netdev - Re: [PATCH net-next 6/6] net_sched: act: remove spinlock in fast path

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Date:	Thu, 02 Jul 2015 22:59:10 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	John Fastabend <john.fastabend@...il.com>
Cc:	Eric Dumazet <edumazet@...gle.com>,
	"David S. Miller" <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	Eric Dumazet <edumazet@...il.com>
Subject: Re: [PATCH net-next 6/6] net_sched: act: remove spinlock in fast
 path

On Thu, 2015-07-02 at 09:35 -0700, John Fastabend wrote:
> > +	if (gact->tcf_tm.lastuse != jiffies)
> > +		gact->tcf_tm.lastuse = jiffies;
> 
> I'm missing the point of the if block. Is that really good enough
> for the 32bit system case? I would have expected some wrapper to
> handle it here something like u64_stats_() maybe _u64_jiffies(). Maybe
> after a coffee I'll make sense of it.
> 

Point is to not dirty cache line for every packet ?

Doing the test means we attempt dirtying only ~HZ times per second,
which really matters to handle millions of packets per second.

My tests show a good enough performance, not sure we want a percpu thing
for this lastuse field.



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