netdev - Re: [PATCH] net: reduce net_rx

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Date:	Thu, 21 Mar 2013 08:27:27 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Paul Gortmaker <paul.gortmaker@...driver.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>, stable <stable@...r.kernel.org>,
	Willy Tarreau <w@....eu>, Tom Herbert <therbert@...gle.com>
Subject: Re: [PATCH] net: reduce net_rx_action() latency to 2 HZ

On Thu, 2013-03-21 at 11:03 -0400, Paul Gortmaker wrote:
> [CC'ing stable & Willy - for the older releases not fed by
> http://patchwork.ozlabs.org/bundle/davem/stable/ ]
> 
> On Tue, Mar 5, 2013 at 12:15 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > From: Eric Dumazet <edumazt@...gle.com>
> >
> > We should use time_after_eq() to get maximum latency of two ticks,
> > instead of three.
> >
> > Bug added in commit 24f8b2385 (net: increase receive packet quantum)
> 
> I'm not sure what applications would notice the extra tick, but 24f8b takes
> us back to 2.6.29.  It cherry picks cleanly onto 2.6.34, so it probably also
> does the same for Willy's 2.6.32 longterm too.
> 
> Commit is now mainline d114a3338747255518 - v3.9-rc3~36^2~34.

BQL (Bytes Queue Limit) relies on TX completion being run often, and
Qdisc being serviced often as well. If net_rx_action() hogs the cpu,
net_tx_action() is delayed and NIC can stall.

I wrote this patch because I was investigating a regression when a
Google application began using BQL enabled kernels.

About the latency in itself, following commit is way more interesting.

commit c10d73671ad30f5 (softirq: reduce latencies)

As without it, I could trigger more than 50ms latencies for the poor
user thread interrupted by softirq processing.


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