Re: [PATCH] don't udelay() in sn_mmiob

[Jack Steiner <steiner@sgi.com>]

On Fri, May 28, 2004 at 05:57:33PM -0700, Seth, Rohit wrote:
> Jack Steiner <> wrote on Friday, May 28, 2004 10:35 AM:
> 
> > On Fri, May 28, 2004 at 12:28:55PM -0500, Jack Steiner wrote:
> >> On Thu, May 27, 2004 at 05:06:31PM -0400, Jesse Barnes wrote:
> >>> On Thursday, May 27, 2004 4:46 pm, David Mosberger wrote:
> >>>> Wouldn't you want at least a cpu_relax() in that loop?
> >>> 
> >>> That'll cause the CPU to switch to the other thread if it's SMT so
> >>> it won't spin waiting for the access to complete?  If so, then yes.
> >>> Here's an updated patch.
> > 
> > Whoops, I posted this response to the wrong mail. I was replying to
> > the question about whether the code should have a "cpu_relax()". 
> > 
> >> 
> >> I didn't think the code would actually spin.
> >> 
> >> sn_mmiob() is something like:
> >> 
> >>     1:	ld.acq r8=...
> >>     	;;
> >> 	cmp.eq p8,p0=r8,r9
> >>    (p8)	br 1b
> >> 
> >> The "load" is an uncached load. Won't the pipeline stall on the cmp
> >> waiting for data to arrive. I would not have expected that a
> >> cpu_relax() would have been needed here.
> >> 
> 
> You are right that pipeline will stall on the cmp instruction.  But if
> you could put the hint@pause between ld and cmp instructions then that
> would allow current execution stream to give up resources to other
> execution stream (if applicable) for the time that data takes to come
> from main memory.
> 
> -rohit

I thought the cpu would automatically switch on a cache miss & I assumed that
a switch would occur on a UC ref. Or am I thinking of a different processor?

If you follow the logic that is cpu_relax is useful in the sn_mmiob()
function, shouldn't there be a cpu_relax() after EVERY uncached load 
that is immediately (or closely) followed by an instruction that 
consumes the data.

What are the conditions that cause a switch between execution streams? 

-- 
Thanks

Jack Steiner (steiner@sgi.com)          651-683-5302
Principal Engineer                      SGI - Silicon Graphics, Inc.