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SubjectRe: [External] Re: [PATCH] cgroup/rstat: record the cumulative per-cpu time of cgroup and its descendants
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On 2023/7/19 Hao Jia wrote:
>
>
> On 2023/7/19 Tejun Heo wrote:
>> On Tue, Jul 18, 2023 at 06:08:50PM +0800, Hao Jia wrote:
>>> https://github.com/jiaozhouxiaojia/cgv2-stat-percpu_test/tree/main
>>
>> So, we run `stress -c 1` for 1 second in the asdf/test0 cgroup and
>> asdf/cpu.stat correctly reports the cumulative usage. After removing
>> asdf/test0 cgroup, asdf's usage_usec is still there. What's missing here?
>
> Sorry, some of my expressions may have misled you.
>
> Yes, cpu.stat will display the cumulative **global** cpu time of the
> cgroup and its descendants (the corresponding kernel variable is
> "cgrp->bstat"), and it will not be lost when the child cgroup is removed.
>
> Similarly, we need a **per-cpu** variable to record the accumulated
> per-cpu time of cgroup and its descendants.
> The existing kernel variable "cgroup_rstat_cpu(cgrp, cpu)->bstat" is not
> satisfied, it only records the per-cpu time of cgroup itself,
> So I try to add "cgroup_rstat_cpu(cgrp, cpu)->cumul_bstat" to record
> per-cpu time of cgroup and its descendants.
>
> In order to verify the correctness of my patch, I wrote a kernel module
> to compare the results of calculating the per-cpu time of cgroup and its
> descendants in two ways:
>   Method 1. Traverse and add the per-cpu rstatc->bstat of cgroup and
> each of its descendants.
>   Method 2. Directly read "cgroup_rstat_cpu(cgrp, cpu)->cumul_bstat" in
> the kernel.
>
> When the child cgroup is not removed, the results calculated by the two
> methods should be equal.
>
>> What are you adding?
> I want to add a **per-cpu variable** to record the cumulative per-cpu
> time of cgroup and its descendants, which is similar to the variable
> "cgrp->bstat", but it is a per-cpu variable.
> It is very useful and convenient for calculating the usage of cgroup on
> each cpu, and its behavior is similar to the "cpuacct.usage*" interface
> of cgroup v1.
>

Hello Tejun,

I don't know if I explained it clearly, and do you understand what I mean?

Would you mind adding a variable like this to facilitate per-cpu usage
calculations and migration from cgroup v1 to cgroup v2?

Thanks,
Hao

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