On Wed, Dec 21, 2005 at 12:20:20PM -0800, Christoph Lameter wrote:
> 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.
Yes.
>
> 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.
We did look at that. Cases RUSAGE_CHILDREN and RUSAGE_SELF are always called by the
current task, so we can avoid tasklist locking there.
getrusage for non-current tasks are always called with RUSAGE_BOTH.
We ensure we always take the siglock for RUSAGE_BOTH case, so that the
p->signal* fields are protected and take the tasklist_lock only if we have
to traverse the tasklist hashlist. Isn't this safe?
Thanks,
Kiran
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