Re: [rfc patch] i386: don't save eflags on task switch

[Zachary Amsden]

Andi Kleen wrote:
> > For me, when compiled with -O2, it results in
> >
> > 	84
> > 	88
> > 	132
> >
> > which basically says: a "rdtsc->rdtsc" is 84 cycles, putting a "pushfl" in 
> > between is another _4_ cycles, and putting a "popfl" in between is about 
> > another 48 cycles. 
> >     
>
> This means we should definitely change restore_flags() to only STI, 
> never popf
>   

sti is expensive as well; iirc just as expensive on most processors as 
popf, although I don't have hard numbers to back this up on hand.  An 
_unlikely_ jcc + popf? is better than a sti, for sure, but a likely 
jcc+popf could cost more than a jcc+sti, depending on model.

Zach
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