diff mbox series

[17/30] x86/thread_info: define TIF_NEED_RESCHED_LAZY

Message ID 20240213055554.1802415-18-ankur.a.arora@oracle.com
State New
Headers
Series PREEMPT_AUTO: support lazy rescheduling |

Commit Message

Ankur Arora Feb. 13, 2024, 5:55 a.m. UTC 
  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(-)

Comments

Mark Rutland Feb. 14, 2024, 1:25 p.m. UTC | #1 
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.

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
>
Ankur Arora Feb. 14, 2024, 8:31 p.m. UTC | #2 
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
Mark Rutland Feb. 19, 2024, 12:32 p.m. UTC | #3 
On Wed, Feb 14, 2024 at 12:31:29PM -0800, Ankur Arora wrote:
> Mark Rutland <mark.rutland@arm.com> writes:
> > On Mon, Feb 12, 2024 at 09:55:41PM -0800, Ankur Arora wrote:

> >> 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.

Great!

BTW, the series overall looks to be in very good shape; thanks a lot for
working on this!

Mark.
diff mbox series

Patch

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 */
 #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)