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

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Message-ID: <AANLkTinFyHV1HdX1TcOX_-sqf_5-Xdqo0q-z9HZo4qqz@mail.gmail.com>
Date:	Tue, 25 May 2010 14:45:03 -0700
From:	Venkatesh Pallipadi <venki@...gle.com>
To:	balbir@...ux.vnet.ibm.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

On Tue, May 25, 2010 at 2:13 AM, Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> * 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.
>

Haven't noticed any significant overhead yet. But, I am yet to run this on wide
array of systems. Will post the data once I have them.

Thanks,
Venki
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