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Linux 2.6.6 et Nvidia


Linux 2.6.6 et Nvidia

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Comme vous savez tous, le noyau Linux 2.6.6 vient de sortir. C'est une
patch d'environ 2.4MiB (hmm 2.4). J'aimerais juste vous avertir de faire
attention si vous utiliser le driver closed-source de Nvidia, il n'est
pas compatible avec cette version du kernel à cause d'une modification.
Voici le texte du changelog qui explique la modif:

Below is a patch to enable 4Kb stacks for x86. The goal of this is to
1) Reduce footprint per thread so that systems can run many more
threads (for the java people)
2) Reduce the pressure on the VM for order > 0 allocations. We see real
life workloads (granted with 2.4 but the fundamental fragmentation issue
isn't solved in 2.6 and isn't solvable in theory) where this can be a
problem. In addition order > 0 allocations can make the VM "stutter" and
give more latency due to having to do much much more work trying to

The first 2 bits of the patch actually affect compiler options in a
generic way: I propose to disable the -funit-at-a-time feature from gcc.
With this enabled (and it's default with -O2), gcc will very
agressively inline functions, which is nice and all for userspace, but
for the kernel this makes us suffer a gcc deficiency more: gcc is
extremely bad at sharing stackslots, for example a situation like this:

if (some_condition)

with -funit-at-a-time, both function_A() and _B() might get inlined,
however the stack usage of both functions of the parent function grows
the stack usage of both functions COMBINED instead of the maximum of the
two. Even with the normal 8Kb stacks this is a danger since we see some
functions grow 3Kb to 4Kb of stack use this way. With 4Kb stacks, 4Kb
of stack usage growth obviously is deadly ;-( but even with 8Kb stacks
it's pure lottery. Disabling -funit-at-a-time also exposes another thing
in the -mm tree; the attribute always_inline is considered harmful by
gcc folks in that when gcc makes a decision to NOT inline a function
marked this way, it throws an error. Disabling -funit-at-a-time
disables some of the agressive inlining (eg of large functions that come
later in the .c file) so this would make your tree not compile.

The 4k stackness of the kernel is included in modversions, so people
don't load 4k-stack modules into 8k-stack kernels.

At present 4k stacks are selectable in config. When the feature has
settled in we should remove the 8k option. This will break the nvidia
modules. But Fedora uses 4k stacks so a new nvidia driver is expected soon.

Théoriquement, il serait possible de choisir un stack de 8kB et de faire
fonctionner le driver Nvidia avec le nouveau kernel.


Stefan "Mitch" Michalowski
a.k.a. DjDelovsky
Email: mitch(à)


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