Re: [PATCH 1/2] perf/core: Share an event with multiple cgroups

[Song Liu <songliubraving@fb.com>]

> On Mar 23, 2021, at 9:21 AM, Namhyung Kim <namhyung@kernel.org> wrote:
> 
> As we can run many jobs (in container) on a big machine, we want to
> measure each job's performance during the run.  To do that, the
> perf_event can be associated to a cgroup to measure it only.
> 
> However such cgroup events need to be opened separately and it causes
> significant overhead in event multiplexing during the context switch
> as well as resource consumption like in file descriptors and memory
> footprint.
> 
> As a cgroup event is basically a cpu event, we can share a single cpu
> event for multiple cgroups.  All we need is a separate counter (and
> two timing variables) for each cgroup.  I added a hash table to map
> from cgroup id to the attached cgroups.
> 
> With this change, the cpu event needs to calculate a delta of event
> counter values when the cgroups of current and the next task are
> different.  And it attributes the delta to the current task's cgroup.
> 
> This patch adds two new ioctl commands to perf_event for light-weight
> cgroup event counting (i.e. perf stat).
> 
> * PERF_EVENT_IOC_ATTACH_CGROUP - it takes a buffer consists of a
>     64-bit array to attach given cgroups.  The first element is a
>     number of cgroups in the buffer, and the rest is a list of cgroup
>     ids to add a cgroup info to the given event.
> 
> * PERF_EVENT_IOC_READ_CGROUP - it takes a buffer consists of a 64-bit
>     array to get the event counter values.  The first element is size
>     of the array in byte, and the second element is a cgroup id to
>     read.  The rest is to save the counter value and timings.
> 
> This attaches all cgroups in a single syscall and I didn't add the
> DETACH command deliberately to make the implementation simple.  The
> attached cgroup nodes would be deleted when the file descriptor of the
> perf_event is closed.

This is very interesting idea! 

Could you please add some description of the relationship among 
perf_event and contexts? The code is a little confusing. For example,
why do we need cgroup_ctx_list? 

Thanks,
Song 

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