Re: [Xen-devel] [PATCH 8/9] qspinlock: Generic paravirt support

[Waiman Long <waiman.long@xxxxxx>]

On 04/02/2015 03:48 PM, Peter Zijlstra wrote:
> On Thu, Apr 02, 2015 at 07:20:57PM +0200, Peter Zijlstra wrote:
> > pv_wait_head():
> >
> >         pv_hash()
> >         /* MB as per cmpxchg */
> >         cmpxchg(&l->locked, _Q_LOCKED_VAL, _Q_SLOW_VAL);
> >
> > VS
> >
> > __pv_queue_spin_unlock():
> >
> >         if (xchg(&l->locked, 0) != _Q_SLOW_VAL)
> >                 return;
> >
> >         /* MB as per xchg */
> >         pv_hash_find(lock);
> Something like so.. compile tested only.
>
> I took out the LFSR because that was likely over engineering from my
> side :-)
>
> --- a/kernel/locking/qspinlock_paravirt.h
> +++ b/kernel/locking/qspinlock_paravirt.h
> @@ -2,6 +2,8 @@
>   #error "do not include this file"
>   #endif
>
> +#include<linux/hash.h>
> +
>   /*
>    * Implement paravirt qspinlocks; the general idea is to halt the vcpus 
> instead
>    * of spinning them.
> @@ -107,7 +109,84 @@ static void pv_kick_node(struct mcs_spin
>                 pv_kick(pn->cpu);
>   }
>
> -static DEFINE_PER_CPU(struct qspinlock *, __pv_lock_wait);
> +/*
> + * Hash table using open addressing with an linear probe sequence.
> + *
> + * Since we should not be holding locks from NMI context (very rare indeed) the
> + * max load factor is 0.75, which is around the point where open adressing
> + * breaks down.
> + *
> + * Instead of probing just the immediate bucket we probe all buckets in the
> + * same cacheline.
> + *
> + * http://en.wikipedia.org/wiki/Hash_table#Open_addressing
> + *
> + */
> +
> +struct pv_hash_bucket {
> +       struct qspinlock *lock;
> +       int cpu;
> +};
> +
> +/*
> + * XXX dynamic allocate using nr_cpu_ids instead...
> + */
> +#define PV_LOCK_HASH_BITS      (2 + NR_CPUS_BITS)
> +
> +#if PV_LOCK_HASH_BITS<  6
> +#undef PV_LOCK_HASH_BITS
> +#define PB_LOCK_HASH_BITS      6
> +#endif
> +
> +#define PV_LOCK_HASH_SIZE      (1<<  PV_LOCK_HASH_BITS)
> +
> +static struct pv_hash_bucket __pv_lock_hash[PV_LOCK_HASH_SIZE] 
> ____cacheline_aligned;
> +
> +#define PV_HB_PER_LINE         (SMP_CACHE_BYTES / sizeof(struct 
> pv_hash_bucket))
> +
> +static inline u32 hash_align(u32 hash)
> +{
> +       return hash&  ~(PV_HB_PER_LINE - 1);
> +}
> +
> +#define for_each_hash_bucket(hb, off, hash)                                    
> \
> +       for (hash = hash_align(hash), off = 0, hb =&__pv_lock_hash[hash + off];\
> +           off<  PV_LOCK_HASH_SIZE;                                         \
> +           off++, hb =&__pv_lock_hash[(hash + off) % PV_LOCK_HASH_SIZE])
> +
> +static struct pv_hash_bucket *pv_hash_insert(struct qspinlock *lock)
> +{
> +       u32 offset, hash = hash_ptr(lock, PV_LOCK_HASH_BITS);
> +       struct pv_hash_bucket *hb;
> +
> +       for_each_hash_bucket(hb, offset, hash) {
> +               if (!cmpxchg(&hb->lock, NULL, lock)) {
> +                       WRITE_ONCE(hb->cpu, smp_processor_id());
> +                       return hb;
> +               }
> +       }
> +
> +       /*
> +        * Hard assumes there is an empty bucket somewhere.
> +        */
> +       BUG();
> +}
> +
> +static struct pv_hash_bucket *pv_hash_find(struct qspinlock *lock)
> +{
> +       u32 offset, hash = hash_ptr(lock, PV_LOCK_HASH_BITS);
> +       struct pv_hash_bucket *hb;
> +
> +       for_each_hash_bucket(hb, offset, hash) {
> +               if (READ_ONCE(hb->lock) == lock)
> +                       return hb;
> +       }
> +
> +       /*
> +        * Hard assumes we _WILL_ find a match.
> +        */
> +       BUG();
> +}
>
>   /*
>    * Wait for l->locked to become clear; halt the vcpu after a short spin.
> @@ -116,7 +195,9 @@ static DEFINE_PER_CPU(struct qspinlock *
>   static void pv_wait_head(struct qspinlock *lock)
>   {
>         struct __qspinlock *l = (void *)lock;
> +       struct pv_hash_bucket *hb = NULL;
>         int loop;
> +       u8 o;
>
>         for (;;) {
>                 for (loop = SPIN_THRESHOLD; loop; loop--) {
> @@ -126,29 +207,47 @@ static void pv_wait_head(struct qspinloc
>                         cpu_relax();
>                 }
>
> -               this_cpu_write(__pv_lock_wait, lock);
> -               /*
> -                * __pv_lock_wait must be set before setting _Q_SLOW_VAL
> -                *
> -                * [S] __pv_lock_wait = lock    [RmW] l = l->locked = 0
> -                *     MB                             MB
> -                * [S] l->locked = _Q_SLOW_VAL  [L]   __pv_lock_wait
> -                *
> -                * Matches the xchg() in pv_queue_spin_unlock().
> -                */
> -               if (!cmpxchg(&l->locked, _Q_LOCKED_VAL, _Q_SLOW_VAL))
> -                       goto done;
> +               if (!hb) {
> +                       hb = pv_hash_insert(lock);
> +                       /*
> +                        * hb  must be set before setting _Q_SLOW_VAL
> +                        *
> +                        * [S]   hb<- lock               [RmW] l = l->locked = 0
> +                        *       MB                             MB
> +                        * [RmW] l->locked ?= _Q_SLOW_VAL [L]   hb
> +                        *
> +                        * Matches the xchg() in pv_queue_spin_unlock().
> +                        */
> +                       o = cmpxchg(&l->locked, _Q_LOCKED_VAL, _Q_SLOW_VAL);
> +                       if (!o) {
> +                               /*
> +                                * The lock got unlocked before we could set
> +                                * _Q_SLOW_VAL, we must unhash ourselves.
> +                                */
> +                               WRITE_ONCE(hb->lock, NULL);
> +                               goto done;
> +                       }
> +                       BUG_ON(o != _Q_LOCKED_VAL);
> +                       /*
> +                        * At this point _Q_SLOW_VAL is visible and the unlock
> +                        * will do the lookup. The lookup hard relies on the
> +                        * entry being visible -- which it should be. Unlock
> +                        * will unhash for us.
> +                        */
> +               }
>
>                 pv_wait(&l->locked, _Q_SLOW_VAL);
> +               /*
> +                * We can get spurious wakeups from interrupts, cycle back.
> +                */
>         }
>   done:
> -       this_cpu_write(__pv_lock_wait, NULL);
> -
>         /*
>          * Lock is unlocked now; the caller will acquire it without waiting.
>          * As with pv_wait_node() we rely on the caller to do a load-acquire
>          * for us.
>          */
> +       return;
>   }
>
>   /*
> @@ -158,20 +257,20 @@ static void pv_wait_head(struct qspinloc
>   void __pv_queue_spin_unlock(struct qspinlock *lock)
>   {
>         struct __qspinlock *l = (void *)lock;
> -       int cpu;
> +       struct pv_hash_bucket *hb;
>
>         if (xchg(&l->locked, 0) != _Q_SLOW_VAL)
>                 return;
>
>         /*
>          * At this point the memory pointed at by lock can be freed/reused,
> -        * however we can still use the pointer value to search in our cpu
> -        * array.
> +        * however we can still use the pointer value to search in our hash
> +        * table.
>          *
> -        * XXX: get rid of this loop
> +        * Also, if we observe _Q_SLOW_VAL we _must_ now observe 'our' hash
> +        * bucket. See pv_wait_head().
>          */
> -       for_each_possible_cpu(cpu) {
> -               if (per_cpu(__pv_lock_wait, cpu) == lock)
> -                       pv_kick(cpu);
> -       }
> +       hb = pv_hash_find(lock);
> +       pv_kick(hb->cpu);
> +       WRITE_ONCE(hb->lock, NULL); /* unhash */
>   }
Thanks for the code. I am working on my own version, too. Will need to 
run some tests to verify the correctness of the code. Hopefully I have 
something for you to review early next week.
Cheers,
Longman

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