Re: [PATCH] Make math_state_restore() save and restore the interruptflag

[Pekka Riikonen]

On Sat, 1 Feb 2014, Linus Torvalds wrote:

> We could do that with the whole "task_work" thing (or perhaps just
> do_notify_resume(), especially after merging the "don't necessarily
> return with iret" patch I sent out earlier), with additionally making
> sure that scheduling does the right thing wrt a "currently dirty math
> state due to kernel use".
>
> The advantage of that would be that we really could do a *lot* of FP
> math very cheaply in the kernel, because we'd pay the overhead of
> kernel_fpu_begin/end() just once (well, the "end" part would be just
> setting the bit that we now have dirty state, the cost would be in the
> return-to-user-space-and-restore-fp-state part).
>
> Comments? That would be much more invasive than just changing
> __kernel_fpu_end(), but would bring in possibly quite noticeable
> advantages under loads that use the FP/vector resources in the kernel.
This would be very good and it needs to work in interrupt context 
(softirq) also, and when we interrupt idle task.  It's with networking we 
can really hit kernel_fpu_begin()/end() millions of times per second and 
there's really only need to do it once per interrupt.  This is actually 
similar what I was doing (in do_softirq)) when I noticed eagerfpu was 
broken and now Nate's bug AFAICS happens there as well.

	Pekka
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