linux-kernel - Re: [PATCH 09/40] autonuma: introduce kthread_bind

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Date:	Fri, 29 Jun 2012 12:58:01 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
CC:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Hillf Danton <dhillf@...il.com>, Dan Smith <danms@...ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, Paul Turner <pjt@...gle.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Mike Galbraith <efault@....de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Bharata B Rao <bharata.rao@...il.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Johannes Weiner <hannes@...xchg.org>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>,
	Alex Shi <alex.shi@...el.com>,
	Mauricio Faria de Oliveira <mauricfo@...ux.vnet.ibm.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Don Morris <don.morris@...com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [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...

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