linux-kernel - Fast path efficiency (Re: [rfc][patch] dynamic data structure switching)

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Message-Id: <E1IT3wK-0006Bh-22@flower>
Date:	Thu, 6 Sep 2007 01:05:40 +0200
From:	Oleg Verych <olecom@...wer.upol.cz>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Paul McKenney <paulmck@...ibm.com>
Subject: Fast path efficiency (Re: [rfc][patch] dynamic data structure switching)

* Sun, 2 Sep 2007 20:36:19 +0200
>

I see, that in many places all pre-checks are done in negative form
with resulting return or jump out. In this case, if function was called,
what likely() path is?

> +static void resize_pid_hash(void)
> +{
> +	unsigned int old_shift, new_shift;
> +
> +	if (system_state != SYSTEM_RUNNING)
> +		return;
> +
> +	old_shift = cur_pid_hash->shift;
> +	new_shift = ilog2(nr_pids * 2 - 1);
> +	if (new_shift == old_shift)
> +		return;
> +
> +	if (!mutex_trylock(&dyn_pidhash.resize_mutex))
> +		return;

that one or this?

==
	if (system_state == SYSTEM_RUNNING) {
		old_shift = cur_pid_hash->shift;
		new_shift = ilog2(nr_pids * 2 - 1);
		if (new_shift != old_shift && mutex_trylock(&dyn_pidhash.resize_mutex)) {
==
		> +	old_shift = cur_pid_hash->shift;
		> +	new_shift = ilog2(nr_pids * 2 - 1);

/* hope this repetition is needed by design */

		...
		
		> +	mutex_unlock(&dyn_pidhash.resize_mutex);
		}

What is more efficient in general sense,
as opposed to s,3,2,1,0 Optimized?

> +	if (new_shift != old_shift) {
> +		struct pid_hash *ph, *ret;
> +		unsigned int idx = ph_cur_idx ^ 1;
> +		ph = &pid_hashes[idx];
> +		if (!init_pid_hash(ph, new_shift)) {
> +			ph_cur_idx = idx;
> +
> +			ret = dyn_data_replace(&dyn_pidhash, dd_transfer_pids, ph);
> +			BUG_ON(ret == ph);
> +			BUG_ON(ret != &pid_hashes[idx ^ 1]);
> +			/* XXX: kfree(ret->table) */
> +			ret->shift = -1;
> +			ret->table = NULL;
> +		}
> +	}
> +	mutex_unlock(&dyn_pidhash.resize_mutex);
> +}
> +

Thanks.
____
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