Re: [patch] SMP alternatives

From: Gerd Knorr
Date: Wed Nov 23 2005 - 10:28:49 EST

Andi Kleen wrote:
Gerd Knorr <kraxel@xxxxxxx> writes:

Modules are supported now, fully modularized distro kernel works fine
with it. If you have a kernel with HOTPLUG_CPU compiled you can
shutdown the second CPU of your dual-processor system via sysfs (echo
0 > /sys/devices/system/cpu/cpu1/online) and watch the kernel switch
over to UP code without lock-prefixed instructions and simplified
spinlocks, then power up the second CPU again (echo 1 > /sys/...) and
watch it patching back in the SMP locking.

This looks like total overkill to me. Who needs to optimize
CPU hotplug this way? If you really need this just do it at boot time with the existing mechanisms.

Sure, for real hardware doing that at boot time is be perfectly fine. In a virtual environment it's very useful to be able to plug in one more virtual CPU on demand without rebooting though. The patch isn't very useful alone, it's more one step on the road of getting the xen bits merged mainline.

+ /* Paranoia */
+ asm volatile ("jmp 1f\n1:");
+ mb();

That would be totally obsolete 386 era paranoia. If anything then use a CLFLUSH (but not available on all x86s)

Ok, dropped. I've just copyed that from the original, pretty ugly xen patch.


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