linux-kernel - Re: Make yield_task

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Message-ID: <20080221205544.GV23197@kernel.dk>
Date:	Thu, 21 Feb 2008 21:55:45 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	balbir@...ux.vnet.ibm.com, 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
Subject: Re: Make yield_task_fair more efficient

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.

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 :-)

-- 
Jens Axboe

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