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qsp: use atomic64 accessors 

With the seqlock, we either have to use atomics to remain
within defined behaviour (and note that 64-bit atomics aren't
always guaranteed to compile, irrespective of __nocheck), or
drop the atomics and be in undefined behaviour territory.

Fix it by dropping the seqlock and using atomic64 accessors.
This will limit scalability when !CONFIG_ATOMIC64, but those
machines (1) don't have many users and (2) are unlikely to
have many cores.

- With CONFIG_ATOMIC64:
$ tests/atomic_add-bench -n 1 -m -p
 Throughput:         13.00 Mops/s

- Forcing !CONFIG_ATOMIC64:
$ tests/atomic_add-bench -n 1 -m -p
 Throughput:         10.89 Mops/s

Signed-off-by: Emilio G. Cota <cota@braap.org>
Message-Id: <20180910232752.31565-5-cota@braap.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Emilio G. Cota 2018-09-10 19:27:44 -04:00 committed by Paolo Bonzini
parent 82fdfcbe64
commit ac8c77486c
1 changed files with 8 additions and 41 deletions
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49
util/qsp.c
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@ -84,13 +84,6 @@ struct QSPEntry {
uint64_t n_acqs;
uint64_t ns;
unsigned int n_objs; /* count of coalesced objs; only used for reporting */
#ifndef CONFIG_ATOMIC64
/*
* If we cannot update the counts atomically, then use a seqlock.
* We don't need an associated lock because the updates are thread-local.
*/
QemuSeqLock sequence;
#endif
};
typedef struct QSPEntry QSPEntry;
@ -344,47 +337,16 @@ static QSPEntry *qsp_entry_get(const void *obj, const char *file, int line,
return qsp_entry_find(&qsp_ht, &orig, hash);
}
/*
* @from is in the global hash table; read it atomically if the host
* supports it, otherwise use the seqlock.
*/
static void qsp_entry_aggregate(QSPEntry *to, const QSPEntry *from)
{
#ifdef CONFIG_ATOMIC64
to->ns += atomic_read__nocheck(&from->ns);
to->n_acqs += atomic_read__nocheck(&from->n_acqs);
#else
unsigned int version;
uint64_t ns, n_acqs;
do {
version = seqlock_read_begin(&from->sequence);
ns = atomic_read__nocheck(&from->ns);
n_acqs = atomic_read__nocheck(&from->n_acqs);
} while (seqlock_read_retry(&from->sequence, version));
to->ns += ns;
to->n_acqs += n_acqs;
#endif
}
/*
* @e is in the global hash table; it is only written to by the current thread,
* so we write to it atomically (as in "write once") to prevent torn reads.
* If the host doesn't support u64 atomics, use the seqlock.
*/
static inline void do_qsp_entry_record(QSPEntry *e, int64_t delta, bool acq)
{
#ifndef CONFIG_ATOMIC64
seqlock_write_begin(&e->sequence);
#endif
atomic_set__nocheck(&e->ns, e->ns + delta);
atomic_set_u64(&e->ns, e->ns + delta);
if (acq) {
atomic_set__nocheck(&e->n_acqs, e->n_acqs + 1);
atomic_set_u64(&e->n_acqs, e->n_acqs + 1);
}
#ifndef CONFIG_ATOMIC64
seqlock_write_end(&e->sequence);
#endif
}
static inline void qsp_entry_record(QSPEntry *e, int64_t delta)
@ -550,7 +512,12 @@ static void qsp_aggregate(void *p, uint32_t h, void *up)
hash = qsp_entry_no_thread_hash(e);
agg = qsp_entry_find(ht, e, hash);
qsp_entry_aggregate(agg, e);
/*
* The entry is in the global hash table; read from it atomically (as in
* "read once").
*/
agg->ns += atomic_read_u64(&e->ns);
agg->n_acqs += atomic_read_u64(&e->n_acqs);
}
static void qsp_iter_diff(void *p, uint32_t hash, void *htp)