Re: [PATCH 1/2] x86-64 TIF flags for debug regs and io bitmap in ctxsw

[Stephane Eranian <eranian@hpl.hp.com>]

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?

-- 
-Stephane