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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
