linux-kernel - Re: [PATCH] Reserve N process to root

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Message-Id: <200710120929.10770.a1426z@gawab.com>
Date:	Fri, 12 Oct 2007 09:29:10 +0300
From:	Al Boldi <a1426z@...ab.com>
To:	Kyle Moffett <mrmacman_g4@....com>
Cc:	LKML Kernel <linux-kernel@...r.kernel.org>, g@...f.cl,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>
Subject: Re: [PATCH] Reserve N process to root

Kyle Moffett wrote:
> On Oct 12, 2007, at 01:37:23, Al Boldi wrote:
> > You have a point, and resource-controllers can probably control DoS
> > a lot better, but the they also incur more overhead.  Think of this
> > "lockout prevention" patch as a near zero overhead safety valve.
>
> But why do you need to add "lockout prevention" if it already
> exists?

I said this before, but I'll say it again: it's about overhead!

> With CFS' extremely efficient per-user-scheduling (hopefully
> soon to be the default) there are only two forms of lockout by non-
> root processes:  (1) Running out of PIDs in the box's PID-space
> (think tens or hundreds of thousands of processes), or (2) Swap-
> storming the box to death.  To put it bluntly trying to reserve free
> PID slots is attacking the wrong end of the problem and your so
> called "lockout prevention" could very easily ensure that 10 PIDs are
> available even if the user has swapstormed the box with the PIDs he
> does have.

I think you are reading this wrong.  It's not about reserving PIDs, it's 
about exceeding the max-threads limit.  This limit is global and affects 
every user including root, which is good, as this allows the sysadmin to 
fence the system into a controllable state.  So once the system reaches the 
fence, sysadmin-intervention allows root to exceed the fence.

Again, this is much nicer with real resource-controllers, but again it's also 
more overhead.


Thanks!

--
Al

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