Setting CPU affinity for current process on kernel module?

Setting CPU affinity for current process on kernel module?

[Martin Galvan]

Hi,

I have a kernel module which needs to perform CPU core-specific
operations. I know that there's a kernel mode version of
sched_setaffinity, however it's not exported to modules and can only
be accessed through e.g. kallsyms_lookup_name, which is ugly. The
alternative is to spawn a new kthread and bind it to the desired CPU
before starting it, but I was wondering if there's a more
straightforward method of doing this. Thanks!

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Re: Setting CPU affinity for current process on kernel module?

[Bharath Vedartham]

On Wed, Sep 11, 2019 at 03:52:06PM -0300, Martin Galvan wrote:
> Hi,
> 
> I have a kernel module which needs to perform CPU core-specific
> operations. I know that there's a kernel mode version of
> sched_setaffinity, however it's not exported to modules and can only
> be accessed through e.g. kallsyms_lookup_name, which is ugly. The
> alternative is to spawn a new kthread and bind it to the desired CPU
> before starting it, but I was wondering if there's a more
> straightforward method of doing this. Thanks!
> 
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies@kernelnewbies.org
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So one thing that pops to my head is using cgroups. Create a cgroup,
modify the cpu file in cpusets cgroup and add the required task to it.
But I am not sure whether you can do all of this from a kernel module...

Thanks
Bharath

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Re: Setting CPU affinity for current process on kernel module?

[Martin Galvan]

El mié., 11 sept. 2019 a las 16:17, Bharath Vedartham
(<linux.bhar@gmail.com>) escribió:
> So one thing that pops to my head is using cgroups. Create a cgroup,
> modify the cpu file in cpusets cgroup and add the required task to it.
> But I am not sure whether you can do all of this from a kernel module...

Thanks, but that's not feasible. This should all be done
programmatically from the module, and be as minimally invasive as
possible.

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Re: Setting CPU affinity for current process on kernel module?

[Bharath Vedartham]

On Wed, Sep 11, 2019 at 04:19:41PM -0300, Martin Galvan wrote:
> El mié., 11 sept. 2019 a las 16:17, Bharath Vedartham
> (<linux.bhar@gmail.com>) escribió:
> > So one thing that pops to my head is using cgroups. Create a cgroup,
> > modify the cpu file in cpusets cgroup and add the required task to it.
> > But I am not sure whether you can do all of this from a kernel module...
> 
> Thanks, but that's not feasible. This should all be done
> programmatically from the module, and be as minimally invasive as
> possible.
Just to be clear, What do you mean by current process. Is it the process
which is executing the kernel module? That would be a kernel thread I
believe or do you want to be able to set the cpu affinity for any
process given its PID? 
And are you looking at setting CPU affinity of kernel threads or user
threads. 

Thank you
Bharath

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Re: Setting CPU affinity for current process on kernel module?

[Martin Galvan]

El jue., 12 sept. 2019 a las 6:26, Bharath Vedartham
(<linux.bhar@gmail.com>) escribió:
> Just to be clear, What do you mean by current process. Is it the process
> which is executing the kernel module? That would be a kernel thread I
> believe or do you want to be able to set the cpu affinity for any
> process given its PID?
> And are you looking at setting CPU affinity of kernel threads or user
> threads.

Sorry, I wasn't clear: I want to set the affinity of 'current', which
is a kernel thread.

El jue., 12 sept. 2019 a las 6:38, Piotr Figiel (<figiel@gmail.com>) escribió:
> set_cpus_allowed_ptr() is exported, maybe this is what you're looking for?

Yes, that's exactly what I needed. Thanks! I actually ended up using
work_on_cpu, which fits my use case better.

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Re: Setting CPU affinity for current process on kernel module?

[Greg KH]

On Thu, Sep 12, 2019 at 11:03:20AM -0300, Martin Galvan wrote:
> El jue., 12 sept. 2019 a las 6:26, Bharath Vedartham
> (<linux.bhar@gmail.com>) escribió:
> > Just to be clear, What do you mean by current process. Is it the process
> > which is executing the kernel module? That would be a kernel thread I
> > believe or do you want to be able to set the cpu affinity for any
> > process given its PID?
> > And are you looking at setting CPU affinity of kernel threads or user
> > threads.
> 
> Sorry, I wasn't clear: I want to set the affinity of 'current', which
> is a kernel thread.

I really hate to ask, but _why_ do you feel that this is the correct
solution to your problem?  What is the problem you are attempting to
solve by doing this?

thanks,

greg k-h

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Re: Setting CPU affinity for current process on kernel module?

[Martin Galvan]

El vie., 13 sept. 2019 a las 1:50, Greg KH (<greg@kroah.com>) escribió:
> I really hate to ask, but _why_ do you feel that this is the correct
> solution to your problem?  What is the problem you are attempting to
> solve by doing this?

I'm reading and comparing per-core MSR values.

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Re: Setting CPU affinity for current process on kernel module?

[Greg KH]

On Fri, Sep 13, 2019 at 09:43:33AM -0300, Martin Galvan wrote:
> El vie., 13 sept. 2019 a las 1:50, Greg KH (<greg@kroah.com>) escribió:
> > I really hate to ask, but _why_ do you feel that this is the correct
> > solution to your problem?  What is the problem you are attempting to
> > solve by doing this?
> 
> I'm reading and comparing per-core MSR values.

And doing what with that information?  We have a msr driver for
userspace access to those values, does that not work correctly for you?

greg k-h

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Re: Setting CPU affinity for current process on kernel module?

[Martin Galvan]

El vie., 13 sept. 2019 a las 9:55, Greg KH (<greg@kroah.com>) escribió:
> And doing what with that information?  We have a msr driver for
> userspace access to those values, does that not work correctly for you?

My use case requires this to be done in kernel space.

In any case, thanks for the help, this seems to be working just fine.

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Re: Setting CPU affinity for current process on kernel module?

[Greg KH]

On Fri, Sep 13, 2019 at 09:56:24AM -0300, Martin Galvan wrote:
> El vie., 13 sept. 2019 a las 9:55, Greg KH (<greg@kroah.com>) escribió:
> > And doing what with that information?  We have a msr driver for
> > userspace access to those values, does that not work correctly for you?
> 
> My use case requires this to be done in kernel space.

So, again, I hate to ask but _why_ do you need access to the msr from
within the kernel?  What useful information can you do with that?  Do
you have a pointer to your code anywhere?

thanks,

greg k-h

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Re: Setting CPU affinity for current process on kernel module?

[R. Engür Pişirici]

I'm sure that some of us(readers of the kernelNewbies e-mail list) 
really wants to understand the same thing Greg was asking.
Best Regards,

-engur



On 2019-09-13 17:13, Greg KH wrote:
> On Fri, Sep 13, 2019 at 09:56:24AM -0300, Martin Galvan wrote:
>> El vie., 13 sept. 2019 a las 9:55, Greg KH (<greg@kroah.com>) 
>> escribió:
>> > And doing what with that information?  We have a msr driver for
>> > userspace access to those values, does that not work correctly for you?
>> 
>> My use case requires this to be done in kernel space.
> 
> So, again, I hate to ask but _why_ do you need access to the msr from
> within the kernel?  What useful information can you do with that?  Do
> you have a pointer to your code anywhere?
> 
> thanks,
> 
> greg k-h
> 
> _______________________________________________
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