Re: [patch 2.6.13-rc3a] i386: inline restore_fpu

From: Chuck Ebbert
Date: Tue Jul 26 2005 - 16:36:26 EST

On Fri, 22 Jul 2005 at 11:13:21 -0700 (PDT), Linus Torvalds wrote:

> Something like the following (totally untested) should make it be
> non-lazy. It's going to slow down normal task switches, but might speed up
> the "restoring FP context all the time" case.
> Chuck? This should work fine with or without your inline thing. Does it
> make any difference?

Now that I am looking at the right output file -- yes, it does (after fixing
one small bug.) Volanomark set a new record of 6125 messages/sec and the
profile shows almost zero hits in math_state_restore.

Since fxsave leaves the FPU state intact, there ought to be a better way to do
this but it gets tricky. Maybe using the TSC to put a timestamp in every thread
save area?

when saving FPU state:
put cpu# and timestamp in thread state info
also store timestamp in per-cpu data

on task switch:
compare cpu# and timestamps for next task
if equal, clear TS and set TS_USEDFPU

when state becomes invalid for some reason:
zero cpu's timestamp

But the extra overhead might be too much in many cases.

(Below is what I changed in the original patch, if anyone wants to try it.)

+ if (tsk_used_math(next_p))

should be:

+ if (!tsk_used_math(next_p))

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