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SubjectRe: [RFC][PATCH 0/5] arch: atomic rework
On Thu, Feb 27, 2014 at 09:01:40AM -0800, Linus Torvalds wrote:
> On Thu, Feb 27, 2014 at 7:37 AM, Torvald Riegel <triegel@redhat.com> wrote:
> >
> > I agree that just considering syntactic properties of the program seems
> > to be insufficient. Making it instead depend on whether there is a
> > "semantic" dependency due to a value being "necessary" to compute a
> > result seems better. However, whether a value is "necessary" might not
> > be obvious, and I understand Paul's argument that he does not want to
> > have to reason about all potential compiler optimizations. Thus, I
> > believe we need to specify when a value is "necessary".
>
> I suspect it's hard to really strictly define, but at the same time I
> actually think that compiler writers (and users, for that matter) have
> little problem understanding the concept and intent.
>
> I do think that listing operations might be useful to give good
> examples of what is a "necessary" value, and - perhaps more
> importantly - what can break the value from being "necessary".
> Especially the gotchas.
>
> > I have a suggestion for a somewhat different formulation of the feature
> > that you seem to have in mind, which I'll discuss below. Excuse the
> > verbosity of the following, but I'd rather like to avoid
> > misunderstandings than save a few words.
>
> Ok, I'm going to cut most of the verbiage since it's long and I'm not
> commenting on most of it.
>
> But
>
> > Based on these thoughts, we could specify the new mo_consume guarantees
> > roughly as follows:
> >
> > An evaluation E (in an execution) has a value dependency to an
> > atomic and mo_consume load L (in an execution) iff:
> > * L's type holds more than one value (ruling out constants
> > etc.),
> > * L is sequenced-before E,
> > * L's result is used by the abstract machine to compute E,
> > * E is value-dependency-preserving code (defined below), and
> > * at the time of execution of E, L can possibly have returned at
> > least two different values under the assumption that L itself
> > could have returned any value allowed by L's type.
> >
> > If a memory access A's targeted memory location has a value
> > dependency on a mo_consume load L, and an action X
> > inter-thread-happens-before L, then X happens-before A.
>
> I think this mostly works.
>
> > Regarding the latter, we make a fresh start at each mo_consume load (ie,
> > we assume we know nothing -- L could have returned any possible value);
> > I believe this is easier to reason about than other scopes like function
> > granularities (what happens on inlining?), or translation units. It
> > should also be simple to implement for compilers, and would hopefully
> > not constrain optimization too much.
> >
> > [...]
> >
> > Paul's litmus test would work, because we guarantee to the programmer
> > that it can assume that the mo_consume load would return any value
> > allowed by the type; effectively, this forbids the compiler analysis
> > Paul thought about:
>
> So realistically, since with the new wording we can ignore the silly
> cases (ie "p-p") and we can ignore the trivial-to-optimize compiler
> cases ("if (p == &variable) .. use p"), and you would forbid the
> "global value range optimization case" that Paul bright up, what
> remains would seem to be just really subtle compiler transformations
> of data dependencies to control dependencies.

FWIW, I am looking through the kernel for instances of your first
"if (p == &variable) .. use p" limus test. All the ones I have found
thus far are OK for one of the following reasons:

1. The comparison was against NULL, so you don't get to dereference
the pointer anyway. About 80% are in this category.

2. The comparison was against another pointer, but there were no
dereferences afterwards. Here is an example of what these
can look like:

list_for_each_entry_rcu(p, &head, next)
if (p == &variable)
return; /* "p" goes out of scope. */

3. The comparison was against another RCU-protected pointer,
where that other pointer was properly fetched using one
of the RCU primitives. Here it doesn't matter which pointer
you use. At least as long as the rcu_assign_pointer() for
that other pointer happened after the last update to the
pointed-to structure.

I am a bit nervous about #3. Any thoughts on it?

Some other reasons why it would be OK to dereference after a comparison:

4. The pointed-to data is constant: (a) It was initialized at
boot time, (b) the update-side lock is held, (c) we are
running in a kthread and the data was initialized before the
kthread was created, (d) we are running in a module, and
the data was initialized during or before module-init time
for that module. And many more besides, involving pretty
much every kernel primitive that makes something run later.

5. All subsequent dereferences are stores, so that a control
dependency is in effect.

Thoughts?

FWIW, no arguments with the following.

Thanx, Paul

> And the only such thing I can think of is basically compiler-initiated
> value-prediction, presumably directed by PGO (since now if the value
> prediction is in the source code, it's considered to break the value
> chain).
>
> The good thing is that afaik, value-prediction is largely not used in
> real life, afaik. There are lots of papers on it, but I don't think
> anybody actually does it (although I can easily see some
> specint-specific optimization pattern that is build up around it).
>
> And even value prediction is actually fine, as long as the compiler
> can see the memory *source* of the value prediction (and it isn't a
> mo_consume). So it really ends up limiting your value prediction in
> very simple ways: you cannot do it to function arguments if they are
> registers. But you can still do value prediction on values you loaded
> from memory, if you can actually *see* that memory op.
>
> Of course, on more strongly ordered CPU's, even that "register
> argument" limitation goes away.
>
> So I agree that there is basically no real optimization constraint.
> Value-prediction is of dubious value to begin with, and the actual
> constraint on its use if some compiler writer really wants to is not
> onerous.
>
> > What I have in mind is roughly the following (totally made-up syntax --
> > suggestions for how to do this properly are very welcome):
> > * Have a type modifier (eg, like restrict), that specifies that
> > operations on data of this type are preserving value dependencies:
>
> So I'm not violently opposed, but I think the upsides are not great.
> Note that my earlier suggestion to use "restrict" wasn't because I
> believed the annotation itself would be visible, but basically just as
> a legalistic promise to the compiler that *if* it found an alias, then
> it didn't need to worry about ordering. So to me, that type modifier
> was about conceptual guarantees, not about actual value chains.
>
> Anyway, the reason I don't believe any type modifier (and
> "[[carries_dependency]]" is basically just that) is worth it is simply
> that it adds a real burden on the programmer, without actually giving
> the programmer any real upside:
>
> Within a single function, the compiler already sees that mo_consume
> source, and so doing a type-based restriction doesn't really help. The
> information is already there, without any burden on the programmer.
>
> And across functions, the compiler has already - by definition -
> mostly lost sight of all the things it could use to reduce the value
> space. Even Paul's example doesn't really work if the use of the
> "mo_consume" value has been passed to another function, because inside
> a separate function, the compiler couldn't see that the value it uses
> comes from only two possible values.
>
> And as mentioned, even *if* the compiler wants to do value prediction
> that turns a data dependency into a control dependency, the limitation
> to say "no, you can't do it unless you saw where the value got loaded"
> really isn't that onerous.
>
> I bet that if you ask actual production compiler people (as opposed to
> perhaps academia), none of them actually really believe in value
> prediction to begin with.
>
> > What do you think?
> >
> > Is this meaningful regarding what current hardware offers, or will it do
> > (or might do in the future) value prediction on it's own?
>
> I can pretty much guarantee that when/if hardware does value
> prediction on its own, it will do so without exposing it as breaking
> the data dependency.
>
> The thing is, a CPU is actually *much* better situated at doing
> speculative memory accesses, because a CPU already has all the
> infrastructure to do speculation in general.
>
> And for a CPU, once you do value speculation, guaranteeing the memory
> ordering is *trivial*: all you need to do is to track the "speculated"
> memory instruction until you check the value (which you obviously have
> to do anyway, otherwise you're not doing value _prediction_, you're
> just doing "value wild guessing" ;^), and when you check the value you
> also check that the cacheline hasn't been evicted out-of-order.
>
> This is all stuff that CPU people already do. If you have
> transactional memory, you already have all the resources to do this.
> Or, even without transactional memory, if like Intel you have a memory
> model that says "loads are done in order" but you actually wildly
> speculate loads and just check before retiring instructions that the
> cachelines didn't get evicted out of order, you already have all the
> hardware to do value prediction *without* making it visible in the
> memory order.
>
> This, btw, is one reason why people who think that compilers should be
> overly smart and do fancy tricks are incompetent. People who thought
> that Itanium was a great idea ("Let's put the complexity in the
> compiler, and make a simple CPU") are simply objectively *wrong*.
> People who think that value prediction by a compiler is a good idea
> are not people you should really care about.
>
> Linus
>



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