Re: [PATCH 17/30] x86/thread_info: define TIF_NEED_RESCHED_LAZY

[Mark Rutland]

Hi Ankur,

On Mon, Feb 12, 2024 at 09:55:41PM -0800, Ankur Arora wrote:
> Define TIF_NEED_RESCHED_LAZY which, with TIF_NEED_RESCHED provides the
> scheduler with two kinds of rescheduling intent: TIF_NEED_RESCHED,
> for the usual rescheduling at the next safe preemption point;
> TIF_NEED_RESCHED_LAZY expressing an intent to reschedule at some
> time in the future while allowing the current task to run to
> completion.
> 
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Originally-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Link: https://lore.kernel.org/lkml/87jzshhexi.ffs@tglx/
> Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
> ---
>  arch/x86/Kconfig                   |  1 +
>  arch/x86/include/asm/thread_info.h | 10 ++++++++--
>  2 files changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 5edec175b9bf..ab58558068a4 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -275,6 +275,7 @@ config X86
>  	select HAVE_STATIC_CALL
>  	select HAVE_STATIC_CALL_INLINE		if HAVE_OBJTOOL
>  	select HAVE_PREEMPT_DYNAMIC_CALL
> +	select HAVE_PREEMPT_AUTO
>  	select HAVE_RSEQ
>  	select HAVE_RUST			if X86_64
>  	select HAVE_SYSCALL_TRACEPOINTS
> diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
> index d63b02940747..88c1802185fc 100644
> --- a/arch/x86/include/asm/thread_info.h
> +++ b/arch/x86/include/asm/thread_info.h
> @@ -81,8 +81,11 @@ struct thread_info {
>  #define TIF_NOTIFY_RESUME	1	/* callback before returning to user */
>  #define TIF_SIGPENDING		2	/* signal pending */
>  #define TIF_NEED_RESCHED	3	/* rescheduling necessary */
> -#define TIF_SINGLESTEP		4	/* reenable singlestep on user return*/
> -#define TIF_SSBD		5	/* Speculative store bypass disable */
> +#ifdef CONFIG_PREEMPT_AUTO
> +#define TIF_NEED_RESCHED_LAZY	4	/* Lazy rescheduling */
> +#endif
> +#define TIF_SINGLESTEP		5	/* reenable singlestep on user return*/
> +#define TIF_SSBD		6	/* Speculative store bypass disable */

It's a bit awkward/ugly to conditionally define the TIF_* bits in arch code,
and we don't do that for other bits that are only used in some configurations
(e.g. TIF_UPROBE). That's not just for aesthetics -- for example, on arm64 we
try to keep the TIF_WORK_MASK bits contiguous, which is difficult if a bit in
the middle doesn't exist in some configurations.

Is it painful to organise the common code so that arch code can define
TIF_NEED_RESCHED_LAZY regardless of whether CONFIG_PREEMPT_AUTO is selected?

Mark.

>  #define TIF_SPEC_IB		9	/* Indirect branch speculation mitigation */
>  #define TIF_SPEC_L1D_FLUSH	10	/* Flush L1D on mm switches (processes) */
>  #define TIF_USER_RETURN_NOTIFY	11	/* notify kernel of userspace return */
> @@ -104,6 +107,9 @@ struct thread_info {
>  #define _TIF_NOTIFY_RESUME	(1 << TIF_NOTIFY_RESUME)
>  #define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
>  #define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)
> +#ifdef CONFIG_PREEMPT_AUTO
> +#define _TIF_NEED_RESCHED_LAZY	(1 << TIF_NEED_RESCHED_LAZY)
> +#endif
>  #define _TIF_SINGLESTEP		(1 << TIF_SINGLESTEP)
>  #define _TIF_SSBD		(1 << TIF_SSBD)
>  #define _TIF_SPEC_IB		(1 << TIF_SPEC_IB)
> -- 
> 2.31.1
>