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

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Date:	Fri, 04 Jan 2013 00:23:06 -0800
From:	Eric Dumazet <erdnetdev@...il.com>
To:	Joe Perches <joe@...ches.com>
Cc:	Ben Hutchings <bhutchings@...arflare.com>,
	David Miller <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	netdev <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Tom Herbert <therbert@...gle.com>
Subject: 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:
> > 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.



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