netdev - Re: [PATCH v6] net: batch skb dequeueing from softnet input_pkt

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Date:	Mon, 03 May 2010 10:45:07 -0400
From:	Brian Bloniarz <bmb@...enacr.com>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	Andi Kleen <andi@...stfloor.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	David Miller <davem@...emloft.net>, hadi@...erus.ca,
	xiaosuo@...il.com, therbert@...gle.com, shemminger@...tta.com,
	netdev@...r.kernel.org, lenb@...nel.org
Subject: Re: [PATCH v6] net: batch skb dequeueing from softnet input_pkt_queue

Arjan van de Ven wrote:
> On Mon, 3 May 2010 12:34:26 +0200
> Andi Kleen <andi@...stfloor.org> wrote:
> 
>>>> Maybe its low cost, (apparently, it is, since I can reach ~900.000
>>>> ipis on my 16 cores machine) but multiply this by 16 or 32 or 64
>>>> cpus, and clockevents_notify() cost appears to be a killer, all
>>>> cpus compete on a single lock.
>>>>
>>>> Maybe this notifier could use RCU ?
>>> could this be an artifact of the local apic stopping in deeper C
>>> states? (which is finally fixed in the Westmere generation)
>> Yes it is I think.
>>
>> But I suspect Eric wants a solution for Nehalem.
> 
> sure ;-)
> 
> 
> so the hard problem is that on going idle, the local timers need to be
> funneled to the external HPET. Afaik right now we use one channel of
> the hpet, with the result that we have one global lock for this.

Does the HPET only need to be programmed when going idle?
That could mean that this isn't a big performance issue.
cares if you spin for a while when you're about to sleep for
at least 60usec?

> HPETs have more than one channel (2 or 3 historically, newer chipsets
> iirc have a few more), so in principle we can split this lock at least
> a little bit... if we can get to one hpet channel per level 3 cache
> domain we'd already make huge progress in terms of cost of the
> contention....

Another possible approach: if a core needs the HPET and finds it
locked, it could queue up its request to a backlog which the
locking core will service.
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