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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Wed, 20 May 2015 18:05:37 +0800
From:	Zefan Li <lizefan@...wei.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Peter Zijlstra <peterz@...radead.org>, <cgroups@...r.kernel.org>,
	<mingo@...hat.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] sched, cgroup: replace signal_struct->group_rwsem
 with a global percpu_rwsem

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.
> 

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/