linux-kernel - Re: [RFC] Thread Migration Preemption

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: <46942F45.2090502@yahoo.com.au>
Date:	Wed, 11 Jul 2007 11:15:49 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
CC:	Matt Mackall <mpm@...enic.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC] Thread Migration Preemption

Mathieu Desnoyers wrote:
> * Matt Mackall (mpm@...enic.com) wrote:
> 
>>On Wed, Jul 11, 2007 at 10:02:23AM +1000, Nick Piggin wrote:
>>
>>>>I like this patch a lot. Even if we don't add the underlying mechanism
>>>>right now, adding migration_disable as an alias for preempt_disable
>>>>will much better document quite a number of the users.
>>>
>>>I'd have no problem with that, and it might make it easier in future to
>>>justify a more complex scheme.
>>
>>What do you think, Mathieu?
>>
> 
> 
> That's an excellent idea. It should probably come with some
> documentation explaining what difference should be expected between
> preempt_disable and migrate_disable in the future so that people can
> choose the right alternative for their code.
> 
> 
>>Also, small nit: it ought to be migrate_disable to match the form of
>>preempt_disable.
> 
> 
> Yes, that's fine with me. :)
> 
> I though a little more about this whole concept of migrate_disable, and
> I think it could be brought further. One of the main problems with per
> cpu variables is that this concept cannot be ported to this grey place
> called "user-space" because preemption cannot be disabled. The
> equivalent (kind of) is currently per thread variables, but it can
> consume a lot of space if many threads are running.
> 
> However, it could be possible, if we implement a vsyscall based
> migration preemption counter accessible with read/write access from
> user-space, to tie a thread to a CPU during a user-space critical path.
> If we combine this with local atomic operations done in user-space, we
> could have highly scalable access to per cpu data structures reentrant
> with respect to signal handlers.

That's all well and good, but for most non-trivial stuff, you
have to disable preemption as well which you cannot do in
userspace. Which I suspect is why there is not a great deal
that can use it in kernelspace either.

So it will remain to be seen what kind of per-cpu data structures
you can access in a highly scalable way, and how big the niche is
between real per-thread data structures and real locking. I'm
skeptical :)

-- 
SUSE Labs, Novell Inc.
-
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/