linux-kernel - Re: [RFC PATCH 1/4] sched: Track and export per task [hard|soft]irq time

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Date:	Tue, 25 May 2010 14:43:00 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Venkatesh Pallipadi <venki@...gle.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul Menage <menage@...gle.com>, linux-kernel@...r.kernel.org,
	Paul Turner <pjt@...gle.com>
Subject: Re: [RFC PATCH 1/4] sched: Track and export per task [hard|soft]irq
 time

* Venkatesh Pallipadi <venki@...gle.com> [2010-05-24 17:11:19]:

> Currently, kernel does not have accounting mechanism for softirq and hardirq
> times at the task level. There is irq time info in kstat_cpu which is
> accumulated at the cpu level.
> 
> Without the task level information, the non irq run time of task(s) would
> have to be guessed based on their exec time and CPU on which they were
> running recently and assuming that the CPU irq time reported are spread
> across all the tasks running there. And this guess can be widely off the mark.
> 
> Sample case, considering just the softirq:
> 
> If there are varied workloads running on a CPU, say a CPU bound task (loop)
> and a network IO bound task (nc) along with the network softirq load,
> there is no way for the administrator/user to know the non-irq runtime of each
> of these tasks. Only information available is the total runtime for each of the
> tasks and kstat_cpu softirq time for the CPU.
> 
> In this example, considering a 10 second sample, both loop and nc would have
> total run time of ~5s. And kstat_cpu softirq on this cpu increase was
> 355 (~3.5s).
> 
> So, all the information the user gets is that both the tasks are running for
> roughly the same amount of time and softirq is around 35%. As a result user
> may conclude that irq overhead for both tasks are equal (1.75s) and the
> non-irq runtime of both the tasks are around ~3.25s. Yes. There is another
> factor of system and user time reported for these tasks that I am ignoring
> as that is tough to correlate with irq time, in cases where the tasks have
> significant non-irq system time.
> 
> This change adds tracking of softirq time on each task and task group.
> This information is exported in /proc/<pid>/stat.
> 
> So, the user can get info like below, looking at exec_time and si_time in
> appropriate /proc/<pid>/stat.
> (Taken for a 10s interval)
> task exec_time softirqtime (in USER_HZ)
> (loop)  (nc)
> 505 0   500 359
> 502 1   501 363
> 503 0   502 354
> 504 0   499 359
> 503 3   500 360
> 
> with this, user can get the non-irq run time as 5s and ~1.45s for
> loop and nc, respectively.

Have you noticed any overheads after these changes? Otherwise, the
changes look correct to me.


-- 
	Three Cheers,
	Balbir
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