linux-kernel - Re: [PATCH 0/4] Finer granularity and task/cgroup irq time accounting

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Date:	Mon, 23 Aug 2010 17:56:42 -0700
From:	Venkatesh Pallipadi <venki@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/4] Finer granularity and task/cgroup irq time accounting

Peter,

Ping.
Does the patchset look sane.

Thanks,
Venki


On Mon, Jul 19, 2010 at 4:57 PM, Venkatesh Pallipadi <venki@...gle.com> wrote:
>
> Earlier version of this patchset here -
> lkml subject:
> "[RFC PATCH 0/4] Finer granularity and task/cgroup irq time accounting"
> http://marc.info/?l=linux-kernel&m=127474630527689&w=2
>
> Currently, the softirq and hardirq time reporting is only done at the
> CPU level. There are usecases where reporting this time against task
> or task groups or cgroups will be useful for user/administrator
> in terms of resource planning and utilization charging. Also, as the
> accoounting is already done at the CPU level, reporting the same at
> the task level does not add any significant computational overhead
> other than task level storage (patch 1).
>
> The softirq/hardirq statistics commonly done based on tick based sampling.
> Though some archs have CONFIG_VIRT_CPU_ACCOUNTING based fine granularity
> accounting. Having similar mechanism to get fine granularity accounting
> on x86 will be a major challenge, given the state of TSC reliability
> on various platforms and also the overhead it may add in common paths
> like syscall entry exit.
>
> An alternative is to have a generic (sched_clock based) and configurable
> fine-granularity accounting of si and hi time which can be reported
> over the /proc/<pid>/stat API (patch 2).
>
> Patch 3 and 4 are exporting this info at the cgroup level.
>
> Changes since the original RFC -
> * General code cleanup and documentation for new APIs added.
> * Handle notsc option by having a runtime flag sched_clock_irqtime, along
>  with the original CONFIG_IRQ_TIME_ACCOUNTING option.
>  Peter Zijlstra suggested the use of alternate instruction kind of mechanism
>  here. But, that is mostly x86 specific and not generic. The irq time
>  accounting code is mostly generic.
> * Did performance runs with various systems with tsc based sched_clock -
>  both with and without sched_clock_stable - running tbench, dbench, SPECjbb
>  and did not notice any measurable slowness when this option is enabled.
> Todo -
> * Peter Zijlstra suggested modifying scale_rt_power to account for
>  irq time. I have a patch for that and have been testing that right now.
>  But, that change is not very pretty as yet and also will need some more
>  testing. Feels better to make that a separate change. Will follow up
>  on that soon.
>
> Thanks,
> Venki
>
>
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