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

On Fri, 2013-01-04 at 00:15 -0800, Joe Perches wrote:

On Thu, 2013-01-03 at 23:49 -0800, Eric Dumazet wrote:

quoted

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.

[]

quoted

diff --git a/kernel/softirq.c b/kernel/softirq.c

[]

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+#define MAX_SOFTIRQ_TIME  max(1, (2*HZ/1000))

And if HZ is 10000?
 

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

quoted

 asmlinkage void __do_softirq(void)
 {

[]

quoted

+	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.