Re: [PATCH v2 04/35] preempt: introduce CONFIG_PREEMPT_AUTO

Shrikanth Hegde
2024-06-03T15:05:32Z

On 5/28/24 6:04 AM, Ankur Arora wrote:
> PREEMPT_AUTO adds a new scheduling model which, like PREEMPT_DYNAMIC,
> allows dynamic switching between a none/voluntary/full preemption
> model. However, unlike PREEMPT_DYNAMIC, it doesn't use explicit
> preemption points for the voluntary models.
>
> It works by depending on CONFIG_PREEMPTION (and thus PREEMPT_COUNT),
> allowing the scheduler to always know when it is safe to preempt
> for all three preemption models.
>
> In addition, it uses an additional need-resched bit
> (TIF_NEED_RESCHED_LAZY) which, with TIF_NEED_RESCHED allows the
> scheduler to express two kinds of rescheduling intent: schedule at
> the earliest opportunity (the usual TIF_NEED_RESCHED semantics), or
> express a need for rescheduling while allowing the task on the
> runqueue to run to timeslice completion (TIF_NEED_RESCHED_LAZY).
>
> The scheduler chooses the specific need-resched flag based on
> the preemption model:
>
> TIF_NEED_RESCHED TIF_NEED_RESCHED_LAZY
>
> none never always [*]
> voluntary higher sched class other tasks [*]
> full always never
>
> [*] when preempting idle, or for kernel tasks that are 'urgent' in
> some way (ex. resched_cpu() used as an RCU hammer), we use
> TIF_NEED_RESCHED.
>
> The other part is when preemption happens -- or, when are the
> need-resched flags checked:
>
> exit-to-user ret-to-kernel preempt_count()
> NEED_RESCHED_LAZY Y N N
> NEED_RESCHED Y Y Y
>
> Exposed under CONFIG_EXPERT for now.
>
> Cc: Peter Ziljstra <peterz@infradead.org>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Originally-by: Thomas Gleixner <tglx@linutronix.de>
> Link: https://lore.kernel.org/lkml/87jzshhexi.ffs@tglx/
> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
> ---
> .../admin-guide/kernel-parameters.txt | 1 +
> include/linux/thread_info.h | 12 ++++++
> init/Makefile | 1 +
> kernel/Kconfig.preempt | 37 +++++++++++++++++--
> 4 files changed, 48 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 2d693300ab57..16a91090d167 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -4719,6 +4719,7 @@
>
> preempt= [KNL]
> Select preemption mode if you have CONFIG_PREEMPT_DYNAMIC
> + or CONFIG_PREEMPT_AUTO.
> none - Limited to cond_resched() calls
> voluntary - Limited to cond_resched() and might_sleep() calls
> full - Any section that isn't explicitly preempt disabled
> diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
> index 9ea0b28068f4..06e13e7acbe2 100644
> --- a/include/linux/thread_info.h
> +++ b/include/linux/thread_info.h
> @@ -59,6 +59,18 @@ enum syscall_work_bit {
>
> #include <asm/thread_info.h>
>
> +/*
> + * Fall back to the default need-resched flag when an architecture does not
> + * define TIF_NEED_RESCHED_LAZY.
> + *
> + * Note: with !PREEMPT_AUTO, code should not be setting TIF_NEED_RESCHED_LAZY
> + * anywhere. Define this here because we will explicitly test for this bit.
> + */

Is this comment still valid?
I see that flag has been set without any checks in arch file.

> +#ifndef TIF_NEED_RESCHED_LAZY
> +#define TIF_NEED_RESCHED_LAZY TIF_NEED_RESCHED
> +#define _TIF_NEED_RESCHED_LAZY _TIF_NEED_RESCHED
> +#endif
> +
> #ifdef __KERNEL__
>
> #ifndef arch_set_restart_data