Re: [RFC] X+SELinux performance work

[Eamon Walsh <ewalsh@tycho.nsa.gov>]

Eric Paris wrote:
> (sorry for everyone who gets this twice, I misspelled the list address
> the first time)
>
> As a great example of how much selinux is killing X performance ajax
> showed me the x11perf -create test.
>
> Without the SELinux extensions: 175,000 events per second.
> With the SELinux extensions:      1,000 events per second.
>
> First thing I did to try to help was to implement creation caching in the AVC:
> http://people.redhat.com/~eparis/xorg-selinux/libselinux-create-cache.patch
>
> This resulted in about a 4.5-5x performance improvement
>   

First glance looks good.


> ---
>
> Next thing to try was to stop regularly calling recv on the netlink
> socket to find policy update information.  So I hacked (poorly)
> libselinux and the xselinux.c extention to try to create a seperate
> thread which does nothing but listen on the netlink socket (I just stole
> most of the code from dbus.)  X people on irc told me they didn't like
> the idea of a thread in the X server and what I did to libselinux is
> quite hideous.  I can certainly clean up libselinux (do the thread
> callback as an selinux_set_callback() rather than this new
> avc_open_init() interface.)  

Yeah, I'd do it via selinux_set_callback() or just forget the callbacks
entirely and hardcode it with pthreads and add a new option to turn on
the thread.

> As long as the X people object to a
> separate thread I guess these might be going into the bit bucket...
>   

Well, I don't see a reasonable alternative -- the netlink check is
required to catch policy reloads, and a separate thread is the only way
to take the recv() call out of the main code path.

I can only think of two other possible ways to handle this:

- - Use a signal handler, which would require support for the kernel
sending a signal on a policy reload (or enforcing change).

- - Form an expedition and venture deep down into the far recesses of
the X server, delving into the OS layer, maybe even Xtrans, and find the
select() call at the very core, and destroy it, freeing us...I mean, add
the netlink socket to it, which may require new interfaces to send the
fd all the way down there and then bubble the notification all the way
back up or call a callback or whatever.


> http://people.redhat.com/~eparis/xorg-selinux/libselinux-avc-open-init.patch
> http://people.redhat.com/~eparis/xorg-selinux/xserver-selinux-netlink-faster.patch
>
> ---
>
> Last thing was that translating from raw to whatever looked to be taking
> up tons of syscalls, open a socket, bind, fail, close over and over and
> over.  So I added new hook where X can just disable translations
> altogether.  What does X care if it has raw strings?  I think as soon as
> we have things to "display" strings to users they should take care of
> translation and just let X internally hand things back and forth the way
> the AVC can use them.
>   

Yeah this makes sense, it should all be raw, no reason to be calling
translate at all.  I tried to go through and fix this earlier, guess I
missed some call sites.

> http://people.redhat.com/~eparis/xorg-selinux/libselinux-use-raw.patch
> http://people.redhat.com/~eparis/xorg-selinux/xserver-selinux-raw.patch
>
> So I've at least got us within one order of magnitude of the non-selinux
> case.
>
> Without the SELinux extensions:   175,000 events per second.
> With the SELinux extensions:        1,000 events per second.
> create-cache:                       5,000 events per second.
> create-cache + nl-thread:           5,500 events per second.
> create-cache + all-raw:            35,000 events per second.
> create-cache + all-raw + nl-thead  65,000 events per second.
>
> So initially the nl-thread didn't seem like a big win (only 10% perf
> increase) but as I cut back on the setrans stuff the enforcing netlink
> stuff started to dominate and it became a 2x perf win.
>
> If I had a box with oprofile that worked I'd try to run it down more,
> but I don't have any hardware with working performance counters.
>
> Anyone how thoughts, comments, complaints, reviews, ideas, pointers,
> direction, insults, compliments, etc etc, please share
>   

Think overall it's good work and I hope the impact can be reduced.  Just
gotta figure out the thread thing.

> -Eric
>
>
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>   


-- 
Eamon Walsh <ewalsh@tycho.nsa.gov>
National Security Agency


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