Information for setting up SMT related parameters on linux 2.6.16 on POWER5

[Meswani, Mitesh]

Thanks guys 
 
That answered so many of my questions. 
 
If I were to use these macros from user space, would they remain set until next reboot or change ? POWER5 allows priorities 2 through 4 for user apps, so considering this, and the fact that the normal prioirity is level 4, if a user app resets it to say 2 and then finishes without changing it back to 4 , would all the subsequent user apps run at the new level 2. I wonder what I am saying even makes sense, because the kernel internally throttles the priority for various sections of the kernel code and it may even overwrite it. 
 
 
On a slightly unrelated note, I appended some boot parameters like smt-enabled=on/off to /etc/lilo.conf and unfortunately I am not able to see any effect and it boots the same way. I am switching from the AIX world so I maybe doing something dumb, please point out if I am !  This kind of seems to effect the bind processor calls using sys_setaffinity when there are 4 logical processors 0-3 on two physical processors, bind only allows me to set affinity to either cpu 0 or 2, this seems weird to me because my system is booting with two logical cpus and then I set online bit to 1 to turn the remaining on, thereafter I try binding and havent been very successful. 
 
 
Thanks for all your replies. 
 
 
 

Mitesh R. Meswani 
Ph.D. Candidate 
Research Associate, PLS2 Group
Room 106 F, Department of Computer Science
The University of Texas at El Paso, 
El Paso, Texas 79968
Tel: 915 747 8012 (O)
Email: mmeswani at utep.edu

________________________________

From: Segher Boessenkool [mailto:segher at kernel.crashing.org]
Sent: Mon 5/8/2006 5:04 PM
To: will_schmidt at vnet.ibm.com
Cc: Meswani, Mitesh; linuxppc-dev at ozlabs.org; Arnd Bergmann; linux-kernel at vger.kernel.org; cbe-oss-dev at ozlabs.org
Subject: Re: Information for setting up SMT related parameters on linux 2.6.16 on POWER5



> the HMT_* macros are telling firmware that "this processor thread 
> should
> run at this priority".  Typically used when we're waiting on a 
> spinlock.
> I.e. When we are waiting on a spinlock, we hit the HMT_low macro to 
> drop
> our threads priority, allowing the other thread to use those extra
> cycles finish it's stuff quicker, and maybe even release the lock 
> we're
> waiting for.          HMT_* is all within the kernel though, no 
> exposure
> to userspace apps.

Actually, those macros translate straight into a single machine insn.
No firmware is involved.  See include/asm-powerpc/processor.h.  For
example:

#define HMT_very_low()   asm volatile("or 31,31,31   # very low 
priority")

You can use those same macros from user space, although it is CPU
implementation dependent which priorities you can actually set (you
probably can do low and medium priority).


Segher



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