Re: Kernel broken on processors without performance counters

[Peter Zijlstra]

On Fri, Jul 24, 2015 at 12:56:02PM +0200, Peter Zijlstra wrote:
> On Thu, Jul 23, 2015 at 03:14:13PM -0400, Jason Baron wrote:
> > > +static enum jump_label_type jump_label_type(struct jump_entry *entry)
> > > +{
> > > +	struct static_key *key = static_key_cast(iter->key);
> > > +	bool true_branch = jump_label_get_branch_default(key);
> > > +	bool state = static_key_enabled(key);
> > > +	bool inv = static_key_inv(iter->key);
> > > +
> > > +	return (true_branch ^ state) ^ inv;
> > 
> > iiuc...this allows both the old style keys to co-exist with the
> > new ones. IE state wouldn't be set for the new ones. And inv
> > shouldn't be set for the old ones. Is that correct?
> 
> @state is the dynamic variable here, the other two are compile time.
> @true_branch denotes the default (compile time) value, and @inv denotes
> the (compile time) branch preference.

Ha!, so that wasn't entirely correct, it turned out @inv means
arch_static_branch_jump().

That would let us remove the whole argument to the arch functions.

That said, I generated the logic table for @inv meaning the branch type
and then found a logic similar to what you outlined:


/*
 * Combine the right initial value (type) with the right branch order
 * to generate the desired result.
 *
 *
 *  type	likely (1)		unlikely (0)
 * -----------+-----------------------+------------------
 *            |                       |
 *  true (1)  |	   ...		      |	   ...
 *            |    NOP		      |	   JMP L
 *            |    <br-stmts>	      |	1: ...
 *            |	L: ...		      |
 *            |			      |
 *            |			      |	L: <br-stmts>
 *            |			      |	   jmp 1b
 *            |                       |
 * -----------+-----------------------+------------------
 *            |                       |
 *  false (0) |	   ...		      |	   ...
 *            |    JMP L	      |	   NOP
 *            |    <br-stmts>	      |	1: ...
 *            |	L: ...		      |
 *            |			      |
 *            |			      |	L: <br-stmts>
 *            |			      |	   jmp 1b
 *            |                       |
 * -----------+-----------------------+------------------
 *
 * The initial value is encoded in the LSB of static_key::entries,
 * type: 0 = false, 1 = true.
 *
 * The branch type is encoded in the LSB of jump_entry::key,
 * branch: 0 = unlikely, 1 = likely.
 *
 * This gives the following logic table:
 *
 *	enabled	type	branch	  instuction
 * -----------------------------+-----------
 *	0	0	0	| NOP
 *	0	0	1	| JMP
 *	0	1	0	| NOP
 *	0	1	1	| JMP
 *
 *	1	0	0	| JMP
 *	1	0	1	| NOP
 *	1	1	0	| JMP
 *	1	1	1	| NOP
 *
 */

This gives a map: ins = enabled ^ branch, which shows @type to be
redundant.

And we can trivially switch over the old static_key_{true,false}() to
emit the right branch type.

Whcih would mean we could remove the type encoding entirely, but I'll
leave that be for now, maybe it'll come in handy later or whatnot.

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