Re: [PATCH next 5/5] locking/osq_lock: Optimise vcpu_is_preempted() check.

[Waiman Long]

On 12/29/23 22:13, Waiman Long wrote:
> On 12/29/23 15:58, David Laight wrote:
> > The vcpu_is_preempted() test stops osq_lock() spinning if a virtual
> >    cpu is no longer running.
> > Although patched out for bare-metal the code still needs the cpu number.
> > Reading this from 'prev->cpu' is a pretty much guaranteed have a 
> > cache miss
> > when osq_unlock() is waking up the next cpu.
> >
> > Instead save 'prev->cpu' in 'node->prev_cpu' and use that value instead.
> > Update in the osq_lock() 'unqueue' path when 'node->prev' is changed.
> >
> > This is simpler than checking for 'node->prev' changing and caching
> > 'prev->cpu'.
> >
> > Signed-off-by: David Laight <david.laight@xxxxxxxxxx>
> > ---
> >   kernel/locking/osq_lock.c | 14 ++++++--------
> >   1 file changed, 6 insertions(+), 8 deletions(-)
> >
> > diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> > index b60b0add0161..89be63627434 100644
> > --- a/kernel/locking/osq_lock.c
> > +++ b/kernel/locking/osq_lock.c
> > @@ -14,8 +14,9 @@
> >     struct optimistic_spin_node {
> >       struct optimistic_spin_node *self, *next, *prev;
> > -    int locked; /* 1 if lock acquired */
> > -    int cpu; /* encoded CPU # + 1 value */
> > +    int locked;    /* 1 if lock acquired */
> > +    int cpu;       /* encoded CPU # + 1 value */
> > +    int prev_cpu;  /* actual CPU # for vpcu_is_preempted() */
> >   };
> >     static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, 
> > osq_node);
> > @@ -29,11 +30,6 @@ static inline int encode_cpu(int cpu_nr)
> >       return cpu_nr + 1;
> >   }
> >   -static inline int node_cpu(struct optimistic_spin_node *node)
> > -{
> > -    return node->cpu - 1;
> > -}
> > -
> >   static inline struct optimistic_spin_node *decode_cpu(int 
> > encoded_cpu_val)
> >   {
> >       int cpu_nr = encoded_cpu_val - 1;
> > @@ -114,6 +110,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
> >       if (old == OSQ_UNLOCKED_VAL)
> >           return true;
> >   +    node->prev_cpu = old - 1;
> >       prev = decode_cpu(old);
> >       node->prev = prev;
> >       node->locked = 0;
> > @@ -148,7 +145,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
> >        * polling, be careful.
> >        */
> >       if (smp_cond_load_relaxed(&node->locked, VAL || need_resched() ||
> > -                  vcpu_is_preempted(node_cpu(node->prev))))
> > +                  vcpu_is_preempted(node->prev_cpu)))

On second thought, I believe it is more correct to use READ_ONCE() to 
access "node->prev_cpu" as this field is subjected to change by a 
WRITE_ONCE().

Cheers,
Longman