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Subject[PATCH 0/2] perf_events: add support for per-cpu per-cgroup monitoring (v8)
This series of patches adds per-container (cgroup) filtering capability
to per-cpu monitoring. In other words, we can monitor all threads belonging
to a specific cgroup and running on a specific CPU.

This is useful to measure what is going on inside a cgroup. Something that
cannot easily and cheaply be achieved with either per-thread or per-cpu mode.
Cgroups can span multiple CPUs. CPUs can be shared between cgroups. Cgroups
can have lots of threads. Threads can come and go during a measurement.

To measure per-cgroup today requires using per-thread mode and attaching to
all the current threads inside a cgroup and tracking new threads. That would
require scanning of /proc/PID, which is subject to race conditions, and
creating an event for each thread, each event requiring kernel memory.

The approach taken by this patch is to leverage the per-cpu mode by simply
adding a filtering capability on context switch only when necessary. That
way the amount of kernel memory used remains bound by the number of CPUs.
We also do not have to scan /proc. We are only interested in cgroup level
counts, samples and not thread level.

The cgroup to monitor is designated by passing a file descriptor opened
on the cgroup directory name in the cgroup filesystem. The cgroup mode
is activated by passing a flag value in the perf_event syscall.

The patch also includes changes to the perf tool to make use of cgroup
filtering. Both perf stat and perf record have been extended to support
cgroup via a new -G option. The cgroup is specified per event:

$ perf stat -B -a -e cycles:u,cycles:u,cycles:u -G test1,,test2 -- sleep 1
Performance counter stats for 'sleep 1':

2,368,667,414 cycles test1
2,369,661,459 cycles
<not counted> cycles test2

1.001856890 seconds time elapsed

Here, we measure cycles in 3 different cgroups. When a cgroup is omitted,
the "root" cgroup is used, i.e., all threads executing on the monitored
CPUs are measured.

In the second version, time tracking has been updated. In cgroup mode,
time_enabled tracks the time during which the cgroup was active, i.e., threads
from the cgroup executed on the monitored CPU. The meaning of time_running
is unchanged. In non-cgroup mode, time_enabled still tracks wall-clock time
for per-cpu events. Here is an example:

In one shell, I do:
$ echo $$ >/cgroup/test1/perf_events.perf
$ taskset -c 1 noploop 600

In another shell, I do:
$ taskset -c 1 noploop 600

Both noploops are competing on CPU1 (part of cgroup test1)

$ perf stat -B -a -e cycles:u,cycles:u,cycles:u -G test1,,test2 -- sleep 1
Performance counter stats for 'sleep 1':

1,190,595,954 cycles test1
2,372,471,023 cycles
<not counted> cycles test2

1.001845567 seconds time elapsed

The second count reflects activity across all CPUs and cgroups.
The first count reflects what happened inside cgroup test1. As shown,
the noploop running inside test1, only got half the CPU cycles.

In the third version, we have dropped dependency on NR_CPUS
in favor of dynamic allocation with alloc_percpu(). We have
also renamed get_event_time() to something more explicit:
perf_event_time(). We cleaned the code so it compiles with
CONFIG_CGROUPS disabled. We have also fixed a bug in the
perf tool sampling module builtin-record.c

In this fourth version, we have dropped changes to perf_event_attr.
Instead we pass the cgroup file descriptor in the pid argument to
the syscall and we active cgroup mode by passing PERF_FLAG_PID_CGROUP
into flags.

The fifth version updates the patch to 2.6.37-rc2 and contains improved
time tracking support. Several bug fixes and cleanups. Context switch
impact is also mitigated with dynamic jump labels.

The sixth version breaks the series of patches into 5 parts and
also updates the patch to 2.6.37-rc4-tip. It also includes a few
bug fixes.

In the seventh version, we upate the patch to 2.6.37-rc8-tip, and
cleanup the code some more based on LKML comments. This patch adds
a CONFIG_CGROUP_PERF option dependent on PERF_EVENTS && CGROUPS.

In the eighth version, we have fixed some important issues related
to cgroup switches and the way we handle exiting tasks (do_exit()).
We have also updated the perf tool patch to support a variable number
of events (and thus cgroups).

PATCH 0/2: introduction
PATCH 1/2: actual cgroup support
PATCH 2/2: perf tool changes for cgroup

Signed-off-by: Stephane Eranian <eranian@google.com>


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