RE: [PATCH 0/5] sched: Lazy preemption muck

[David Laight <David.Laight@ACULAB.COM>]

...
> And once all the problems with LAZY are sorted then this cond_resched()
> line just goes away and the loop looks like this:
> 
>     while ($cond) {
>           spin_lock(L);
>           do_stuff();
>           spin_unlock(L);
>     }

The problem with that pattern is the cost of the atomics.
Thay can easily be significant especially if there are
a lot of iterations and do_stuff() is cheap;

If $cond needs the lock, the code is really:
	spin_lock(L);
	while ($cond) {
		do_stuff();
		spin_unlock(L);
		spin_lock(L);
	}
	spin_unlock(L);
which make it even more obvious that you need a cheap
test to optimise away the unlock/lock pair.

Perhaps it could be wrapped inside a spin_relax(L)?

	David

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