Re: [PATCH 09/40] autonuma: introduce kthread_bind_node()

[Rik van Riel]

On 06/29/2012 12:38 PM, Andrea Arcangeli wrote:
> On Fri, Jun 29, 2012 at 11:36:26AM -0400, Rik van Riel wrote:
> > On 06/28/2012 08:55 AM, Andrea Arcangeli wrote:
> >
> > > --- a/include/linux/sched.h
> > > +++ b/include/linux/sched.h
> > > @@ -1792,7 +1792,7 @@ extern void thread_group_times(struct task_struct *p, cputime_t *ut, cputime_t *
> > >    #define PF_SWAPWRITE	0x00800000	/* Allowed to write to swap */
> > >    #define PF_SPREAD_PAGE	0x01000000	/* Spread page cache over cpuset */
> > >    #define PF_SPREAD_SLAB	0x02000000	/* Spread some slab caches over cpuset */
> > > -#define PF_THREAD_BOUND	0x04000000	/* Thread bound to specific cpu */
> > > +#define PF_THREAD_BOUND	0x04000000	/* Thread bound to specific cpus */
> > >    #define PF_MCE_EARLY    0x08000000      /* Early kill for mce process policy */
> > >    #define PF_MEMPOLICY	0x10000000	/* Non-default NUMA mempolicy */
> > >    #define PF_MUTEX_TESTER	0x20000000	/* Thread belongs to the rt mutex tester */
> >
> > Changing the semantics of PF_THREAD_BOUND without so much as
> > a comment in your changelog or buy-in from the scheduler
> > maintainers is a big no-no.
> >
> > Is there any reason you even need PF_THREAD_BOUND in your
> > kernel numa threads?
> >
> > I do not see much at all in the scheduler code that uses
> > PF_THREAD_BOUND and it is not clear at all that your
> > numa threads get any benefit from them...
> >
> > Why do you think you need it?

> This flag is only used to prevent userland to mess with the kernel CPU
> binds of kernel threads. It is used to avoid the root user to shoot
> itself in the foot.
>
> So far it has been used to prevent changing bindings to a single
> CPU. I'm setting it also after making a multiple-cpu bind (all CPUs of
> the node, instead of just 1 CPU).

Fair enough.  Looking at the scheduler code some more, I
see that all PF_THREAD_BOUND seems to do is block userspace
from changing a thread's CPU bindings.

Peter and Ingo, what is the special magic in PF_THREAD_BOUND
that should make it only apply to kernel threads that are bound
to a single CPU?

Allowing it for threads that are bound to a NUMA node
could make some sense for eg. kswapd...

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