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Date:   Mon, 24 Jul 2023 10:59:42 +0100
From:   Conor Dooley <conor.dooley@microchip.com>
To:     Jisheng Zhang <jszhang@kernel.org>
CC:     Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Arnd Bergmann <arnd@arndb.de>,
        <linux-riscv@lists.infradead.org>, <linux-kernel@vger.kernel.org>,
        <linux-arch@vger.kernel.org>
Subject: Re: [PATCH v2] riscv: support PREEMPT_DYNAMIC with static keys
Message-ID: <20230724-work-headboard-b1a4f5286ada@wendy>
References: <20230716164925.1858-1-jszhang@kernel.org>
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On Mon, Jul 17, 2023 at 12:49:25AM +0800, Jisheng Zhang wrote:
> Currently, each architecture can support PREEMPT_DYNAMIC through
> either static calls or static keys. To support PREEMPT_DYNAMIC on
> riscv, we face three choices:
>=20
> 1. only add static calls support to riscv
> As Mark pointed out in commit 99cf983cc8bc ("sched/preempt: Add
> PREEMPT_DYNAMIC using static keys"), static keys "...should have
> slightly lower overhead than non-inline static calls, as this
> effectively inlines each trampoline into the start of its callee. This
> may avoid redundant work, and may integrate better with CFI schemes."
> So even we add static calls(without inline static calls) to riscv,
> static keys is still a better choice.
>=20
> 2. add static calls and inline static calls to riscv
> Per my understanding, inline static calls requires objtool support
> which is not easy.
>=20
> 3. use static keys
>=20
> While riscv doesn't have static calls support, it supports static keys
> perfectly. So this patch selects HAVE_PREEMPT_DYNAMIC_KEY to enable
> support for PREEMPT_DYNAMIC on riscv, so that the preemption model can
> be chosen at boot time. It also patches asm-generic/preempt.h, mainly
> to add __preempt_schedule() and __preempt_schedule_notrace() macros
> for PREEMPT_DYNAMIC case. Other architectures which use generic
> preempt.h can also benefit from this patch by simply selecting
> HAVE_PREEMPT_DYNAMIC_KEY to enable PREEMPT_DYNAMIC if they supports
> static keys.
>=20
> Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
> ---
> since v1:
>  - keep Kconfig entries sorted
>  - group asm-generic modifications under CONFIG_PREEMPT_DYNAMIC &&
>    CONFIG_HAVE_PREEMPT_DYNAMIC_KEY)
>=20
>  arch/riscv/Kconfig            |  1 +
>  include/asm-generic/preempt.h | 14 +++++++++++++-
>  2 files changed, 14 insertions(+), 1 deletion(-)
>=20
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 4c07b9189c86..686df6902947 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -130,6 +130,7 @@ config RISCV
>  	select HAVE_PERF_REGS
>  	select HAVE_PERF_USER_STACK_DUMP
>  	select HAVE_POSIX_CPU_TIMERS_TASK_WORK
> +	select HAVE_PREEMPT_DYNAMIC_KEY if !XIP_KERNEL

Had a go of this, and it seems fine to me, as do the asm-generic bits
seem fine from a single arch perspective.
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>

Thanks,
Conor.

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