linux-kernel - [PATCH] sched: Simplify move

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Date:	Sat, 04 Aug 2007 12:31:24 +1000
From:	Peter Williams <pwil3058@...pond.net.au>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Nick Piggin <npiggin@...e.de>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [PATCH] sched: Simplify move_tasks()

The move_tasks() function is currently multiplexed with two distinct 
capabilities:

1. attempt to move a specified amount of weighted load from one run 
queue to another; and
2. attempt to move a specified number of tasks from one run queue to 
another.

The first of these capabilities is used in two places, load_balance() 
and load_balance_idle(), and in both of these cases the return value of 
move_tasks() is used purely to decide if tasks/load were moved and no 
notice of the actual number of tasks moved is taken.

The second capability is used in exactly one place, 
active_load_balance(), to attempt to move exactly one task and, as 
before, the return value is only used as an indicator of success or failure.

This multiplexing of sched_task() was introduced, by me, as part of the 
smpnice patches and was motivated by the fact that the alternative, one 
function to move specified load and one to move a single task, would 
have led to two functions of roughly the same complexity as the old 
move_tasks() (or the new balance_tasks()).  However, the new modular 
design of the new CFS scheduler allows a simpler solution to be adopted 
and this patch addresses that solution by:

1. adding a new function, move_one_task(), to be used by 
active_load_balance(); and
2. making move_tasks() a single purpose function that tries to move a 
specified weighted load and returns 1 for success and 0 for failure.

One of the consequences of these changes is that neither move_one_task()
or the new move_tasks() care how many tasks sched_class.load_balance() 
moves and this enables its interface to be simplified by returning the 
amount of load moved as its result and removing the load_moved pointer 
from the argument list.  This helps simplify the new move_tasks() and 
slightly reduces the amount of work done in each of 
sched_class.load_balance()'s implementations.

Further simplification, e.g. changes to balance_tasks(), are possible 
but (slightly) complicated by the special needs of load_balance_fair() 
so I've left them to a later patch (if this one gets accepted).

NB Since move_tasks() gets called with two run queue locks held even 
small reductions in overhead are worthwhile.

Signed-off-by: Peter Williams <pwil3058@...pond.net.au>

-- 
Peter Williams                                   pwil3058@...pond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
  -- Ambrose Bierce

View attachment "simplify-move_tasks.patch" of type "text/x-patch" (9427 bytes)