diff options
| author | Devin Kim <dojip.kim@lge.com> | 2014-10-14 22:00:26 +0900 |
|---|---|---|
| committer | Ziyan <jaraidaniel@gmail.com> | 2015-04-11 23:02:39 +0200 |
| commit | ea168ae3b076cb687bc35e7d3ca7189c4dbffc96 (patch) | |
| tree | 51741ce67f7f0c6480bfc04ad975e96fd43ea11a /kernel | |
| parent | ca319511ae38fa8d3c0ae980f424c5f56c208b06 (diff) | |
| download | kernel_samsung_tuna-ea168ae3b076cb687bc35e7d3ca7189c4dbffc96.zip kernel_samsung_tuna-ea168ae3b076cb687bc35e7d3ca7189c4dbffc96.tar.gz kernel_samsung_tuna-ea168ae3b076cb687bc35e7d3ca7189c4dbffc96.tar.bz2 | |
cgroup: remove synchronize_rcu() from cgroup_attach_{task|proc}()
These 2 syncronize_rcu()s make attaching a task to a cgroup
quite slow, and it can't be ignored in some situations.
A real case from Colin Cross: Android uses cgroups heavily to
manage thread priorities, putting threads in a background group
with reduced cpu.shares when they are not visible to the user,
and in a foreground group when they are. Some RPCs from foreground
threads to background threads will temporarily move the background
thread into the foreground group for the duration of the RPC.
This results in many calls to cgroup_attach_task.
In cgroup_attach_task() it's task->cgroups that is protected by RCU,
and put_css_set() calls kfree_rcu() to free it.
If we remove this synchronize_rcu(), there can be threads in RCU-read
sections accessing their old cgroup via current->cgroups with
concurrent rmdir operation, but this is safe.
# time for ((i=0; i<50; i++)) { echo $$ > /mnt/sub/tasks; echo $$ > /mnt/tasks; }
real 0m2.524s
user 0m0.008s
sys 0m0.004s
With this patch:
real 0m0.004s
user 0m0.004s
sys 0m0.000s
tj: These synchronize_rcu()s are utterly confused. synchornize_rcu()
necessarily has to come between two operations to guarantee that
the changes made by the former operation are visible to all rcu
readers before proceeding to the latter operation. Here,
synchornize_rcu() are at the end of attach operations with nothing
beyond it. Its only effect would be delaying completion of
write(2) to sysfs tasks/procs files until all rcu readers see the
change, which doesn't mean anything.
cherry-picked from:
https://android.googlesource.com/kernel/common/+/5d65bc0ca1bceb73204dab943922ba3c83276a8c
Bug: 17709419
Change-Id: I98dacd6c13da27cb3496fe4a24a24084e46bdd9c
Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Colin Cross <ccross@google.com>
Signed-off-by: Devin Kim <dojip.kim@lge.com>
Diffstat (limited to 'kernel')
| -rw-r--r-- | kernel/cgroup.c | 1 |
1 files changed, 0 insertions, 1 deletions
diff --git a/kernel/cgroup.c b/kernel/cgroup.c index 2005981..842f8a9 100644 --- a/kernel/cgroup.c +++ b/kernel/cgroup.c @@ -2168,7 +2168,6 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader) /* * step 5: success! and cleanup */ - synchronize_rcu(); cgroup_wakeup_rmdir_waiter(cgrp); retval = 0; out_list_teardown: |