linux-kernel - Re: posible latency issues in seq

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Date:	Sat, 21 Jul 2007 05:46:45 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Chris Friesen <cfriesen@...tel.com>
CC:	Lee Revell <rlrevell@...-job.com>, linux-kernel@...r.kernel.org,
	linux-net@...r.kernel.org
Subject: Re: posible latency issues in seq_read

Chris Friesen a écrit :
> Lee Revell wrote:
>> On 7/20/07, Chris Friesen <cfriesen@...tel.com> wrote:
> 
>>> We've run into an issue (on 2.6.10) where calling "lsof" triggers lost
>>> packets on our server.  Preempt is disabled, and NAPI is enabled.
> 
>> Can you reproduce with a recent kernel?  Lots of latency issues have
>> been fixed since then.
> 
> Unfortunately I have to fix it on this version (the bug was found on 
> shipped product), so if there was a difference I'd have to isolate the 
> changes and backport them.  Also, I can't run the software that triggers 
> the problem on a newer kernel as it has dependencies on various patches 
> that are not in mainline.
> 
> Basically what I'd like to know is whether calling schedule() in 
> seq_read() is safe or whether it would break assumptions made by 
> seq_file users.
> 

It wont help much. seq_read() is fine in itself.

The problem is in established_get_next() and established_get_first() not 
allowing softirq processing, while scanning a possibly huge hash table, even 
if few sockets are hashed in.

As cond_resched_softirq() was added in linux-2.6.11, you probably *need* to 
check the diffs between linux-2.6.10 & linux-2.6.11

files :

include/linux/sched.h
net/core/sock.c      (__release_sock() latency)
net/ipv4/tcp_ipv4.c  (/proc/net/tcp latency)


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