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Date:   Thu, 27 Jul 2023 20:05:44 +0800
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Subject: Re: [External] Re: [PATCH] cgroup/rstat: record the cumulative
 per-cpu time of cgroup and its descendants
From:   Hao Jia <jiahao.os@bytedance.com>
To:     Tejun Heo <tj@kernel.org>
Cc:     lizefan.x@bytedance.com, hannes@cmpxchg.org, mkoutny@suse.com,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org
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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