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

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Date:   Mon, 8 Nov 2021 00:34:00 +0800
From:   Tao Zhou <tao.zhou@...ux.dev>
To:     Peter Oskolkov <posk@...k.io>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Paul Turner <pjt@...gle.com>, Ben Segall <bsegall@...gle.com>,
        Peter Oskolkov <posk@...gle.com>,
        Andrei Vagin <avagin@...gle.com>, Jann Horn <jannh@...gle.com>,
        Thierry Delisle <tdelisle@...terloo.ca>,
        Tao Zhou <tao.zhou@...ux.dev>
Subject: Re: [PATCH v0.8 4/6] sched/umcg, lib/umcg: implement libumcg

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);
> +}