Hi Song, On Wed, Mar 24, 2021 at 9:30 AM Song Liu <songliubrav...@fb.com> wrote: > > > > > On Mar 23, 2021, at 9:21 AM, Namhyung Kim <namhy...@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!
Thanks! > > 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? Sure, a perf_event belongs to an event context (hw or sw, mostly) which takes care of multiplexing, timing, locking and so on. So many of the fields in the perf_event are protected by the context lock. A context has a list of perf_events and there are per-cpu contexts and per-task contexts. The cgroup_ctx_list is to traverse contexts (in that cpu) only having perf_events with attached cgroups. Hope this makes it clear. Please let me know if you need more. :) Thanks, Namhyung
