Re: [RFC][PATCH 1/6] Tunable structure and registration routines

[Nadia Derbey <Nadia.Derbey@bull.net>]

Randy Dunlap wrote:
> On Thu, 25 Jan 2007 17:26:31 +0100 Nadia Derbey wrote:
> > > > +Any kernel subsystem that has registered a tunable should call
> > > > +auto_tune_func() as follows:
> > > > +
> > > > ++-------------------------+--------------------------------------------+
> > > > +| Step                    | Routine to call                            |
> > > > ++-------------------------+--------------------------------------------+
> > > > +| Declaration phase       | DEFINE_TUNABLE(name, values...);           |
> > > > ++-------------------------+--------------------------------------------+
> > > > +| Initialization routine  | set_tunable_min_max(name, min, max);       |
> > > > +|                         | set_autotuning_routine(name, routine);     |
> > > > +|                         | register_tunable(&name);                   |
> > > > +| Note: the 1st 2 calls   |                                            |
> > > > +|       are optional      |                                            |
> > > > ++-------------------------+--------------------------------------------+
> > > > +| Alloc                   | activate_auto_tuning(AKT_UP, &name);       |
> > > > ++-------------------------+--------------------------------------------+
> > > > +| Free                    | activate_auto_tuning(AKT_DOWN, &name);     |
> > >
> > >
> > > So does Free always use AKT_DOWN?  why does it matter?
> > > Seems unneeded and inconsistent.
> >
> > Tuning down is recommended in order to come back to the default tunable 
> > value.
>
>
> Let me try to be clearer.  What is Alloc?  and why is AKT_UP
> associated with Alloc and AFK_DOWN associated with Free (whatever
> that means)?

Alloc stands for resource allocation: in a subsystem where resource 
allocation depends on a tunable value, we should tune up that value 
prior to the alllocation itself. Let's come back to the ipc subsystem 
example: ipc_addid() is the routine that adds an entry to an ipc array. 
The 1st thing it does (via grow_ary()) is to allocate some more space 
for the ipc array if needed, i.e. if the ipc tunable value has 
increased. That's why the tunable should be tuned up before calling 
ipc_addid().

AKT_DOWN is the reverse operation: we are freeing resources, so the 
tunble has no reason to remain with a big value.

> > I agree with you: today it has quite no effect, except on the tunable 
> > value. If we take the ipc's example, grow_ary() just returns if the new 
> > tunable value happens to be lower than the previous one.
> > But we can imagine, in the future, that grow_ary could deallocate the 
> > unused memory.
> > + in that particular case, lowering the tunable value makes the 1st loop 
> > in ipc_addid() shorter.
> >
> >
> > > How does one activate a tunable for downward adjustment?
> >
> > Actually a tunable is activated to be dynamically adjusted (whatever the 
> > direction).
> > But you are giving me an idea for a future enhancement: we can imagine a 
> > tunable that could be allowed to increase only (or decrease only). In 
> > that case, we should move the autotune sysfs attribute into an 'up' and 
> > a 'down' attribute?
>
>
> Couldn't the tunable owner just adjust the min value to a new
> (larger) min value, e.g.?

You're completely right: setting the min value to the default one should 
be enough!

> > > > +extern void fork_late_init(void);
> > >
> > >
> > > Looks like the wrong header file for that extern.
> >
> > Actually, I wanted the changes to the existing kernel files to be as 
> > small as possible. That's why everything is concentrated, whenever 
> > possible, in the added files.
>
>
> I suppose that's OK for review, but it shouldn't be merged that way.
>
> ---
> ~Randy


Regards,
Nadia

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/