linux-kernel - Re: [PATCH v5 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock

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Message-ID: <6ce50aeb-6b87-5d1c-9011-4329e8dadfec@redhat.com>
Date:   Fri, 18 Oct 2019 12:03:10 -0400
From:   Waiman Long <longman@...hat.com>
To:     Alex Kogan <alex.kogan@...cle.com>, linux@...linux.org.uk,
        peterz@...radead.org, mingo@...hat.com, will.deacon@....com,
        arnd@...db.de, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, bp@...en8.de, hpa@...or.com, x86@...nel.org,
        guohanjun@...wei.com, jglauber@...vell.com
Cc:     steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        dave.dice@...cle.com, rahul.x.yadav@...cle.com
Subject: Re: [PATCH v5 3/5] locking/qspinlock: Introduce CNA into the slow
 path of qspinlock

On 10/16/19 12:29 AM, Alex Kogan wrote:
> +static inline void cna_pass_lock(struct mcs_spinlock *node,
> +				 struct mcs_spinlock *next)
> +{
> +	struct cna_node *cn = (struct cna_node *)node;
> +	struct mcs_spinlock *next_holder = next, *tail_2nd;
> +	u32 val = 1;
> +
> +	u32 scan = cn->pre_scan_result;
> +
> +	/*
> +	 * check if a successor from the same numa node has not been found in
> +	 * pre-scan, and if so, try to find it in post-scan starting from the
> +	 * node where pre-scan stopped (stored in @pre_scan_result)
> +	 */
> +	if (scan > 0)
> +		scan = cna_scan_main_queue(node, decode_tail(scan));
> +
> +	if (!scan) { /* if found a successor from the same numa node */
> +		next_holder = node->next;
> +		/*
> +		 * make sure @val gets 1 if current holder's @locked is 0 as
> +		 * we have to store a non-zero value in successor's @locked
> +		 * to pass the lock
> +		 */
> +		val = node->locked + (node->locked == 0);

node->locked can be 0 when the cpu enters into an empty MCS queue. We
could unconditionally set node->locked to 1 for this case in qspinlock.c
or with your above code. Perhaps, a comment about when node->locked will
be 0.

It may be easier to understand if you just do

    val = node->locked ? node->locked : 1;

Cheers,
Longman