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

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Date:	Fri, 04 Jan 2013 00:15:51 -0800
From:	Joe Perches <joe@...ches.com>
To:	Eric Dumazet <eric.dumazet@...il.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 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?
 
>  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.


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