Re: [PATCH v0.8 4/6] sched/umcg, lib/umcg: implement libumcg

Tao Zhou
2021-11-07T16:33:24Z

On Thu, Nov 04, 2021 at 12:58:02PM -0700, Peter Oskolkov wrote:

> +/* 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.

> +
> + 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);
> +}
> +

> +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.

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
> + 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);
> +}