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Date: Thu, 02 Apr 2015 23:39:04 -0400
From: Waiman Long <waiman.long@...com>
To: Peter Zijlstra <peterz@...radead.org>
CC: tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
paolo.bonzini@...il.com, konrad.wilk@...cle.com,
boris.ostrovsky@...cle.com, paulmck@...ux.vnet.ibm.com,
riel@...hat.com, torvalds@...ux-foundation.org,
raghavendra.kt@...ux.vnet.ibm.com, david.vrabel@...rix.com,
oleg@...hat.com, scott.norton@...com, doug.hatch@...com,
linux-arch@...r.kernel.org, x86@...nel.org,
linux-kernel@...r.kernel.org,
virtualization@...ts.linux-foundation.org,
xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
luto@...capital.net
Subject: Re: [PATCH 8/9] qspinlock: Generic paravirt support
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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