linux-kernel - Re: [PATCH 1/3 v2] softirq: Introduce statistics for softirq

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Message-Id: <20081126191400.3CCF.KOSAKI.MOTOHIRO@jp.fujitsu.com>
Date:	Wed, 26 Nov 2008 19:14:58 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Keika Kobayashi <kobayashi.kk@...s.nec.co.jp>
Cc:	kosaki.motohiro@...fujitsu.com, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, h-shimamoto@...jp.nec.com
Subject: Re: [PATCH 1/3 v2] softirq: Introduce statistics for softirq

> On Sat, 22 Nov 2008 14:36:34 +0900 (JST)
> KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:
> 
> > > Statistics for softirq doesn't exist.
> > > It will be helpful like statistics for interrupts.
> > > This patch introduces counting the number of softirq,
> > > which will be exported in /proc/softirqs.
> > > 
> > > When softirq handler consumes much CPU time,
> > > /proc/stat is like the following.
> > > 
> > > $ while :; do  cat /proc/stat | head -n1 ; sleep 10 ; done
> > > cpu  88 0 408 739665 583 28 2 0 0
> > > cpu  450 0 1090 740970 594 28 1294 0 0
> > >                               ^^^^
> > >                              softirq
> > > 
> > > In such a situation,
> > > /proc/softirqs shows us which softirq handler is invoked.
> > > We can see the increase rate of softirqs.
> > > 
> > > <before>
> > > $ cat /proc/softirqs
> > >                 CPU0       CPU1       CPU2       CPU3
> > > HI                 0          0          0          0
> > > TIMER         462850     462805     462782     462718
> > > NET_TX             0          0          0        365
> > > NET_RX          2472          2          2         40
> > > BLOCK              0          0        381       1164
> > > TASKLET            0          0          0        224
> > > SCHED         462654     462689     462698     462427
> > > RCU             3046       2423       3367       3173
> > > 
> > > <after>
> > > $ cat /proc/softirqs
> > >                 CPU0       CPU1       CPU2       CPU3
> > > HI                 0          0          0          0
> > > TIMER         463361     465077     465056     464991
> > > NET_TX            53          0          1        365
> > > NET_RX          3757          2          2         40
> > > BLOCK              0          0        398       1170
> > > TASKLET            0          0          0        224
> > > SCHED         463074     464318     464612     463330
> > > RCU             3505       2948       3947       3673
> > 
> > Maybe, this printing format don't works well on >100 CPUS.
> > 
> > Do you have any experience of trouble-shooting by this patch?
> > Actually, I don't understand this patch usuful working situation.
> 
> When reported that CPU TIME of softirq went up,
> we had to investigate the cause.
> In that case, kind of network looked suspicious.
> We added such a function to confirm.
> 
> Original patch shows ticks of softirqs.
> But /proc/interrups have counts of interrupts.
> So we changed from ticks to counts.

ok. it seems reasonable reason.
and this patch looks good to me.

	Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>


>  
> > Honestly, I think HI and TASKLET are not beneficialness.
> > end user can't know what tasklet spent time.
> > TIMER also are not so much useful because it always have the same rate.
> > 
> > So.. guessing...
> > 
> > ultra band-width networking problem?


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