On Tue, 2010-12-21 at 18:28 +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
> To use the generic percpu counter infrastructure for counters that
> require conditional addition based on a threshold value we need
> special handling of the counter. Further, the caller needs to know
> the status of the conditional addition to determine what action to
> take depending on whether the addition occurred or not. Examples of
> this sort of usage are resource counters that cannot go below zero
> (e.g. filesystem free blocks).
> To allow XFS to replace it's complex roll-your-own per-cpu
> superblock counters, a single generic conditional function is
> required: percpu_counter_add_unless_lt(). This will add the amount
> to the counter unless the result would be less than the given
> threshold. A caller supplied threshold is required because XFS does
> not necessarily use the same threshold for every counter.
> percpu_counter_add_unless_lt() attempts to minimise counter lock
> traversals by only taking the counter lock when the threshold is
> within the error range of the current counter value. Hence when the
> threshold is not within the counter error range, the counter will
> still have the same scalability characteristics as the normal
> percpu_counter_add() function.
> Adding this functionality to the generic percpu counters allows us
> to remove the much more complex and less efficient XFS percpu
> counter code (~700 lines of code) and replace it with generic
> percpu counters.
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
I want to look at this one more closely again in the
morning, but for now I'll just mention one nit, and
one easily fixed problem.
. . .
> + * Add @amount to @fdc if and only if result of addition is greater than or
^^^ should be fbc
This has to be:
(with leading underscores).