linux-kernel - Re: [ 146/184] softirq: reduce latencies

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Date:	Fri, 2 Aug 2013 16:14:13 +0800
From:	Li Zefan <lizefan@...wei.com>
To:	Willy Tarreau <w@....eu>
CC:	<linux-kernel@...r.kernel.org>, <stable@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	David Miller <davem@...emloft.net>,
	Tom Herbert <therbert@...gle.com>,
	Ben Hutchings <bhutchings@...arflare.com>,
	Ben Greear <greearb@...delatech.com>, Tejun Heo <tj@...nel.org>
Subject: Re: [ 146/184] softirq: reduce latencies

Cc: Ben Greear
Cc: Tejun

Hi Willy,

This patch introduced a bug, which was then fixed by commit 34376a50fb1f
("Fix lockup related to stop_machine being stuck in __do_softirq."),
do we need this fix for 2.6.32 ?

On 2013/6/5 1:23, Willy Tarreau wrote:
> 2.6.32-longterm review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Eric Dumazet <edumazet@...gle.com>
> 
> In various network workloads, __do_softirq() latencies can be up
> to 20 ms if HZ=1000, and 200 ms if HZ=100.
> 
> This is because we iterate 10 times in the softirq dispatcher,
> and some actions can consume a lot of cycles.
> 
> This patch changes the fallback to ksoftirqd condition to :
> 
> - A time limit of 2 ms.
> - need_resched() being set on current task
> 
> When one of this condition is met, we wakeup ksoftirqd for further
> softirq processing if we still have pending softirqs.
> 
> Using need_resched() as the only condition can trigger RCU stalls,
> as we can keep BH disabled for too long.
> 
> I ran several benchmarks and got no significant difference in
> throughput, but a very significant reduction of latencies (one order
> of magnitude) :
> 
> In following bench, 200 antagonist "netperf -t TCP_RR" are started in
> background, using all available cpus.
> 
> Then we start one "netperf -t TCP_RR", bound to the cpu handling the NIC
> IRQ (hard+soft)
> 
> Before patch :
> 
> RT_LATENCY,MIN_LATENCY,MAX_LATENCY,P50_LATENCY,P90_LATENCY,P99_LATENCY,MEAN_LATENCY,STDDEV_LATENCY
> MIGRATED TCP REQUEST/RESPONSE TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET
> to 7.7.7.84 () port 0 AF_INET : first burst 0 : cpu bind
> RT_LATENCY=550110.424
> MIN_LATENCY=146858
> MAX_LATENCY=997109
> P50_LATENCY=305000
> P90_LATENCY=550000
> P99_LATENCY=710000
> MEAN_LATENCY=376989.12
> STDDEV_LATENCY=184046.92
> 
> After patch :
> 
> RT_LATENCY,MIN_LATENCY,MAX_LATENCY,P50_LATENCY,P90_LATENCY,P99_LATENCY,MEAN_LATENCY,STDDEV_LATENCY
> MIGRATED TCP REQUEST/RESPONSE TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET
> to 7.7.7.84 () port 0 AF_INET : first burst 0 : cpu bind
> RT_LATENCY=40545.492
> MIN_LATENCY=9834
> MAX_LATENCY=78366
> P50_LATENCY=33583
> P90_LATENCY=59000
> P99_LATENCY=69000
> MEAN_LATENCY=38364.67
> STDDEV_LATENCY=12865.26
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: David Miller <davem@...emloft.net>
> Cc: Tom Herbert <therbert@...gle.com>
> Cc: Ben Hutchings <bhutchings@...arflare.com>
> Signed-off-by: David S. Miller <davem@...emloft.net>
> (cherry picked from commit c10d73671ad30f54692f7f69f0e09e75d3a8926a)
> Signed-off-by: Willy Tarreau <w@....eu>

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