config SCHED_CORE

config SCHED_CORE

[Hugh Dickins]

Hi Peter,

make oldconfig gave me no help at all on how to decide whether to choose
SCHED_CORE Y or n, beyond it recommending Y.  Maybe you'll delete that
option later, or maybe removing the prompt string would silence it.

I decided that I probably need kernel/sched/core.o, so chose Y :)

Hugh

Re: config SCHED_CORE

[Peter Zijlstra]

On Thu, May 20, 2021 at 08:06:07PM -0700, Hugh Dickins wrote:
> Hi Peter,
> 
> make oldconfig gave me no help at all on how to decide whether to choose
> SCHED_CORE Y or n, beyond it recommending Y.  Maybe you'll delete that
> option later, or maybe removing the prompt string would silence it.

Ah, you're quite right. I never seem to have gotten around to actually
writing anything useful there :/ Similarly the documentation for all
this seems to have gone missing too.

Joel, could I ask you to refresh the document to match the current state
of things and repost? I still whole hartedly despise this RST crud, it
makes it so hard to read / modify the files.

( I think the latest version is here:
  https://lkml.kernel.org/r/20210324214020.34142-8-joel@joelfernandes.org
)

Anyway, how is something like the below, Joel can add a reference to the
document once it's there.

---
 kernel/Kconfig.preempt | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index ea1e3331c0ba..3c4566cd20ef 100644
--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -104,4 +104,16 @@ config SCHED_CORE
 	bool "Core Scheduling for SMT"
 	default y
 	depends on SCHED_SMT
-
+	help
+	  This option enables Core scheduling, a means of coordinated task
+	  selection across SMT siblings with the express purpose of creating a
+	  Core wide privilidge boundary. When enabled -- see prctl(PR_SCHED_CORE)
+	  -- task selection will ensure all SMT siblings will execute a task
+	  from the same 'core group', forcing idle when no matching task is found.
+
+	  This provides means of mitigation against a number of SMT side-channels;
+	  but is, on its own, insufficient to mitigate all known side-channels.
+	  Notable: the MDS class of attacks require more.
+
+	  Default enabled for anything that has SCHED_SMT, when unused there should
+	  be no impact on performance.

Re: config SCHED_CORE

[Joel Fernandes]

On Fri, May 21, 2021 at 3:53 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, May 20, 2021 at 08:06:07PM -0700, Hugh Dickins wrote:
> > Hi Peter,
> >
> > make oldconfig gave me no help at all on how to decide whether to choose
> > SCHED_CORE Y or n, beyond it recommending Y.  Maybe you'll delete that
> > option later, or maybe removing the prompt string would silence it.
>
> Ah, you're quite right. I never seem to have gotten around to actually
> writing anything useful there :/ Similarly the documentation for all
> this seems to have gone missing too.
>
> Joel, could I ask you to refresh the document to match the current state
> of things and repost? I still whole hartedly despise this RST crud, it
> makes it so hard to read / modify the files.
>
> ( I think the latest version is here:
>   https://lkml.kernel.org/r/20210324214020.34142-8-joel@joelfernandes.org
> )
>
> Anyway, how is something like the below, Joel can add a reference to the
> document once it's there.

Sure thing, Peter! I will work on it and post a patch.

- Joel


>
> ---
>  kernel/Kconfig.preempt | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
> index ea1e3331c0ba..3c4566cd20ef 100644
> --- a/kernel/Kconfig.preempt
> +++ b/kernel/Kconfig.preempt
> @@ -104,4 +104,16 @@ config SCHED_CORE
>         bool "Core Scheduling for SMT"
>         default y
>         depends on SCHED_SMT
> -
> +       help
> +         This option enables Core scheduling, a means of coordinated task
> +         selection across SMT siblings with the express purpose of creating a
> +         Core wide privilidge boundary. When enabled -- see prctl(PR_SCHED_CORE)
> +         -- task selection will ensure all SMT siblings will execute a task
> +         from the same 'core group', forcing idle when no matching task is found.
> +
> +         This provides means of mitigation against a number of SMT side-channels;
> +         but is, on its own, insufficient to mitigate all known side-channels.
> +         Notable: the MDS class of attacks require more.
> +
> +         Default enabled for anything that has SCHED_SMT, when unused there should
> +         be no impact on performance.

Re: config SCHED_CORE

[Joel Fernandes]

On Fri, May 21, 2021 at 3:53 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, May 20, 2021 at 08:06:07PM -0700, Hugh Dickins wrote:
> > Hi Peter,
> >
> > make oldconfig gave me no help at all on how to decide whether to choose
> > SCHED_CORE Y or n, beyond it recommending Y.  Maybe you'll delete that
> > option later, or maybe removing the prompt string would silence it.
>
> Ah, you're quite right. I never seem to have gotten around to actually
> writing anything useful there :/ Similarly the documentation for all
> this seems to have gone missing too.
>
> Joel, could I ask you to refresh the document to match the current state
> of things and repost? I still whole hartedly despise this RST crud, it
> makes it so hard to read / modify the files.
>
> ( I think the latest version is here:
>   https://lkml.kernel.org/r/20210324214020.34142-8-joel@joelfernandes.org
> )
>
> Anyway, how is something like the below, Joel can add a reference to the
> document once it's there.
>
> ---
>  kernel/Kconfig.preempt | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
> index ea1e3331c0ba..3c4566cd20ef 100644
> --- a/kernel/Kconfig.preempt
> +++ b/kernel/Kconfig.preempt
> @@ -104,4 +104,16 @@ config SCHED_CORE
>         bool "Core Scheduling for SMT"
>         default y
>         depends on SCHED_SMT
> -
> +       help
> +         This option enables Core scheduling, a means of coordinated task
> +         selection across SMT siblings with the express purpose of creating a
> +         Core wide privilidge boundary. When enabled -- see prctl(PR_SCHED_CORE)
> +         -- task selection will ensure all SMT siblings will execute a task
> +         from the same 'core group', forcing idle when no matching task is found.
> +
> +         This provides means of mitigation against a number of SMT side-channels;
> +         but is, on its own, insufficient to mitigate all known side-channels.
> +         Notable: the MDS class of attacks require more.
> +
> +         Default enabled for anything that has SCHED_SMT, when unused there should
> +         be no impact on performance.

This description sort of makes it sound like security is the only
usecase. Perhaps we can also add here that core-scheduling can help
performance of workloads where hyperthreading is undesired, such as
when VM providers don't want to share hyperthreads.

Thoughts?

Re: config SCHED_CORE

[Peter Zijlstra]

On Fri, May 21, 2021 at 07:57:35AM -0400, Joel Fernandes wrote:

> > ---
> >  kernel/Kconfig.preempt | 14 +++++++++++++-
> >  1 file changed, 13 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
> > index ea1e3331c0ba..3c4566cd20ef 100644
> > --- a/kernel/Kconfig.preempt
> > +++ b/kernel/Kconfig.preempt
> > @@ -104,4 +104,16 @@ config SCHED_CORE
> >         bool "Core Scheduling for SMT"
> >         default y
> >         depends on SCHED_SMT
> > -
> > +       help
> > +         This option enables Core scheduling, a means of coordinated task
> > +         selection across SMT siblings with the express purpose of creating a
> > +         Core wide privilidge boundary. When enabled -- see prctl(PR_SCHED_CORE)
> > +         -- task selection will ensure all SMT siblings will execute a task
> > +         from the same 'core group', forcing idle when no matching task is found.
> > +
> > +         This provides means of mitigation against a number of SMT side-channels;
> > +         but is, on its own, insufficient to mitigate all known side-channels.
> > +         Notable: the MDS class of attacks require more.
> > +
> > +         Default enabled for anything that has SCHED_SMT, when unused there should
> > +         be no impact on performance.
> 
> This description sort of makes it sound like security is the only
> usecase. Perhaps we can also add here that core-scheduling can help
> performance of workloads where hyperthreading is undesired, such as
> when VM providers don't want to share hyperthreads.
> 
> Thoughts?

You're right. And there's this whole class of people who want to use
this to eliminate SMT interference. I'll see if I can work that in
without turning the whole thing into a novella or so ;-/

Re: config SCHED_CORE

[Randy Dunlap]

On 5/21/21 12:53 AM, Peter Zijlstra wrote:
> On Thu, May 20, 2021 at 08:06:07PM -0700, Hugh Dickins wrote:
>> Hi Peter,
>>
>> make oldconfig gave me no help at all on how to decide whether to choose
>> SCHED_CORE Y or n, beyond it recommending Y.  Maybe you'll delete that
>> option later, or maybe removing the prompt string would silence it.
> 
> Ah, you're quite right. I never seem to have gotten around to actually
> writing anything useful there :/ Similarly the documentation for all
> this seems to have gone missing too.
> 
> Joel, could I ask you to refresh the document to match the current state
> of things and repost? I still whole hartedly despise this RST crud, it
> makes it so hard to read / modify the files.
> 
> ( I think the latest version is here:
>   https://lkml.kernel.org/r/20210324214020.34142-8-joel@joelfernandes.org
> )
> 
> Anyway, how is something like the below, Joel can add a reference to the
> document once it's there.
> 
> ---
>  kernel/Kconfig.preempt | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
> index ea1e3331c0ba..3c4566cd20ef 100644
> --- a/kernel/Kconfig.preempt
> +++ b/kernel/Kconfig.preempt
> @@ -104,4 +104,16 @@ config SCHED_CORE
>  	bool "Core Scheduling for SMT"
>  	default y
>  	depends on SCHED_SMT
> -
> +	help
> +	  This option enables Core scheduling, a means of coordinated task
> +	  selection across SMT siblings with the express purpose of creating a
> +	  Core wide privilidge boundary. When enabled -- see prctl(PR_SCHED_CORE)

	            privilege
while you are at it.

> +	  -- task selection will ensure all SMT siblings will execute a task
> +	  from the same 'core group', forcing idle when no matching task is found.
> +
> +	  This provides means of mitigation against a number of SMT side-channels;
> +	  but is, on its own, insufficient to mitigate all known side-channels.
> +	  Notable: the MDS class of attacks require more.
> +
> +	  Default enabled for anything that has SCHED_SMT, when unused there should
> +	  be no impact on performance.
> 


-- 
~Randy

Re: config SCHED_CORE

[Peter Zijlstra]

On Fri, May 21, 2021 at 07:57:35AM -0400, Joel Fernandes wrote:
> On Fri, May 21, 2021 at 3:53 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > +       help
> > +         This option enables Core scheduling, a means of coordinated task
> > +         selection across SMT siblings with the express purpose of creating a
> > +         Core wide privilidge boundary. When enabled -- see prctl(PR_SCHED_CORE)
> > +         -- task selection will ensure all SMT siblings will execute a task
> > +         from the same 'core group', forcing idle when no matching task is found.
> > +
> > +         This provides means of mitigation against a number of SMT side-channels;
> > +         but is, on its own, insufficient to mitigate all known side-channels.
> > +         Notable: the MDS class of attacks require more.
> > +
> > +         Default enabled for anything that has SCHED_SMT, when unused there should
> > +         be no impact on performance.
> 
> This description sort of makes it sound like security is the only
> usecase. Perhaps we can also add here that core-scheduling can help
> performance of workloads where hyperthreading is undesired, such as
> when VM providers don't want to share hyperthreads.

Something like so then?

---
 kernel/Kconfig.preempt | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index ea1e3331c0ba..cd497fecfd43 100644
--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -104,4 +104,18 @@ config SCHED_CORE
 	bool "Core Scheduling for SMT"
 	default y
 	depends on SCHED_SMT
+	help
+	  This option enables Core scheduling, a means of coordinated task
+	  selection across SMT siblings. When enabled -- see
+	  prctl(PR_SCHED_CORE) -- task selection will ensure all SMT siblings
+	  will execute a task from the same 'core group', forcing idle when no
+	  matching task is found.
+
+	  Use of this feature includes:
+	   - mitigation of some (not all) SMT side channels;
+	   - limiting SMT interference to improve determinism and/or performance.
+
+	  Default enabled for anything that has SCHED_SMT, when unused there
+	  should be no impact on performance.
+
 

Re: config SCHED_CORE

[Hugh Dickins]

On Sun, 23 May 2021, Peter Zijlstra wrote:
> On Fri, May 21, 2021 at 07:57:35AM -0400, Joel Fernandes wrote:
> > On Fri, May 21, 2021 at 3:53 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > > +       help
> > > +         This option enables Core scheduling, a means of coordinated task
> > > +         selection across SMT siblings with the express purpose of creating a
> > > +         Core wide privilidge boundary. When enabled -- see prctl(PR_SCHED_CORE)
> > > +         -- task selection will ensure all SMT siblings will execute a task
> > > +         from the same 'core group', forcing idle when no matching task is found.
> > > +
> > > +         This provides means of mitigation against a number of SMT side-channels;
> > > +         but is, on its own, insufficient to mitigate all known side-channels.
> > > +         Notable: the MDS class of attacks require more.
> > > +
> > > +         Default enabled for anything that has SCHED_SMT, when unused there should
> > > +         be no impact on performance.
> > 
> > This description sort of makes it sound like security is the only
> > usecase. Perhaps we can also add here that core-scheduling can help
> > performance of workloads where hyperthreading is undesired, such as
> > when VM providers don't want to share hyperthreads.
> 
> Something like so then?

Much more helpful, thanks. And I agree that you have to keep it fairly
brief here: I think you've struck the right balance.  Some nits below.

> 
> ---
>  kernel/Kconfig.preempt | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
> index ea1e3331c0ba..cd497fecfd43 100644
> --- a/kernel/Kconfig.preempt
> +++ b/kernel/Kconfig.preempt
> @@ -104,4 +104,18 @@ config SCHED_CORE
>  	bool "Core Scheduling for SMT"
>  	default y
>  	depends on SCHED_SMT
> +	help
> +	  This option enables Core scheduling, a means of coordinated task

Maybe s/scheduling/Scheduling/ to match the title?

I think I got the picture once I reached the end, but was confused here
by the stages of enablement.  s/This option enables/This option permits/
would be clearer, I think.


> +	  selection across SMT siblings. When enabled -- see
> +	  prctl(PR_SCHED_CORE) -- task selection will ensure all SMT siblings

s/will ensure/ensures that/ (it felt like too many "will"s before)

> +	  will execute a task from the same 'core group', forcing idle when no
> +	  matching task is found.
> +
> +	  Use of this feature includes:
> +	   - mitigation of some (not all) SMT side channels;
> +	   - limiting SMT interference to improve determinism and/or performance.
> +
> +	  Default enabled for anything that has SCHED_SMT, when unused there

"SCHED_CORE is default enabled when SCHED_SMT is enabled - when unused there"
would be better.

> +	  should be no impact on performance.
> +

Thanks,
Hugh

Re: config SCHED_CORE

[Randy Dunlap]

On 5/23/21 1:30 PM, Hugh Dickins wrote:
> On Sun, 23 May 2021, Peter Zijlstra wrote:
>> On Fri, May 21, 2021 at 07:57:35AM -0400, Joel Fernandes wrote:
>>> On Fri, May 21, 2021 at 3:53 AM Peter Zijlstra <peterz@infradead.org> wrote:
>>>> +       help
>>>> +         This option enables Core scheduling, a means of coordinated task
>>>> +         selection across SMT siblings with the express purpose of creating a
>>>> +         Core wide privilidge boundary. When enabled -- see prctl(PR_SCHED_CORE)
>>>> +         -- task selection will ensure all SMT siblings will execute a task
>>>> +         from the same 'core group', forcing idle when no matching task is found.
>>>> +
>>>> +         This provides means of mitigation against a number of SMT side-channels;
>>>> +         but is, on its own, insufficient to mitigate all known side-channels.
>>>> +         Notable: the MDS class of attacks require more.
>>>> +
>>>> +         Default enabled for anything that has SCHED_SMT, when unused there should
>>>> +         be no impact on performance.
>>>
>>> This description sort of makes it sound like security is the only
>>> usecase. Perhaps we can also add here that core-scheduling can help
>>> performance of workloads where hyperthreading is undesired, such as
>>> when VM providers don't want to share hyperthreads.
>>
>> Something like so then?
> 
> Much more helpful, thanks. And I agree that you have to keep it fairly
> brief here: I think you've struck the right balance.  Some nits below.
> 
>>
>> ---
>>  kernel/Kconfig.preempt | 14 ++++++++++++++
>>  1 file changed, 14 insertions(+)
>>
>> diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
>> index ea1e3331c0ba..cd497fecfd43 100644
>> --- a/kernel/Kconfig.preempt
>> +++ b/kernel/Kconfig.preempt
>> @@ -104,4 +104,18 @@ config SCHED_CORE
>>  	bool "Core Scheduling for SMT"
>>  	default y
>>  	depends on SCHED_SMT
>> +	help
>> +	  This option enables Core scheduling, a means of coordinated task
> 
> Maybe s/scheduling/Scheduling/ to match the title?
> 
> I think I got the picture once I reached the end, but was confused here
> by the stages of enablement.  s/This option enables/This option permits/
> would be clearer, I think.
> 

I like all of Hugh's suggestions...

> 
>> +	  selection across SMT siblings. When enabled -- see
>> +	  prctl(PR_SCHED_CORE) -- task selection will ensure all SMT siblings
> 
> s/will ensure/ensures that/ (it felt like too many "will"s before)

especially that one. ^^^

>> +	  will execute a task from the same 'core group', forcing idle when no
>> +	  matching task is found.
>> +
>> +	  Use of this feature includes:
>> +	   - mitigation of some (not all) SMT side channels;
>> +	   - limiting SMT interference to improve determinism and/or performance.
>> +
>> +	  Default enabled for anything that has SCHED_SMT, when unused there
> 
> "SCHED_CORE is default enabled when SCHED_SMT is enabled - when unused there"
> would be better.
> 
>> +	  should be no impact on performance.


thanks.
-- 
~Randy

Re: config SCHED_CORE

[Joel Fernandes]

On Fri, May 21, 2021 at 9:36 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Fri, May 21, 2021 at 07:57:35AM -0400, Joel Fernandes wrote:
>
> > > ---
> > >  kernel/Kconfig.preempt | 14 +++++++++++++-
> > >  1 file changed, 13 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
> > > index ea1e3331c0ba..3c4566cd20ef 100644
> > > --- a/kernel/Kconfig.preempt
> > > +++ b/kernel/Kconfig.preempt
> > > @@ -104,4 +104,16 @@ config SCHED_CORE
> > >         bool "Core Scheduling for SMT"
> > >         default y
> > >         depends on SCHED_SMT
> > > -
> > > +       help
> > > +         This option enables Core scheduling, a means of coordinated task
> > > +         selection across SMT siblings with the express purpose of creating a
> > > +         Core wide privilidge boundary. When enabled -- see prctl(PR_SCHED_CORE)
> > > +         -- task selection will ensure all SMT siblings will execute a task
> > > +         from the same 'core group', forcing idle when no matching task is found.
> > > +
> > > +         This provides means of mitigation against a number of SMT side-channels;
> > > +         but is, on its own, insufficient to mitigate all known side-channels.
> > > +         Notable: the MDS class of attacks require more.
> > > +
> > > +         Default enabled for anything that has SCHED_SMT, when unused there should
> > > +         be no impact on performance.
> >
> > This description sort of makes it sound like security is the only
> > usecase. Perhaps we can also add here that core-scheduling can help
> > performance of workloads where hyperthreading is undesired, such as
> > when VM providers don't want to share hyperthreads.
> >
> > Thoughts?
>
> You're right. And there's this whole class of people who want to use
> this to eliminate SMT interference.

That's good, even more core scheduling users ;-)

> I'll see if I can work that in
> without turning the whole thing into a novella or so ;-/

Your patch looked good to me (delta Randy/Hugh's comments). I still
owe you the documentation patch refresh, I'll send it out soon.
Thanks,

- Joel

[PATCH] sched: Add CONFIG_SCHED_CORE help text

[Peter Zijlstra]

Subject: sched: Add CONFIG_SCHED_CORE help text
From: Peter Zijlstra <peterz@infradead.org>
Date: Tue May 25 08:53:28 CEST 2021

Hugh noted that the SCHED_CORE Kconfig option could do with a help
text.

Requested-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/Kconfig.preempt |   14 ++++++++++++++
 1 file changed, 14 insertions(+)

--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -104,4 +104,18 @@ config SCHED_CORE
 	bool "Core Scheduling for SMT"
 	default y
 	depends on SCHED_SMT
+	help
+	  This option permits Core Scheduling, a means of coordinated task
+	  selection across SMT siblings. When enabled -- see
+	  prctl(PR_SCHED_CORE) -- task selection ensures that all SMT siblings
+	  will execute a task from the same 'core group', forcing idle when no
+	  matching task is found.
+
+	  Use of this feature includes:
+	   - mitigation of some (not all) SMT side channels;
+	   - limiting SMT interference to improve determinism and/or performance.
+
+	  SCHED_CORE is default enabled when SCHED_SMT is enabled -- when
+	  unused there should be no impact on performance.
+
 

Re: [PATCH] sched: Add CONFIG_SCHED_CORE help text

[Randy Dunlap]

On 5/25/21 12:05 AM, Peter Zijlstra wrote:
> Subject: sched: Add CONFIG_SCHED_CORE help text
> From: Peter Zijlstra <peterz@infradead.org>
> Date: Tue May 25 08:53:28 CEST 2021
> 
> Hugh noted that the SCHED_CORE Kconfig option could do with a help
> text.
> 
> Requested-by: Hugh Dickins <hughd@google.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Reviewed-by: Randy Dunlap <rdunlap@infradead.org>

Thanks.

> ---
>  kernel/Kconfig.preempt |   14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> --- a/kernel/Kconfig.preempt
> +++ b/kernel/Kconfig.preempt
> @@ -104,4 +104,18 @@ config SCHED_CORE
>  	bool "Core Scheduling for SMT"
>  	default y
>  	depends on SCHED_SMT
> +	help
> +	  This option permits Core Scheduling, a means of coordinated task
> +	  selection across SMT siblings. When enabled -- see
> +	  prctl(PR_SCHED_CORE) -- task selection ensures that all SMT siblings
> +	  will execute a task from the same 'core group', forcing idle when no
> +	  matching task is found.
> +
> +	  Use of this feature includes:
> +	   - mitigation of some (not all) SMT side channels;
> +	   - limiting SMT interference to improve determinism and/or performance.
> +
> +	  SCHED_CORE is default enabled when SCHED_SMT is enabled -- when
> +	  unused there should be no impact on performance.
> +
>  
> 


-- 
~Randy

Re: [PATCH] sched: Add CONFIG_SCHED_CORE help text

[Hugh Dickins]

On Tue, 25 May 2021, Peter Zijlstra wrote:

> Subject: sched: Add CONFIG_SCHED_CORE help text
> From: Peter Zijlstra <peterz@infradead.org>
> Date: Tue May 25 08:53:28 CEST 2021
> 
> Hugh noted that the SCHED_CORE Kconfig option could do with a help
> text.
> 
> Requested-by: Hugh Dickins <hughd@google.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Thanks,
Acked-by: Hugh Dickins <hughd@google.com>
if necessary (but there's more than enough of me there already).

> ---
>  kernel/Kconfig.preempt |   14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> --- a/kernel/Kconfig.preempt
> +++ b/kernel/Kconfig.preempt
> @@ -104,4 +104,18 @@ config SCHED_CORE
>  	bool "Core Scheduling for SMT"
>  	default y
>  	depends on SCHED_SMT
> +	help
> +	  This option permits Core Scheduling, a means of coordinated task
> +	  selection across SMT siblings. When enabled -- see
> +	  prctl(PR_SCHED_CORE) -- task selection ensures that all SMT siblings
> +	  will execute a task from the same 'core group', forcing idle when no
> +	  matching task is found.
> +
> +	  Use of this feature includes:
> +	   - mitigation of some (not all) SMT side channels;
> +	   - limiting SMT interference to improve determinism and/or performance.
> +
> +	  SCHED_CORE is default enabled when SCHED_SMT is enabled -- when
> +	  unused there should be no impact on performance.
> +

[tip: sched/core] sched: Add CONFIG_SCHED_CORE help text

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     7b419f47facd286c6723daca6ad69ec355473f78
Gitweb:        https://git.kernel.org/tip/7b419f47facd286c6723daca6ad69ec355473f78
Author:        Peter Zijlstra <peterz@infradead.org>
AuthorDate:    Tue, 25 May 2021 08:53:28 +02:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Tue, 01 Jun 2021 16:00:10 +02:00

sched: Add CONFIG_SCHED_CORE help text

Hugh noted that the SCHED_CORE Kconfig option could do with a help
text.

Requested-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Hugh Dickins <hughd@google.com>
Link: https://lkml.kernel.org/r/YKyhtwhEgvtUDOyl@hirez.programming.kicks-ass.net
---
 kernel/Kconfig.preempt | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index ea1e333..bd7c414 100644
--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -104,4 +104,18 @@ config SCHED_CORE
 	bool "Core Scheduling for SMT"
 	default y
 	depends on SCHED_SMT
+	help
+	  This option permits Core Scheduling, a means of coordinated task
+	  selection across SMT siblings. When enabled -- see
+	  prctl(PR_SCHED_CORE) -- task selection ensures that all SMT siblings
+	  will execute a task from the same 'core group', forcing idle when no
+	  matching task is found.
+
+	  Use of this feature includes:
+	   - mitigation of some (not all) SMT side channels;
+	   - limiting SMT interference to improve determinism and/or performance.
+
+	  SCHED_CORE is default enabled when SCHED_SMT is enabled -- when
+	  unused there should be no impact on performance.
+