On Sun, Nov 7, 2021 at 8:33 AM Tao Zhou [off-list ref] wrote:
On Thu, Nov 04, 2021 at 12:58:02PM -0700, Peter Oskolkov wrote:
quoted
+/* Update the state variable, set new timestamp. */
+static bool umcg_update_state(uint64_t *state, uint64_t *prev, uint64_t next)
+{
+ uint64_t prev_ts = (*prev) >> (64 - UMCG_STATE_TIMESTAMP_BITS);
+ struct timespec now;
+ uint64_t next_ts;
+ int res;
+
+ /*
+ * clock_gettime(CLOCK_MONOTONIC, ...) takes less than 20ns on a
+ * typical Intel processor on average, even when run concurrently,
+ * so the overhead is low enough for most applications.
+ *
+ * If this is still too high, `next_ts = prev_ts + 1` should work
+ * as well. The only real requirement is that the "timestamps" are
+ * uniqueue per thread within a reasonable time frame.
+ */
+ res = clock_gettime(CLOCK_MONOTONIC, &now);
+ assert(!res);
+ next_ts = (now.tv_sec * NSEC_PER_SEC + now.tv_nsec) >>
+ UMCG_STATE_TIMESTAMP_GRANULARITY;
+
+ /* Cut higher order bits. */
+ next_ts &= ((1ULL << UMCG_STATE_TIMESTAMP_BITS) - 1);
This is the right cut.. The same to the kernel side.
Yes, thanks!
quoted
+
+ if (next_ts == prev_ts)
+ ++next_ts;
+
+#ifndef NDEBUG
+ if (prev_ts > next_ts) {
+ fprintf(stderr, "%s: time goes back: prev_ts: %lu "
+ "next_ts: %lu diff: %lu\n", __func__,
+ prev_ts, next_ts, prev_ts - next_ts);
+ }
+#endif
+
+ /* Remove old timestamp, if any. */
+ next &= ((1ULL << (64 - UMCG_STATE_TIMESTAMP_BITS)) - 1);
+
+ /* Set the new timestamp. */
+ next |= (next_ts << (64 - UMCG_STATE_TIMESTAMP_BITS));
+
+ /*
+ * TODO: review whether memory order below can be weakened to
+ * memory_order_acq_rel for success and memory_order_acquire for
+ * failure.
+ */
+ return atomic_compare_exchange_strong_explicit(state, prev, next,
+ memory_order_seq_cst, memory_order_seq_cst);
+}
+
quoted
+static void task_unlock(struct umcg_task_tls *task, uint64_t expected_state,
+ uint64_t new_state)
+{
+ bool ok;
+ uint64_t next;
+ uint64_t prev = atomic_load_explicit(&task->umcg_task.state_ts,
+ memory_order_acquire);
+
+ next = ((prev & ~UMCG_TASK_STATE_MASK_FULL) | new_state) & ~UMCG_TF_LOCKED;
Use UMCG_TASK_STATE_MASK instead and the other state flag can be checked.
Why? We want to clear the TF_LOCKED flag and keep every other bit of
state, including other state flags (but excluding timestamp).
All others places that use UMCG_TASK_STATE_MASK_FULL to mask to check
the task state may seems reasonable if the state flag not allowed to
be set when we check that task state, otherwise use UMCG_TASK_STATE_MASK
will be enough.
Not sure.
Thanks,
Tao
quoted
+ assert(next != prev);
+ assert((prev & UMCG_TASK_STATE_MASK_FULL & ~UMCG_TF_LOCKED) == expected_state);
+
+ ok = umcg_update_state(&task->umcg_task.state_ts, &prev, next);
+ assert(ok);
+}