Regarding Linux kernel SMP and task switching CPU

Regarding Linux kernel SMP and task switching CPU

[William Tambe]

When a task is moved from one CPU to another, are there platform
specific functions that are called to detect change ?

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Re: Regarding Linux kernel SMP and task switching CPU

[CVS]

> are there platform specific functions that are called to detect change ?
What is it that you wish to do with this?

- Are you thinking of modifying the thread migration logic before a
thread is migrated?
- Are you looking to trace such thread migrations across CPUs after they happen?
- Something else?...

regards
CVS

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Re: Regarding Linux kernel SMP and task switching CPU

[William Tambe]

On Sun, Jul 12, 2020 at 7:00 AM CVS <cvs268@gmail.com> wrote:
>
> > are there platform specific functions that are called to detect change ?
> What is it that you wish to do with this?

I am doing porting work; I would like to be able to update
mm_struct->context when a task migrates to another CPU.

Another related question to this topic is:
can the same mm_struct be used by two tasks running each on a different CPU ?
if yes, I would probably need to make mm_struct->context an array so
that it can hold information for each CPU.

>
> - Are you thinking of modifying the thread migration logic before a
> thread is migrated?
> - Are you looking to trace such thread migrations across CPUs after they happen?
> - Something else?...
>
> regards
> CVS

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Re: Regarding Linux kernel SMP and task switching CPU

[CVS]

> update mm_struct->context when a task migrates to another CPU
You might be able to do that using rq->prev_mm or  rq->active_mm
in move_queued_task() and ttwu_remote() functions...

Reference :
https://elixir.bootlin.com/linux/v5.7.8/source/kernel/sched/core.c#L1472
https://elixir.bootlin.com/linux/v5.7.8/source/kernel/sched/core.c#L2257

...however, i begin to cringe as I started to write the above.
Why? Because it sounds like a hack to me! :-)

> update mm_struct->context when a task migrates to another CPU
So, again, why do you want to do this?

BTW, in the interest of saving time, let us practice 5-Whys
(Reference: https://en.wikipedia.org/wiki/Five_whys)
and jump straight to the actual use-case / final problem
that you are trying to solve ...and work ahead from there.

May be there are pre-existing cleaner approaches to your problem
that do NOT involve fiddling with the Linux kernel internals.

regards
CVS

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Re: Regarding Linux kernel SMP and task switching CPU

[Valdis Klētnieks]

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On Sun, 12 Jul 2020 12:24:46 -0400, William Tambe said:
> On Sun, Jul 12, 2020 at 7:00 AM CVS <cvs268@gmail.com> wrote:
> >
> > > are there platform specific functions that are called to detect change ?
> > What is it that you wish to do with this?
>
> I am doing porting work; I would like to be able to update
> mm_struct->context when a task migrates to another CPU.

That shouldn't be a big issue - there should already be architecture
independent code that calls a function to do this sort of thing on task
migration, and all you have to do is provide a "Here's how the actual update is
done on the Wombat architecture" function in the appropriate place in the arch/
tree.


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