Messages in this thread | | | Date | Tue, 4 Jul 2006 01:10:55 -0700 | From | Stephane Eranian <> | Subject | Re: [PATCH 1/2] x86-64 TIF flags for debug regs and io bitmap in ctxsw |
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On Tue, Jul 04, 2006 at 09:51:49AM +0200, Arjan van de Ven wrote: > > - } > > - } > > + if (unlikely((task_thread_info(next_p)->flags & _TIF_WORK_CTXSW)) > > + || test_tsk_thread_flag(prev_p, TIF_IO_BITMAP)) > > + __switch_to_xtra(prev_p, next_p, tss); > > well isn't this replacing an if() (which isn't cheap but also not > expensive, due to unlikely()) with an atomic operation (which *is* > expensive) ? > > That to me doesn't make this sound like an actual win.... > Although the two if were marked unlikely, you had to do the test anyway. So you had to touch next->debugreg[7], next->io_bitmap_ptr, and prev->io_bitmap_ptr. Now the first two are collapsed into one cache line in thread_info->flags.
Yet, I see your point about the test_tsk_thread_flag() and I am wondering if we do need the atomicity in this case and whether we could simplify by using the same expression as for next, i.e, task_thread_info(prev_p)->flags & TIF_IO_BITMAP?
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