[PATCH] x86 TSS: io port caching
There's one additional step we can do ontop of the ports-max code to get rid of copying in X.org's case: cache the last task that set up the IO bitmap. This means we can set the offset to invalid and keep the IO bitmap of that task, and switch back to a valid offset (without any copying) when switching back to that task. (or do a copy if there is another ioperm task we switch to.) I've attached ioport-cache-2.6.8.1.patch that implements this. When there's a single active ioperm() using task in the system then the context-switch overhead is very low and constant: # ./ioperm-latency default no ioperm: scheduling latency: 2478 cycles turning on port 80 ioperm: scheduling latency: 2499 cycles turning on port 65535 ioperm: scheduling latency: 2481 cycles This single-ioperm-user situation matches 99% of the actual ioperm() usage scenarios and gets rid of any copying whatsoever - without relying on any fault mechanism. I can see no advantage of the GPF approach over this patch. Signed-off-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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