linux-kernel - Re: [PATCH 5/6] Makes procs file writable to move all threads by tgid at once

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]

Message-ID: <20090724172320.GH5878@count0.beaverton.ibm.com>
Date:	Fri, 24 Jul 2009 10:23:20 -0700
From:	Matt Helsley <matthltc@...ibm.com>
To:	Paul Menage <menage@...gle.com>
Cc:	Matt Helsley <matthltc@...ibm.com>, Ben Blum <bblum@...gle.com>,
	linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, akpm@...ux-foundation.org,
	serue@...ibm.com, lizf@...fujitsu.com
Subject: Re: [PATCH 5/6] Makes procs file writable to move all threads by
	tgid at once

On Fri, Jul 24, 2009 at 09:01:46AM -0700, Paul Menage wrote:
> On Fri, Jul 24, 2009 at 8:50 AM, Matt Helsley<matthltc@...ibm.com> wrote:
> >
> > There is much ado about not taking additional "global locks" in fork()
> > paths.
> >
> > * The fork and exit callbacks cgroup_fork() and cgroup_exit(), don't
> > * (usually) take cgroup_mutex.  These are the two most performance
> > * critical pieces of code here.
> > ...
> >
> > and as I recall cgroup_fork() doesn't ever take cgroup_mutex because it is
> > so performance critical.
> 
> cgroup_mutex is a much bigger and heavier mutex than the new rwsem
> being introduced in this patch. It's sort of the BKL of cgroups,
> although where possible I'm encouraging use of finer-grained
> alternatives (such as subsystem-specific locks, the per-hierarchy
> lock, etc).
> 
> > Assuming the above comments in kernel/cgroup.c
> > are correct then this patch adds a performance regression by introducing a
> > global mutex in the fork path, doesn't it?
> 
> Yes, although to what magnitude isn't clear.

OK.

> Alternatives that we looked at were:
> 
> - add a clone_rwsem to task_struct, and require that a clone operation
> that's adding to the same thread group take a read lock on the
> leader's clone_rwsem; then the effect would be localised to a single
> process; but for a system that has one big multi-threaded server on
> it, the effect would still be similar to a global lock

Except most processes aren't big multi-threaded servers and not everyone
runs such processes. They'll experience the overhead of a global lock
when they don't have to. Again, it's a question of magnitudes we don't
know I think.

> 
> - move the housekeeping done by cgroup_fork() inside the tasklist_lock
> critical section in do_fork(); then cgroup_attach_proc() can rely on
> the existing global tasklist_lock to provide the necessary
> synchronization, rather than introducing a second global lock; the
> downside is that it slightly increases the size of the section where
> tasklist_lock is held for write.

Well, I imagine holding tasklist_lock is worse than cgroup_mutex in some
ways since it's used even more widely. Makes sense not to use it here..

Cheers,
	-Matt Helsley
--
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/