Re: [rfc][patch] Avoid taking global tasklist_lock for single threaded process at getrusage()

[Christoph Lameter <clameter@engr.sgi.com>]

On Wed, 21 Dec 2005, Ravikiran G Thirumalai wrote:

> Following patch avoids taking the global tasklist_lock when possible,
> if a process is single threaded during getrusage().  Any avoidance of 
> tasklist_lock is good for NUMA boxes (and possibly for large SMPs).  We found 
> that this optimization reduces the runtime of a certain scientific application 
> by half on a 16 cpu NUMA box.
> 
> This optimization is similar to the sys_times tasklist_lock optimization.

The optimization of sys_times is only possible because the "current" 
task is running and therefore guarantees that the thread will not be 
exiting.

getrusage and k_getrusage can be called onother tasks than the currently 
executing task and in those cases better take the tasklist lock because 
the task may exit while getrusage runs.

See wait_noreap_copyout() in kernel/exit.c and 
arch/mips/kernel/{sysirix,irixsig}.c for uses of getrusage where the 
struct task_struct * != current.

Maybe you can deal with these and insure that getrusage is always called 
for the current process? In that case the struct task_struct * parameter
needs to be dropped from getrusage and k_getrusage.