Cc: Mark Rutland <mark.rutland@....com>, Mike Galbraith <efault@....de>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Peter Collingbourne <pcc@...gle.com>, Neeraj Upadhyay
 <neeraj.upadhyay@...nel.org>, Thiago Jung Bauermann
 <thiago.bauermann@...aro.org>, Mark Brown <broonie@...nel.org>, Kristina
 Martsenko <kristina.martsenko@....com>, Sebastian Andrzej Siewior
 <bigeasy@...utronix.de>, Petr Tesarik <ptesarik@...e.com>, Jinjie Ruan
 <ruanjinjie@...wei.com>, Juri Lelli <juri.lelli@...hat.com>, Phil Auld
 <pauld@...hat.com>
Subject: Re: [PATCH 1/1] arm64: enable PREEMPT_LAZY

On Wed, Mar 05 2025 at 11:49, Valentin Schneider wrote:
> From: Mark Rutland <mark.rutland@....com>
>
> For an architecture to enable CONFIG_ARCH_HAS_RESCHED_LAZY, two things are
> required:
> 1) Adding a TIF_NEED_RESCHED_LAZY flag definition
> 2) Checking for TIF_NEED_RESCHED_LAZY in the appropriate locations
>
> 2) is handled in a generic manner by CONFIG_GENERIC_ENTRY, which isn't
> (yet) implemented for arm64. However, outside of core scheduler code,
> TIF_NEED_RESCHED_LAZY only needs to be checked on a kernel exit, meaning:
> o return/entry to userspace.
> o return/entry to guest.
>
> The return/entry to a guest is all handled by xfer_to_guest_mode_handle_work()
> which already does the right thing, so it can be left as-is.
>
> arm64 doesn't use common entry's exit_to_user_mode_prepare(), so update its
> return to user path to check for TIF_NEED_RESCHED_LAZY and call into
> schedule() accordingly.
>
> Link: https://lore.kernel.org/linux-rt-users/20241216190451.1c61977c@mordecai.tesarici.cz/
> Link: https://lore.kernel.org/all/xhsmh4j0fl0p3.mognet@vschneid-thinkpadt14sgen2i.remote.csb/
> Signed-off-by: Mark Rutland <mark.rutland@....com>
> [testdrive, _TIF_WORK_MASK fixlet and changelog.]
> Signed-off-by: Mike Galbraith <efault@....de>
> [Another round of testing; changelog faff]
> Signed-off-by: Valentin Schneider <vschneid@...hat.com>

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>