linux-kernel - Re: [PATCH v8 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock

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Date:   Mon, 6 Jan 2020 10:02:26 -0500
From:   Alex Kogan <alex.kogan@...cle.com>
To:     Waiman Long <longman@...hat.com>
Cc:     linux@...linux.org.uk, Peter Zijlstra <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, steven.sistare@...cle.com,
        daniel.m.jordan@...cle.com, dave.dice@...cle.com
Subject: Re: [PATCH v8 3/5] locking/qspinlock: Introduce CNA into the slow
 path of qspinlock



> On Jan 3, 2020, at 5:14 PM, Waiman Long <longman@...hat.com> wrote:
> 
> On 12/30/19 2:40 PM, Alex Kogan wrote:
>> +/*
>> + * cna_scan_main_queue - scan the main waiting queue looking for the first
>> + * thread running on the same NUMA node as the lock holder. If found (call it
>> + * thread T), move all threads in the main queue between the lock holder and
>> + * T to the end of the secondary queue and return 0
>> + * (=SUCCESSOR_FROM_SAME_NUMA_NODE_FOUND); otherwise, return the encoded
> Are you talking about LOCAL_WAITER_FOUND?
Ahh, yes — good catch!

>> + * pointer of the last scanned node in the primary queue (so a subsequent scan
>> + * can be resumed from that node).
>> + *
>> + * Schematically, this may look like the following (nn stands for numa_node and
>> + * et stands for encoded_tail).
>> + *
>> + *   when cna_scan_main_queue() is called (the secondary queue is empty):
>> + *
>> + *  A+------------+   B+--------+   C+--------+   T+--------+
>> + *   |mcs:next    | -> |mcs:next| -> |mcs:next| -> |mcs:next| -> NULL
>> + *   |mcs:locked=1|    |cna:nn=0|    |cna:nn=2|    |cna:nn=1|
>> + *   |cna:nn=1    |    +--------+    +--------+    +--------+
>> + *   +----------- +
>> + *
>> + *   when cna_scan_main_queue() returns (the secondary queue contains B and C):
>> + *
>> + *  A+----------------+    T+--------+
>> + *   |mcs:next        | ->  |mcs:next| -> NULL
>> + *   |mcs:locked=C.et | -+  |cna:nn=1|
>> + *   |cna:nn=1        |  |  +--------+
>> + *   +--------------- +  +-----+
>> + *                             \/
>> + *          B+--------+   C+--------+
>> + *           |mcs:next| -> |mcs:next| -+
>> + *           |cna:nn=0|    |cna:nn=2|  |
>> + *           +--------+    +--------+  |
>> + *               ^                     |
>> + *               +---------------------+
>> + *
>> + * The worst case complexity of the scan is O(n), where n is the number
>> + * of current waiters. However, the amortized complexity is close to O(1),
>> + * as the immediate successor is likely to be running on the same node once
>> + * threads from other nodes are moved to the secondary queue.
>> + *
>> + * @node      : Pointer to the MCS node of the lock holder
>> + * @pred_start: Pointer to the MCS node of the waiter whose successor should be
>> + *              the first node in the scan
>> + * Return     : LOCAL_WAITER_FOUND or encoded tail of the last scanned waiter
>> + */
>> +static u32 cna_scan_main_queue(struct mcs_spinlock *node,
>> +			       struct mcs_spinlock *pred_start)
>> +{
>> +	struct cna_node *cn = (struct cna_node *)node;
>> +	struct cna_node *cni = (struct cna_node *)READ_ONCE(pred_start->next);
>> +	struct cna_node *last;
>> +	int my_numa_node = cn->numa_node;
>> +
>> +	/* find any next waiter on 'our' NUMA node */
>> +	for (last = cn;
>> +	     cni && cni->numa_node != my_numa_node;
>> +	     last = cni, cni = (struct cna_node *)READ_ONCE(cni->mcs.next))
>> +		;
>> +
>> +	/* if found, splice any skipped waiters onto the secondary queue */
>> +	if (cni) {
>> +		if (last != cn)	/* did we skip any waiters? */
>> +			cna_splice_tail(node, node->next,
>> +					(struct mcs_spinlock *)last);
>> +		return LOCAL_WAITER_FOUND;
>> +	}
>> +
>> +	return last->encoded_tail;
>> +}
>> +
>> 
>> +/*
>> + * Switch to the NUMA-friendly slow path for spinlocks when we have
>> + * multiple NUMA nodes in native environment, unless the user has
>> + * overridden this default behavior by setting the numa_spinlock flag.
>> + */
>> +void cna_configure_spin_lock_slowpath(void)
> Nit: There should be a __init.
True. I will fix that.

>> +{
>> +	if ((numa_spinlock_flag == 1) ||
>> +	    (numa_spinlock_flag == 0 && nr_node_ids > 1 &&
>> +		    pv_ops.lock.queued_spin_lock_slowpath ==
>> +			native_queued_spin_lock_slowpath)) {
>> +		pv_ops.lock.queued_spin_lock_slowpath =
>> +		    __cna_queued_spin_lock_slowpath;
>> +
>> +		pr_info("Enabling CNA spinlock\n");
>> +	}
>> +}
> 
> Other than these two minor nits, the rests looks good to me.
Great. I will revise and resubmit.

Best regards,
— Alex