Messages in this thread |  | | Date | Wed, 14 Feb 2007 16:35:44 -0800 (PST) | From | Davide Libenzi <> | Subject | Re: [patch 06/11] syslets: core, documentation |
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On Wed, 14 Feb 2007, Benjamin LaHaise wrote:
> On Wed, Feb 14, 2007 at 03:17:59PM -0800, Davide Libenzi wrote: > > > That's an incorrect assumption. Every task/thread in the system has FPU > > > state associated with it, in part due to the fact that glibc has to change > > > some of the rounding mode bits, making them different than the default from > > > a freshly initialized state. > > > > IMO I still belive this is not a huge problem. FPU state propagation/copy > > can be done in a clever way, once we detect the in-async condition. > > Show me. clts() and stts() are expensive hardware operations which there > is no means of avoiding as control register writes impact the CPU in a not > trivial manner. I've spent far too much time staring at profiles of what > goes on in the context switch code in the process of looking for optimizations > on this very issue to be ignored on this point.
The trivial case is the cachehit case. Everything flows like usual since we don't swap threads. If we're going to sleep, __async_schedule has to save/copy (depending if TS_USEDFPU is set) the current FPU state to the newly selected service thread (return-to-userspace thread). When a fault eventually happen in the new userspace thread, context is restored.
- Davide
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