Re: [PATCH v2] sched: update_top_cache_domain only at the times ofbuilding sched domain.

From: Rakib Mullick
Date: Wed Jul 24 2013 - 09:57:26 EST

On Wed, Jul 24, 2013 at 2:34 PM, Michael Wang
<wangyun@xxxxxxxxxxxxxxxxxx> wrote:
> On 07/24/2013 04:01 PM, Rakib Mullick wrote:
>> On Wed, Jul 24, 2013 at 9:26 AM, Michael Wang
>> <wangyun@xxxxxxxxxxxxxxxxxx> wrote:
>>> Hi, Rakib
>>> On 07/24/2013 01:42 AM, Rakib Mullick wrote:
>>>> Currently, update_top_cache_domain() is called whenever schedule domain is built or destroyed. But, the following
>>>> callpath shows that they're at the same callpath and can be avoided update_top_cache_domain() while destroying schedule
>>>> domain and update only at the times of building schedule domains.
>>>> partition_sched_domains()
>>>> detach_destroy_domain()
>>>> cpu_attach_domain()
>>>> update_top_cache_domain()
>>> IMHO, cpu_attach_domain() and update_top_cache_domain() should be
>>> paired, below patch will open a window which 'rq->sd == NULL' while
>>> 'sd_llc != NULL', isn't it?
>>> I don't think we have the promise that before we rebuild the stuff
>>> correctly, no one will utilize 'sd_llc'...
>> I never said it. My point is different. partition_sched_domain works as -
>> - destroying existing schedule domain (if previous domain and new
>> domain aren't same)
>> - building new partition
>> while doing the first it needs to detach all the cpus on that domain.
>> By detaching what it does,
>> it fall backs to it's root default domain. In this context (which i've
>> proposed to skip), by means
>> of updating top cache domain it takes the highest flag domain to setup
>> it's sd_llc_id or cpu itself.
>> Whatever done above gets overwritten (updating top cache domain),
>> while building new partition.
>> Then, why we did that before? Hope you understand my point.
> I think you missed this in PeterZ's suggestion:
> - cpu_attach_domain(NULL, &def_root_domain, i);
> With this change, it will be safe since you will still get an un-freed
> sd, although it's an old one.
I never meant it and clearly I missed it. If you remove
cpu_attach_domain(), then detach_destroy_domain() becomes meaningless.
And I don't have any intent to remove cpu_attach_domain from
detach_destroy_domain() at all.

> But your patch will run the risk to get a freed sd, since you make
> 'sd_llc' wrong for a period of time (between destroy and rebuild) IMO.
Building 'sd_llc' depends on schedule domain. If you don't have sd,
sd_llc will point to NULL and sd_llc_id is
the CPU itself. Since, we're trying to re-construing so for this time
being it doesn't matter, cause we're building
it again. Now, please just note what you're saying, on last thread
you've said -

"I don't think we have the promise that before we rebuild the stuff
correctly, no one will utilize 'sd_llc'..."

If that is the case, then we shouldn't worry about it at all. And this
above comments (from previous thread I've quoted and this thread I'm
replying) they're just self contradictory.

> I guess I get you point, you are trying to save one time update since
> you think this will be done twice, but actually the result of this two
> time update was different, it's not redo and it's in order to sync
> 'sd_llc' with 'rq->sd'.
Yes, you got my point now, but I don't understand your points. Anyway,
I'm not going to argue with this
anymore, this stuff isn't much of an issue, but removing this sorts of
stuff is typical in kernel development.

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