Re: [PATCH] net: refactor icmp_global_allow to improve readability and performance.

[Eric Dumazet]

On Thu, 2015-12-31 at 23:58 -0800, Mike Danese wrote:
> We can reduce the number of operations performed by icmp_global_allow
> and make the routine more readable by refactoring it in two ways:
> 
> First, this patch refactors the meaning of the "delta" variable. Before
> this change, it meant min("time since last refill of token bucket", HZ).
> After this change, it means "time since last refill". The original
> definition is required only once but was being calculated twice. The new
> meaning is also more intuitive for a variable named "delta".
> 
> Second, by calculating "delta" (time since last refill of token bucket)
> and "cbr" (token bucket can be refilled) at the beginning of the
> routine, we reduce the number of repeated calculations of these two
> variables.
> 
> There should be no functional difference.
> 
> Signed-off-by: Mike Danese <mikedanese@xxxxxxxxxx>
> ---
>  net/ipv4/icmp.c | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)

Hi Mike

Sorry, this is a very broken patch.

There is a comment you apparently missed completely :

/* Check if token bucket is empty and cannot be refilled
 * without taking the spinlock.
 */

There is a reason we compute 'delta' two times.

One without the spinlock held, and a second time with the spinlock held.

This is an opportunistic way to exit early without false sharing in the
stress case where many cpus might enter this code.

Really I do not think current code needs any 'refactoring', especially
around December 31th at midnight ;)



--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/