sched_* implemented?

sched_* implemented?

[James Marshall]

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Hi,

I'm working on a realtime application and am trying to use musl. I would
like to call sched_setscheduler, but it looks like this is just a stub in
musl.

However,
http://nsz.repo.hu/git/?p=musl-tables;a=blob_plain;f=tab_posix.html;hb=HEAD
has it marked as implemented. Am I missing something? Is there a patch
available?

If there is not, any idea of how difficult it would be for me to add it in?
Or would I be better off using syscall directly instead?

Your help and time is appreciated.

Thanks,
James

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Re: sched_* implemented?

[Rich Felker]

On Mon, Feb 29, 2016 at 11:32:34PM -0500, James Marshall wrote:
> Hi,
> 
> I'm working on a realtime application and am trying to use musl. I would
> like to call sched_setscheduler, but it looks like this is just a stub in
> musl.
> 
> However,
> http://nsz.repo.hu/git/?p=musl-tables;a=blob_plain;f=tab_posix.html;hb=HEAD
> has it marked as implemented. Am I missing something? Is there a patch
> available?
> 
> If there is not, any idea of how difficult it would be for me to add it in?
> Or would I be better off using syscall directly instead?
> 
> Your help and time is appreciated.

The reason it doesn't do anything is that Linux does not provide a way
to set scheduling parameters for a _process_, only for threads. The
sched_setscheduler syscall is documented as taking a pid but actually
takes a thread id and only operates on that thread. glibc just ignores
this and provides sched_* functions that do the wrong thing.

Fortunately there's an easy fix: use pthread_setschedparam, and
pthread_self to get the pthread_t value you need to pass to it.

Rich

Re: sched_* implemented?

[James Marshall]

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That works! Thank you for the help.

The way glibc / sched_setscheduler are setup is very... interesting.

James

On Mon, Feb 29, 2016 at 11:59 PM, Rich Felker <dalias@libc.org> wrote:

> On Mon, Feb 29, 2016 at 11:32:34PM -0500, James Marshall wrote:
> > Hi,
> >
> > I'm working on a realtime application and am trying to use musl. I would
> > like to call sched_setscheduler, but it looks like this is just a stub in
> > musl.
> >
> > However,
> >
> http://nsz.repo.hu/git/?p=musl-tables;a=blob_plain;f=tab_posix.html;hb=HEAD
> > has it marked as implemented. Am I missing something? Is there a patch
> > available?
> >
> > If there is not, any idea of how difficult it would be for me to add it
> in?
> > Or would I be better off using syscall directly instead?
> >
> > Your help and time is appreciated.
>
> The reason it doesn't do anything is that Linux does not provide a way
> to set scheduling parameters for a _process_, only for threads. The
> sched_setscheduler syscall is documented as taking a pid but actually
> takes a thread id and only operates on that thread. glibc just ignores
> this and provides sched_* functions that do the wrong thing.
>
> Fortunately there's an easy fix: use pthread_setschedparam, and
> pthread_self to get the pthread_t value you need to pass to it.
>
> Rich
>

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