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From: Ankur Arora <ankur.a.arora@oracle.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Ankur Arora <ankur.a.arora@oracle.com>, linux-kernel@vger.kernel.org,
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Subject: Re: [PATCH 17/30] x86/thread_info: define TIF_NEED_RESCHED_LAZY
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Date: Wed, 14 Feb 2024 12:31:29 -0800
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Mark Rutland <mark.rutland@arm.com> writes:

> 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@redhat.com>
>> Cc: Borislav Petkov <bp@alien8.de>
>> Cc: Dave Hansen <dave.hansen@linux.intel.com>
>> 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>
>> ---
>>  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.

That's useful to know. And, I think you are right about the
ugliness of this.

> 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?

So, the original reason I did it this way was because I wanted to have
zero performance impact on !CONFIG_PREEMPT_AUTO configurations whether
TIF_NEED_RESCHED_LAZY was defined or not.
(I was doing some computation with TIF_NEED_RESCHED_LAZY at that point.)

Eventually I changed that part of code but this stayed.

Anyway, this should be easy enough to fix with done #ifdefry.

Thanks for reviewing.

--
ankur