Re: [PATCH -mm 1/5] Blackfin: blackfin architecture patch update

[Paul Mundt]

On Mon, Mar 05, 2007 at 11:29:19AM -0500, Robin Getz wrote:
> On Mon 5 Mar 2007 09:00, Paul Mundt pondered:
> > Throwing this all at the user simply shows that the functions being
> > relocated haven't been profiled adequately with real workloads. 
> 
> Actually - that is not true at all - we have profiled these extensively
> across many workloads on various real world applications from fax
> machine to bar code scanner, to software radio, to internet radio - and
> that is the point - it all depends on what the user application/drivers
> that are loaded are doing. I can't decided what is best for the user,
> because I don't know what they are doing.
> 
And there are just as many cases that are common regardless of the
workload that are simply good decisions. Exception vectors are a good
example, either you get a boost from having them in L1 memory or you
don't, the workload really doesn't matter.

Workloads will of course have variations, but it's optimizing the hot
paths that end up being quite common that end up being the most useful.
These are not things that should be required from the end user.

> > If a user wants to use on-chip memory, presumably they have a target
> > application in mind, and they know how much space they need. Beyond
> > that, they expect the kernel to do the best it can with the space
> > that's left over for it to play with. 
> 
> And that is the problem - the kernel doesn't know how much on chip memory an 
> application which will be loaded in the future might need. The user needs to 
> be able to control the amount of on-chip memory that the kernel uses - hence 
> the knobs.
> 
That's crap, for the exact reason that these are not run-time
configurable knobs, they are static build-time components. If a user has
to scale back the kernel users until their application fits, then they
already know how much space they're going to need. Once you know that,
reserving a chunk for userspace and limiting the kernel's usage is quite
simple.

> > If a user has to sit around profiling their workload to 
> > figure out what config options to set to chew through the rest of the L1
> > memory, you've completely lost at intuitive design.
> 
> Embedded has never been intuitive.
> 
Embedded has also never been about throwing everything at the user that's
possible. Flexibility is good. Intelligent design is good. Blindly
tossing everything the user's way and hoping for the best is not.

> Normally, the majority of end users leave things set by the default, meaning 
> their application doesn't use the on-chip memory, and the kernel is allowed 
> to. If they want to start using on-chip memory, they are forced to do a 
> re-compile, with the kernel functions using off-chip memory, and see how much 
> of on-chip memory their application uses. They calculate the difference, and 
> do a little system level profiling, and put the most used kernel functions 
> back.
> 
Yes, precisely. This is all something that should be done by the kernel
_for_ the user. They can of course add their own instrumentation to the
hot paths that their workload hits that aren't generally handled, but
this should not be a required step.

If I'm working on an embedded application, I expect my kernel to do some
sensible things with the hardware available. I don't want to have to
manually configure ever possible thing in order to get a system that
behaves in a reasonable manner, and I certainly don't want to have to do
this multiple times with a profiler for paths that are inevitably going
to be quite common anyways.

> You are asking me to try to decide what is best for all the potential end 
> users, with out knowing what the end user is doing. What ends up happening is 
> that people make modifications (normally poor ones) and then still ask for 
> help when their modifications break.
> 
No, I'm asking you to set some sensible defaults, provide an interface
that works for the common cases, and allow the special cases to remain
simply that.

> I would rather give them a supported (but arguablely confusing) method of 
> customisation. (This is why it was added - end users were asking for it, and 
> when we didn't have it, they would try to make the modifications themselves, 
> break it, and report a bug. After telling 8 different people the same thing, 
> when do you decide this is a feature you need to support?)
> 
There's also a substantial differences between what customers are telling
you they want, what they're trying to accomplish, and what they actually
need. If you want to add something simple, setup an attribute tag for the
section and tell the customers to flag the cases they're most concerned
with. Obviously you expect them to be sitting there with a profiler open
anyways, so this should not be a problem. Also note that we are not
required to accept bad code in the kernel because you happen to have
customer requirements for it.

The point is, doing this as a config option design is just broken. It
doesn't scale for new code, and it doesn't scale as soon as you start
throwing in more blocks of SRAM with varying locality, latencies, etc.
Littering around ifdefs for this stuff all over the code is equally ugly.

I'll reiterate my original suggestion, allow the user to cap the amount
of memory that's used by the kernel, then add something akin to initcall
levels that you can use for prioritizing items to be relocated in on-chip
memory, while spilling the rest to RAM. With this sort of scheme you can
also trivially spill in to additional SRAM pools and whatever else you
have handy as you go along.

You can of course argue that as an embedded platform the user must make
all of these decisions on their own, but there can be a compromise if the
kernel's at least half-way intelligent about matters. Embedded has never
been about hand-waving and expecting the user to solve all of the
difficult problems just because they can.

While it may not count for much, until something is done about this and
the other issues that were raised earlier in the thread, I'd have to NAK
this patch. If not wanting to manually select everything that goes in to
L1 memory makes me too unsophisticated by blackfin standards, so be it
;-) 
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