Subject: [PATCH v2 2/7] sched: rework of sched_domain topology definition

From: [email-masked] (Dietmar Eggemann)
Date: Wed, 19 Mar 2014 15:22:39 +0000

On 19/03/14 13:33, Vincent Guittot wrote:
[...]

>>> Is there a way to check that MC and GMC have to have
>>> SD_SHARE_PKG_RESOURCES set so that this can't happen unnoticed?
>>
>> So from the core codes perspective those names mean less than nothing.
>> Its just a string to carry along for us meat-bags. The string isn't even
>> there when !SCHED_DEBUG.
>>
>> So from this codes POV you told it it had a domain without PKGSHARE,
>> that's fine.
>>
>> That said; yeah the thing isn't the prettiest piece of code. But it has
>> the big advantage of being the one place where we convert topology into
>> behaviour.
> 
> We might add a check of the child in sd_init to ensure that the child
> has at least some properties of the current level.
> I mean that if a level has got the SD_SHARE_PKG_RESOURCES flag, its
> child must also have it. The same for SD_SHARE_CPUPOWER and
> SD_ASYM_PACKING.
> 
> so we can add something like the below in sd_init
> 
> child_flags = SD_SHARE_PKG_RESOURCES | SD_SHARE_CPUPOWER | SD_ASYM_PACKING
> flags = sd->flags & child_flags
> if (sd->child)
>    child_flags &= sd->child->flags
> child_flags &= flags
> if (flags != child_flags)
>     pr_info("The topology description looks strange \n");

I tried it with my faulty set-up on TC2 and I get the info message for
the GMC level for all CPU's in sd_init.

I had to pass an 'struct sched_domain *child' pointer into sd_init()
from build_sched_domain() because inside sd_init() sd->child is always NULL.

So one of the requirements of this approach is that a child level like
GMC (which could potentially replace its parent level or otherwise is
destroyed itself) has to specify all flags of its parent level (MC)?

What about SD_NUMA in child_flags? SD_ASYM_PACKING is also a little bit
different than SD_SHARE_PKG_RESOURCES or SD_SHARE_CPUPOWER because it's
not used in the if ... else statement.

But I'm afraid this only works for this specific case of the MC/GMC
layer and is not scalable. If sd->child is a level for which you don't
want to potentially destroy itself or its parent, then you would get
false alarms. IMHO, sd_init() has no information for which pair of
adjacent levels it should apply this check and for which not. Do I miss
something here?

-- Dietmar

> Vincent