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SubjectRe: rcu-refcount stacker performance
Quoting Paul E. McKenney (paulmck@us.ibm.com):
> On Thu, Jul 14, 2005 at 08:44:50AM -0500, serue@us.ibm.com wrote:
> > Quoting Paul E. McKenney (paulmck@us.ibm.com):
> > > My guess is that the reference count is indeed costing you quite a
> > > bit. I glance quickly at the patch, and most of the uses seem to
> > > be of the form:
> > >
> > > increment ref count
> > > rcu_read_lock()
> > > do something
> > > rcu_read_unlock()
> > > decrement ref count
> > >
> > > Can't these cases rely solely on rcu_read_lock()? Why do you also
> > > need to increment the reference count in these cases?
> >
> > The problem is on module unload: is it possible for CPU1 to be
> > on "do something", and sleep, and, while it sleeps, CPU2 does
> > rmmod(lsm), so that by the time CPU1 stops sleeping, the code it
> > is executing has been freed?
>
> OK, but in the above case, "do something" cannot be sleeping, since
> it is under rcu_read_lock().

Oh, but that's not quite what the code is doing, rather it is doing:

rcu_read_lock
while get next element from list
inc element.refcount
rcu_read_unlock
do something
rcu_read_lock
dec refcount
rcu_read_unlock

What I plan to try next is:

rcu_read_lock
while get next element from list
if (element->owning_module->state != LIVE)
continue
rcu_read_unlock
do something
rcu_read_lock
rcu_read_unlock
> > Because stacker won't remove the lsm from the list of modules
> > until mod->exit() is executed, and module_free(mod) happens
> > immediately after that, the above scenario seems possible.
>
> Right, if you have some other code path that sleeps (outside of
> rcu_read_lock(), right?), then you need the reference count for that
> code path. But the code paths that do not sleep should be able to
> dispense with the reference count, reducing the cache-line traffic.

Most if not all of the codepaths can sleep, however. So unfortunately
that doesn't seem a feasible solution. That's why I'm hoping there is
something inherent in the module unload code that I can take advantage
of to forego my own refcounting.

thanks,
-serge
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