get docker container cpu usage from cpuacct.usage

[CMGS]

This PR for:

• Get cgroup cpu usage from cpuacct.usage, it's a nanoseconds data, so I trans it to seconds.
• Make CgroupCPUWithContext return same unit like cpu.Times. I saw cpu.Times will trans jiffies to seconds, so here should divide CPUTick.

After this change, we can calculate container CPU percent in host view and container view like this:

interval := Seconds
containerCPUStat1 := CgroupCPUDocker(cid)
hostCPUStat1 := cpu.Times(false)[0]
time.sleep(interval)
hostCPUStat2 := cpu.Times(false)[0]
containerCPUStat2 := CgroupCPUDocker(cid)

percentInHostView := (containerCPUStat2.Usage - containerCPUStat1.Usage) / (hostCPUStat2.Total() - hostCPUStat1.Total())

percentInContainerView := percentInHostView * CPUNumbers

[shirou]

Thank you for your PR. but it seems this PR breaks compatibility about CgroupCPU. (*cpu.TimesStat -> CgroupCPUStat) We don't want to change API compatiblity, so it should be merge on v3.

or, because CgroupCPUUsage is just added and not break compatibility, we can merge it if you separate this PR.

[CMGS]

@shirou So, could I add Usage into cpu.TimesStat? In this way, CgroupCPU can return *cpu.TimesStat

[shirou]

No. cpuacct is not same meaning in all platform. If you add only CgroupCPUUsage users who use cgroup can get usage without breaking. compatibility. No need to add usage to CgroupCPU, I think.

[CMGS]

OK, let me think about it...

[CMGS]

separate PR done

[shirou]

Sorry for late response and thank you for your contribution!