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

[Zefan Li]

On 2015/5/19 23:51, Tejun Heo wrote:
> Hello, Peter.
> 
> On Tue, May 19, 2015 at 05:16:59PM +0200, Peter Zijlstra wrote:
>> .gitconfig:
>>
>> [diff "default"]
>>         xfuncname = "^[[:alpha:]$_].*[^:]$"
>>
>> Will avoid keying on labels like that and show us this is
>> __cgroup_procs_write().
> 
> Ah, nice trick.
> 
>> So my only worry with this patch-set is that these operations will be
>> hugely expensive.
>>
>> Now it looks like the cgroup_update_dfl_csses() thing is very rare, its
>> when you change which controllers are active in a given subtree under
>> the uber-l337-super-comount design.
>>
>> The other one, __cgorup_procs_write() is every /procs, /tasks write to a
>> cgroup, and that does worry me, this could be a somewhat common thing.
>>
>> The Changelog states task migration is a cold path, but is tens of
>> miliseconds per task really no problem?
> 
> 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.

> I agree that something which is closer to lglock in characteristics
> would fit the workload better tho.  If this actually becomes a
> problem, we can come up with a different percpu locking scheme which
> puts a bit more overhead on the reader side to reduce the latency /
> overhead on the writer side which shouldn't be that difficult but
> let's see whether we need to get there at all.
> 
> Thanks.
> 

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