linux-kernel - Re: Make yield_task

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Message-ID: <47BE411B.8090904@linux.vnet.ibm.com>
Date:	Fri, 22 Feb 2008 08:57:23 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Jens Axboe <jens.axboe@...cle.com>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, David Woodhouse <dwmw2@...radead.org>
Subject: Re: Make yield_task_fair more efficient

Jens Axboe wrote:
> On Thu, Feb 21 2008, Jens Axboe wrote:
>> On Thu, Feb 21 2008, Peter Zijlstra wrote:
>>> On Thu, 2008-02-21 at 15:37 +0530, Balbir Singh wrote:
>>>
>>>> You use the empty pointer (missing right child), so why do we need a list. May
>>>> be I am missing something.
>>> A fully threaded tree also has back-pointer to traverse backwards
>>> through the ordered elements.
>>>
>>> That said, overloading the right child pointer might not be the best
>>> thing for the linux kernel, as it will impact all the rb-tree lookups
>>> which are open-coded and often performance critical (this is the reason
>>> the colour isn't bit encoded in either of the child pointers either).
>>>
>>> But if you only want a uni directional thread, I guess we can stick it
>>> in the unsigned long we use for the node colour.
>>>
>>> Still, perhaps it's worth it to grow rb_node to 4 words and do the fully
>>> threaded thing as there are also a lot of rb_prev() users in the kernel.
>>> Who knows..
>>>
>>> Anyway, I agree that improving rb_next() is worth looking into for the
>>> scheduler.
>> For the IO scheduler as well, it's used quite extensively! So speeding
>> up rb_next() would definitely help, as it's typically invoked for every
>> bio queued (attempting to back merge with the next request). CFQ and AS
>> additionally does an rb_next() and rb_prev() when trying to decide which
>> request to do next.
> 
> One possible course of action to implement this without eating extra
> space in the rb_node would be:
> 
> - Add rb_right() and rb_set_right() (plus ditto _left variants) to
>   rbtree.h
> - Convert all in-kernel users to use these. Quite extensive, as the
>   rbtree code search/insert functions are coded in situ and not in
>   rbtree.[ch]
> - Now we can overload bit 0 of ->rb_right and ->rb_left to indicate
>   whether this is a node or thread pointer and modify rbtree.c to tag
>   and add the thread links when appropriate.
> 

Exactly along the lines I was thinking of.and discussing with David.

> So we can definitely do this in a compatible fashion. Given that I have
> a flight coming up in a few days time, I may give it a got if no one
> beats me to it :-)
> 

Feel free to do so, please do keep me on the cc. I am very interested in getting
rb threaded trees done, but my bandwidth is a little limited this month.


-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL
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