RE: [PATCH next v2 5/5] locking/osq_lock: Optimise decode_cpu() and per_cpu_ptr().

[David Laight]

From: Ingo Molnar
> Sent: 02 January 2024 09:54
> 
> 
> * David Laight <David.Laight@xxxxxxxxxx> wrote:
> 
> > per_cpu_ptr() indexes __per_cpu_offset[] with the cpu number.
> > This requires the cpu number be 64bit.
> > However the value is osq_lock() comes from a 32bit xchg() and there
> > isn't a way of telling gcc the high bits are zero (they are) so
> > there will always be an instruction to clear the high bits.
> >
> > The cpu number is also offset by one (to make the initialiser 0)
> > It seems to be impossible to get gcc to convert __per_cpu_offset[cpu_p1 - 1]
> > into (__per_cpu_offset - 1)[cpu_p1] (transferring the offset to the address).
> >
> > Converting the cpu number to 32bit unsigned prior to the decrement means
> > that gcc knows the decrement has set the high bits to zero and doesn't
> > add a register-register move (or cltq) to zero/sign extend the value.
> >
> > Not massive but saves two instructions.
> >
> > Signed-off-by: David Laight <david.laight@xxxxxxxxxx>
> > ---
> >  kernel/locking/osq_lock.c | 6 ++----
> >  1 file changed, 2 insertions(+), 4 deletions(-)
> >
> > diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> > index 35bb99e96697..37a4fa872989 100644
> > --- a/kernel/locking/osq_lock.c
> > +++ b/kernel/locking/osq_lock.c
> > @@ -29,11 +29,9 @@ static inline int encode_cpu(int cpu_nr)
> >  	return cpu_nr + 1;
> >  }
> >
> > -static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val)
> > +static inline struct optimistic_spin_node *decode_cpu(unsigned int encoded_cpu_val)
> >  {
> > -	int cpu_nr = encoded_cpu_val - 1;
> > -
> > -	return per_cpu_ptr(&osq_node, cpu_nr);
> > +	return per_cpu_ptr(&osq_node, encoded_cpu_val - 1);
> 
> So why do we 'encode' the CPU number to begin with?
> 
> Why not use -1 as the special value? Checks for negative values
> generates similarly fast machine code compared to checking for 0, if
> the value is also used (which it is in most cases here). What am I
> missing? We seem to be going through a lot of unnecessary hoops, and
> some of that is in the runtime path.

You'd really have to ask the person who did the original patch
that changed lock->tail from a pointer to an int (saving 8 bytes)
in every mutex and rwsem.

I suspect the reason is that it is so much safer to have the
initialiser being zero, rather than a non-zero value with zero
being a valid value.

It is also hard to avoid an extra instruction in the per_cpu_ptr()
code - something has to extend the 32bit result from xchg() to
a 64bit one for the array index.
The asm for an unsigned 32bit exchange could return a 64bit result
(which would have the desired effect), but that won't work for
a signed value.
The '-1' in the vcpu_is_preempted() path will be executed in parallel
with something else and is likely to have no measurable effect.

So it is a slightly risky change that has less benefit than the
other changes (which save cache line accesses).

	David

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