Re: [PATCH 4/5] perf_events: add cgroup support (v7)

[Li Zefan <lizf@cn.fujitsu.com>]

Stephane Eranian wrote:
> This kernel patch adds the ability to filter monitoring based on
> container groups (cgroups). This is for use in per-cpu mode only.
>     
> The cgroup to monitor is passed as a file descriptor in the pid
> argument to the syscall. The file descriptor must be opened to 
> the cgroup name in the cgroup filesystem. For instance, if the
> cgroup name is foo and cgroupfs is mounted in /cgroup, then the
> file descriptor is opened to /cgroup/foo. Cgroup mode is
> activated by passing PERF_FLAG_PID_CGROUP in the flags argument
> to the syscall.
> 
> For instance to measure in cgroup foo on CPU1 assuming
> cgroupfs is mounted under /cgroup:
> 
> struct perf_event_attr attr;
> int cgroup_fd, fd;
> 
> cgroup_fd = open("/cgroup/foo", O_RDONLY);
> fd = perf_event_open(&attr, cgroup_fd, 1, -1, PERF_FLAG_PID_CGROUP);
> close(cgroup_fd);
> 
> Signed-off-by: Stephane Eranian <eranian@google.com>

For the cgroup part:

Acked-by: Li Zefan <lizf@cn.fujitsu.com>

a few comments below:

> 
...
> +config CGROUP_PERF
> +	bool "Enable perf_event per-cpu per-container group (cgroup) monitoring"
> +	depends on PERF_EVENTS && CGROUPS

Depending on CGROUPS is already implicated by "if CGROUPS"

> +	help
> +	  This option extends the per-cpu mode to restrict monitoring to
> +	  threads which belong to the cgroup specificied and run on the
> +	  designated cpu.
> +
> +	  Say N if unsure.
> +
>  menuconfig CGROUP_SCHED
>  	bool "Group CPU scheduler"
>  	depends on EXPERIMENTAL
...
> +static inline int perf_cgroup_connect(int fd, struct perf_event *event,
> +				      struct perf_event_attr *attr,
> +				      struct perf_event *group_leader)
> +{
> +	struct perf_cgroup *cgrp;
> +	struct cgroup_subsys_state *css;
> +	struct file *file;
> +	int ret = 0, fput_needed;
> +
> +	file = fget_light(fd, &fput_needed);
> +	if (!file)
> +		return -EBADF;
> +
> +	css = cgroup_css_from_dir(file, perf_subsys_id);
> +	if (IS_ERR(css))
> +		return PTR_ERR(css);
> +
> +	cgrp = container_of(css, struct perf_cgroup, css);
> +	event->cgrp = cgrp;
> +
> +	/*
> +	 * all events in a group must monitor
> +	 * the same cgroup because a thread belongs
> +	 * to only one perf cgroup at a time
> +	 */
> +	if (group_leader && group_leader->cgrp != cgrp) {
> +		perf_detach_cgroup(event);

This doesn't seem right, because we haven't got the cgroup
refcount.

> +		ret = -EINVAL;
> +	} else {
> +		/* must be done before we fput() the file */
> +		perf_get_cgroup(event);
> +	}
> +	fput_light(file, fput_needed);
> +	return ret;
> +}