Re: [PATCH v2 net-next] softirq: reduce latencies

Eric Dumazet
2013-01-04T08:23:06Z

On Fri, 2013-01-04 at 00:15 -0800, Joe Perches wrote:
> On Thu, 2013-01-03 at 23:49 -0800, Eric Dumazet wrote:
> > In various network workloads, __do_softirq() latencies can be up
> > to 20 ms if HZ=1000, and 200 ms if HZ=100.
> > This patch changes the fallback to ksoftirqd condition to :
> > - A time limit of 2 ms.
>
> []
> > diff --git a/kernel/softirq.c b/kernel/softirq.c
> []
> > +#define MAX_SOFTIRQ_TIME max(1, (2*HZ/1000))
>
> And if HZ is 10000?
>

Then its OK. 2*10000/1000 -> 20 ticks -> 2 ms

> > asmlinkage void __do_softirq(void)
> > {
> []
> > + unsigned long end = jiffies + MAX_SOFTIRQ_TIME;
>
> Perhaps MAX_SOFTIRQ_TIME should be
>
> #define MAX_SOFTIRQ_TIME msecs_to_jiffies(2)
>
> though it would be nicer if it were a compile time constant.

If you send a patch to convert msecs_to_jiffies() to an inline function
when HZ = 1000, I will gladly use it instead of (2*HZ/1000)

Right now, max(1, msecs_to_jiffies(2)) uses way too many instructions,
while it should be the constant 2, known at compile time.