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

All rights reversed

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