[PATCH 0/4] Finer granularity and task/cgroup irq time accounting

From: Venkatesh Pallipadi
Date: Mon Jul 19 2010 - 19:57:42 EST

Earlier version of this patchset here -
lkml subject:
"[RFC PATCH 0/4] Finer granularity and task/cgroup irq time accounting"

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.


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