Re: [PATCH 2/3] sched, cgroup: replace signal_struct->group_rwsem with a global percpu_rwsem

[Zefan Li <lizefan@huawei.com>]

On 2015/5/22 4:39, Tejun Heo wrote:
> Hello, Li.
> 
> On Wed, May 20, 2015 at 06:05:37PM +0800, Zefan Li wrote:
>>> The latency is bound by synchronize_sched_expedited().  Given the way
>>> cgroups are used in majority of setups (process migration happening
>>> only during service / session setups), I think this should be okay.
>>
>> Actually process migration can happen quite frequently, for example in
>> Android phones, and that's why Google had an out-of-tree patch to remove
>> the synchronize_rcu() in that path, which turned out to be buggy.
> 
> It's still not a very frequent operation tho.  We're talking about
> users switching fore/background jobs here and the expedited
> synchronization w/ preemption enabled doesn't take much time.  In
> addition, as it currently stands, android is doing memory charge
> immigration on each fore/background switches.  I'm pretty doubtful
> this would make any difference.
> 

I did some testing with my laptop.

Moving a task between 2 cgroups for 10W times with one or two threads:

                  1T        2T
orig              3.36s     3.65s
orig+tj           3.55s     6.31s
orig+sync_rcu     16.69s    28.47s    (only 1000 times)

The overhead looks acceptable.