Re: [PATCH v9 01/24] mm: Introduce CONFIG_SPECULATIVE_PAGE_FAULT

[David Rientjes]

On Wed, 28 Mar 2018, Laurent Dufour wrote:

> > Putting this in mm/Kconfig is definitely the right way to go about it 
> > instead of any generic option in arch/*.
> > 
> > My question, though, was making this configurable by the user:
> > 
> > config SPECULATIVE_PAGE_FAULT
> > bool "Speculative page faults"
> > depends on X86_64 || PPC
> > default y
> > help
> >   ..
> > 
> > It's a question about whether we want this always enabled on x86_64 and 
> > power or whether the user should be able to disable it (right now they 
> > can't).  With a large feature like this, you may want to offer something 
> > simple (disable CONFIG_SPECULATIVE_PAGE_FAULT) if someone runs into 
> > regressions.
> 
> I agree, but I think it would be important to get the per architecture
> enablement to avoid complex check here. For instance in the case of powerPC
> this is only supported for PPC_BOOK3S_64.
> 
> To avoid exposing such per architecture define here, what do you think about
> having supporting architectures setting ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT
> and the SPECULATIVE_PAGE_FAULT depends on this, like this:
> 
> In mm/Kconfig:
> config SPECULATIVE_PAGE_FAULT
>   bool "Speculative page faults"
>   depends on ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT && SMP
>   default y
>   help
>   ...
> 
> In arch/powerpc/Kconfig:
> config PPC
>   ...
>   select ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT if PPC_BOOK3S_64
> 
> In arch/x86/Kconfig:
> config X86_64
>   ...
>   select ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT
> 
> 

Looks good to me!  It feels like this will add more assurance that if 
things regress for certain workloads that it can be disabled.  I don't 
feel strongly about the default value, I'm ok with it being enabled by 
default.

Re: [PATCH v9 01/24] mm: Introduce CONFIG_SPECULATIVE_PAGE_FAULT

[Laurent Dufour]

On 28/03/2018 12:16, David Rientjes wrote:
> On Wed, 28 Mar 2018, Laurent Dufour wrote:
> 
 This configuration variable will be used to build the code needed to
 handle speculative page fault.

 By default it is turned off, and activated depending on architecture
 support.

 Suggested-by: Thomas Gleixner 
 Signed-off-by: Laurent Dufour 
 ---
  mm/Kconfig | 3 +++
  1 file changed, 3 insertions(+)

 diff --git a/mm/Kconfig b/mm/Kconfig
 index abefa573bcd8..07c566c88faf 100644
 --- a/mm/Kconfig
 +++ b/mm/Kconfig
 @@ -759,3 +759,6 @@ config GUP_BENCHMARK
  performance of get_user_pages_fast().
  
  See tools/testing/selftests/vm/gup_benchmark.c
 +
 +config SPECULATIVE_PAGE_FAULT
 +   bool
>>>
>>> Should this be configurable even if the arch supports it?
>>
>> Actually, this is not configurable unless by manually editing the .config 
>> file.
>>
>> I made it this way on the Thomas's request :
>> https://lkml.org/lkml/2018/1/15/969
>>
>> That sounds to be the smarter way to achieve that, isn't it ?
>>
> 
> Putting this in mm/Kconfig is definitely the right way to go about it 
> instead of any generic option in arch/*.
> 
> My question, though, was making this configurable by the user:
> 
> config SPECULATIVE_PAGE_FAULT
>   bool "Speculative page faults"
>   depends on X86_64 || PPC
>   default y
>   help
> ..
> 
> It's a question about whether we want this always enabled on x86_64 and 
> power or whether the user should be able to disable it (right now they 
> can't).  With a large feature like this, you may want to offer something 
> simple (disable CONFIG_SPECULATIVE_PAGE_FAULT) if someone runs into 
> regressions.

I agree, but I think it would be important to get the per architecture
enablement to avoid complex check here. For instance in the case of powerPC
this is only supported for PPC_BOOK3S_64.

To avoid exposing such per architecture define here, what do you think about
having supporting architectures setting ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT
and the SPECULATIVE_PAGE_FAULT depends on this, like this:

In mm/Kconfig:
config SPECULATIVE_PAGE_FAULT
bool "Speculative page faults"
depends on ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT && SMP
default y
help
...

In arch/powerpc/Kconfig:
config PPC
...
select ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT if PPC_BOOK3S_64

In arch/x86/Kconfig:
config X86_64
...
select ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT

Re: [PATCH v9 01/24] mm: Introduce CONFIG_SPECULATIVE_PAGE_FAULT

[David Rientjes]

On Wed, 28 Mar 2018, Laurent Dufour wrote:

> >> This configuration variable will be used to build the code needed to
> >> handle speculative page fault.
> >>
> >> By default it is turned off, and activated depending on architecture
> >> support.
> >>
> >> Suggested-by: Thomas Gleixner 
> >> Signed-off-by: Laurent Dufour 
> >> ---
> >>  mm/Kconfig | 3 +++
> >>  1 file changed, 3 insertions(+)
> >>
> >> diff --git a/mm/Kconfig b/mm/Kconfig
> >> index abefa573bcd8..07c566c88faf 100644
> >> --- a/mm/Kconfig
> >> +++ b/mm/Kconfig
> >> @@ -759,3 +759,6 @@ config GUP_BENCHMARK
> >>  performance of get_user_pages_fast().
> >>  
> >>  See tools/testing/selftests/vm/gup_benchmark.c
> >> +
> >> +config SPECULATIVE_PAGE_FAULT
> >> +   bool
> > 
> > Should this be configurable even if the arch supports it?
> 
> Actually, this is not configurable unless by manually editing the .config 
> file.
> 
> I made it this way on the Thomas's request :
> https://lkml.org/lkml/2018/1/15/969
> 
> That sounds to be the smarter way to achieve that, isn't it ?
> 

Putting this in mm/Kconfig is definitely the right way to go about it 
instead of any generic option in arch/*.

My question, though, was making this configurable by the user:

config SPECULATIVE_PAGE_FAULT
bool "Speculative page faults"
depends on X86_64 || PPC
default y
help
  ..

It's a question about whether we want this always enabled on x86_64 and 
power or whether the user should be able to disable it (right now they 
can't).  With a large feature like this, you may want to offer something 
simple (disable CONFIG_SPECULATIVE_PAGE_FAULT) if someone runs into 
regressions.

Re: [PATCH v9 01/24] mm: Introduce CONFIG_SPECULATIVE_PAGE_FAULT

[Laurent Dufour]

Hi David,

Thanks a lot for your deep review on this series.

On 25/03/2018 23:50, David Rientjes wrote:
> On Tue, 13 Mar 2018, Laurent Dufour wrote:
> 
>> This configuration variable will be used to build the code needed to
>> handle speculative page fault.
>>
>> By default it is turned off, and activated depending on architecture
>> support.
>>
>> Suggested-by: Thomas Gleixner 
>> Signed-off-by: Laurent Dufour 
>> ---
>>  mm/Kconfig | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/mm/Kconfig b/mm/Kconfig
>> index abefa573bcd8..07c566c88faf 100644
>> --- a/mm/Kconfig
>> +++ b/mm/Kconfig
>> @@ -759,3 +759,6 @@ config GUP_BENCHMARK
>>performance of get_user_pages_fast().
>>  
>>See tools/testing/selftests/vm/gup_benchmark.c
>> +
>> +config SPECULATIVE_PAGE_FAULT
>> +   bool
> 
> Should this be configurable even if the arch supports it?

Actually, this is not configurable unless by manually editing the .config file.

I made it this way on the Thomas's request :
https://lkml.org/lkml/2018/1/15/969

That sounds to be the smarter way to achieve that, isn't it ?

Laurent.

Re: [PATCH v9 01/24] mm: Introduce CONFIG_SPECULATIVE_PAGE_FAULT

[David Rientjes]

On Tue, 13 Mar 2018, Laurent Dufour wrote:

> This configuration variable will be used to build the code needed to
> handle speculative page fault.
> 
> By default it is turned off, and activated depending on architecture
> support.
> 
> Suggested-by: Thomas Gleixner 
> Signed-off-by: Laurent Dufour 
> ---
>  mm/Kconfig | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/Kconfig b/mm/Kconfig
> index abefa573bcd8..07c566c88faf 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -759,3 +759,6 @@ config GUP_BENCHMARK
> performance of get_user_pages_fast().
>  
> See tools/testing/selftests/vm/gup_benchmark.c
> +
> +config SPECULATIVE_PAGE_FAULT
> +   bool

Should this be configurable even if the arch supports it?

[PATCH v9 01/24] mm: Introduce CONFIG_SPECULATIVE_PAGE_FAULT

[Laurent Dufour]

This configuration variable will be used to build the code needed to
handle speculative page fault.

By default it is turned off, and activated depending on architecture
support.

Suggested-by: Thomas Gleixner 
Signed-off-by: Laurent Dufour 
---
 mm/Kconfig | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/Kconfig b/mm/Kconfig
index abefa573bcd8..07c566c88faf 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -759,3 +759,6 @@ config GUP_BENCHMARK
  performance of get_user_pages_fast().
 
  See tools/testing/selftests/vm/gup_benchmark.c
+
+config SPECULATIVE_PAGE_FAULT
+   bool
-- 
2.7.4