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

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Date:	Fri, 04 Jan 2013 09:00:24 -0800
From:	Eric Dumazet <eric.dumazet@...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 01:12 -0800, Joe Perches wrote:
> On Fri, 2013-01-04 at 00:23 -0800, Eric Dumazet wrote:
> > On Fri, 2013-01-04 at 00:15 -0800, Joe Perches wrote:
> > > 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.
> 
> Something like this might work.
> 
> This is incomplete, it just does msecs_to_jiffies,
> and it should convert usecs_to_jiffies and the
> jiffies_to_<foo> types too.
> 
> Maybe it's worthwhile.
> 
> It does reduce object size by 16 bytes per call site
> (x86-32) when the argument is a constant. There are
> about 800 of these jiffies conversions in kernel sources.
> 
> What do you think?
> 

I think this is something to discuss in another thread, and definitely
worth to do, at least for msecs_to_jiffies()

We have many HZ references everywhere that could be cleaned up using
this.



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