Messages in this thread | | | Date | Wed, 20 May 2015 18:05:37 +0800 | From | Zefan Li <> | Subject | Re: [PATCH 2/3] sched, cgroup: replace signal_struct->group_rwsem with a global percpu_rwsem |
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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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