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>
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util/qsp.c
49
util/qsp.c
@ -84,13 +84,6 @@ struct QSPEntry {
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uint64_t n_acqs;
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uint64_t ns;
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unsigned int n_objs; /* count of coalesced objs; only used for reporting */
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#ifndef CONFIG_ATOMIC64
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/*
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* If we cannot update the counts atomically, then use a seqlock.
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* We don't need an associated lock because the updates are thread-local.
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*/
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QemuSeqLock sequence;
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#endif
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};
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typedef struct QSPEntry QSPEntry;
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@ -344,47 +337,16 @@ static QSPEntry *qsp_entry_get(const void *obj, const char *file, int line,
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return qsp_entry_find(&qsp_ht, &orig, hash);
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}
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/*
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* @from is in the global hash table; read it atomically if the host
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* supports it, otherwise use the seqlock.
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*/
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static void qsp_entry_aggregate(QSPEntry *to, const QSPEntry *from)
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{
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#ifdef CONFIG_ATOMIC64
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to->ns += atomic_read__nocheck(&from->ns);
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to->n_acqs += atomic_read__nocheck(&from->n_acqs);
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#else
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unsigned int version;
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uint64_t ns, n_acqs;
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do {
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version = seqlock_read_begin(&from->sequence);
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ns = atomic_read__nocheck(&from->ns);
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n_acqs = atomic_read__nocheck(&from->n_acqs);
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} while (seqlock_read_retry(&from->sequence, version));
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to->ns += ns;
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to->n_acqs += n_acqs;
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#endif
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}
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/*
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* @e is in the global hash table; it is only written to by the current thread,
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* so we write to it atomically (as in "write once") to prevent torn reads.
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* If the host doesn't support u64 atomics, use the seqlock.
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*/
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static inline void do_qsp_entry_record(QSPEntry *e, int64_t delta, bool acq)
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{
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#ifndef CONFIG_ATOMIC64
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seqlock_write_begin(&e->sequence);
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#endif
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atomic_set__nocheck(&e->ns, e->ns + delta);
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atomic_set_u64(&e->ns, e->ns + delta);
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if (acq) {
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atomic_set__nocheck(&e->n_acqs, e->n_acqs + 1);
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atomic_set_u64(&e->n_acqs, e->n_acqs + 1);
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}
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#ifndef CONFIG_ATOMIC64
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seqlock_write_end(&e->sequence);
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#endif
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}
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static inline void qsp_entry_record(QSPEntry *e, int64_t delta)
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@ -550,7 +512,12 @@ static void qsp_aggregate(void *p, uint32_t h, void *up)
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hash = qsp_entry_no_thread_hash(e);
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agg = qsp_entry_find(ht, e, hash);
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qsp_entry_aggregate(agg, e);
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/*
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* The entry is in the global hash table; read from it atomically (as in
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* "read once").
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*/
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agg->ns += atomic_read_u64(&e->ns);
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agg->n_acqs += atomic_read_u64(&e->n_acqs);
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}
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static void qsp_iter_diff(void *p, uint32_t hash, void *htp)
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