Date:   Fri, 6 Dec 2019 14:50:39 -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,
        rahul.x.yadav@...cle.com
Subject: Re: [PATCH v7 3/5] locking/qspinlock: Introduce CNA into the slow
 path of qspinlock

Thanks for the feedback. I will take care of that and resubmit.

Regards,
— Alex

> On Dec 6, 2019, at 12:21 PM, Waiman Long <longman@...hat.com> wrote:
> 
> On 11/25/19 4:07 PM, Alex Kogan wrote:
>> In CNA, spinning threads are organized in two queues, a main queue for
>> threads running on the same node as the current lock holder, and a
>> secondary queue for threads running on other nodes. After acquiring the
>> MCS lock and before acquiring the spinlock, the lock holder scans the
>> main queue looking for a thread running on the same node (pre-scan). If
>> found (call it thread T), all threads in the main queue between the
>> current lock holder and T are moved to the end of the secondary queue.
>> If such T is not found, we make another scan of the main queue when
>> unlocking the MCS lock (post-scan), starting at the position where
>> pre-scan stopped. If both scans fail to find such T, the MCS lock is
>> passed to the first thread in the secondary queue. If the secondary queue
>> is empty, the lock is passed to the next thread in the main queue.
>> For more details, see https://urldefense.proofpoint.com/v2/url?u=https-3A__arxiv.org_abs_1810.05600&d=DwIFaQ&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=Hvhk3F4omdCk-GE1PTOm3Kn0A7ApWOZ2aZLTuVxFK4k&m=FcJA-hpAHs8MxL3rksMfq_GYsA7RIciHEF3AE4ZATOI&s=SR-p0LtEzUCrpwWY2ShRIF9lXod5Wc_NkN9zIOVvfxM&e= .
>> 
>> Note that this variant of CNA may introduce starvation by continuously
>> passing the lock to threads running on the same node. This issue
>> will be addressed later in the series.
>> 
>> Enabling CNA is controlled via a new configuration option
>> (NUMA_AWARE_SPINLOCKS). By default, the CNA variant is patched in at the
>> boot time only if we run on a multi-node machine in native environment and
>> the new config is enabled. (For the time being, the patching requires
>> CONFIG_PARAVIRT_SPINLOCKS to be enabled as well. However, this should be
>> resolved once static_call() is available.) This default behavior can be
>> overridden with the new kernel boot command-line option
>> "numa_spinlock=on/off" (default is "auto").
>> 
>> Signed-off-by: Alex Kogan <alex.kogan@...cle.com>
>> Reviewed-by: Steve Sistare <steven.sistare@...cle.com>
>> ---
>> .../admin-guide/kernel-parameters.txt         |  10 +
>> arch/x86/Kconfig                              |  20 ++
>> arch/x86/include/asm/qspinlock.h              |   4 +
>> arch/x86/kernel/alternative.c                 |  43 +++
>> kernel/locking/mcs_spinlock.h                 |   2 +-
>> kernel/locking/qspinlock.c                    |  34 ++-
>> kernel/locking/qspinlock_cna.h                | 264 ++++++++++++++++++
>> 7 files changed, 372 insertions(+), 5 deletions(-)
>> create mode 100644 kernel/locking/qspinlock_cna.h
>> 
>   :
>> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
>> index 9d3a971ea364..6a4ccbf4e09c 100644
>> --- a/arch/x86/kernel/alternative.c
>> +++ b/arch/x86/kernel/alternative.c
>> @@ -698,6 +698,33 @@ static void __init int3_selftest(void)
>> 	unregister_die_notifier(&int3_exception_nb);
>> }
>> 
>> +#if defined(CONFIG_NUMA_AWARE_SPINLOCKS)
>> +/*
>> + * Constant (boot-param configurable) flag selecting the NUMA-aware variant
>> + * of spinlock.  Possible values: -1 (off) / 0 (auto, default) / 1 (on).
>> + */
>> +static int numa_spinlock_flag;
>> +
>> +static int __init numa_spinlock_setup(char *str)
>> +{
>> +	if (!strcmp(str, "auto")) {
>> +		numa_spinlock_flag = 0;
>> +		return 1;
>> +	} else if (!strcmp(str, "on")) {
>> +		numa_spinlock_flag = 1;
>> +		return 1;
>> +	} else if (!strcmp(str, "off")) {
>> +		numa_spinlock_flag = -1;
>> +		return 1;
>> +	}
>> +
>> +	return 0;
>> +}
>> +
>> +__setup("numa_spinlock=", numa_spinlock_setup);
>> +
>> +#endif
>> +
> 
> This __init function should be in qspinlock_cna.h. We generally like to
> put as much related code into as few places as possible instead of
> spreading them around in different places.
> 
>> void __init alternative_instructions(void)
>> {
>> 	int3_selftest();
>> @@ -738,6 +765,22 @@ void __init alternative_instructions(void)
>> 	}
>> #endif
>> 
>> +#if defined(CONFIG_NUMA_AWARE_SPINLOCKS)
>> +	/*
>> +	 * By default, switch to the NUMA-friendly slow path for
>> +	 * spinlocks when we have multiple NUMA nodes in native environment.
>> +	 */
>> +	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");
>> +	}
>> +#endif
>> +
>> 	apply_paravirt(__parainstructions, __parainstructions_end);
> 
> Encapsulate the logic into another __init function in qspinlock_cna.h
> and just make a function call here. You can declare the function in
> arch/x86/include/asm/qspinlock.h.
> 
> 
>> 
>> 	restart_nmi();
>> diff --git a/kernel/locking/mcs_spinlock.h b/kernel/locking/mcs_spinlock.h
>> index 52d06ec6f525..e40b9538b79f 100644
>> --- a/kernel/locking/mcs_spinlock.h
>> +++ b/kernel/locking/mcs_spinlock.h
>> @@ -17,7 +17,7 @@
>> 
>> struct mcs_spinlock {
>> 	struct mcs_spinlock *next;
>> -	int locked; /* 1 if lock acquired */
>> +	unsigned int locked; /* 1 if lock acquired */
>> 	int count;  /* nesting count, see qspinlock.c */
>> };
>> 
>> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
>> index c06d1e8075d9..6d8c4a52e44e 100644
>> --- a/kernel/locking/qspinlock.c
>> +++ b/kernel/locking/qspinlock.c
>> @@ -11,7 +11,7 @@
>>  *          Peter Zijlstra <peterz@...radead.org>
>>  */
>> 
>> -#ifndef _GEN_PV_LOCK_SLOWPATH
>> +#if !defined(_GEN_PV_LOCK_SLOWPATH) && !defined(_GEN_CNA_LOCK_SLOWPATH)
>> 
>> #include <linux/smp.h>
>> #include <linux/bug.h>
>> @@ -70,7 +70,8 @@
>> /*
>>  * On 64-bit architectures, the mcs_spinlock structure will be 16 bytes in
>>  * size and four of them will fit nicely in one 64-byte cacheline. For
>> - * pvqspinlock, however, we need more space for extra data. To accommodate
>> + * pvqspinlock, however, we need more space for extra data. The same also
>> + * applies for the NUMA-aware variant of spinlocks (CNA). To accommodate
>>  * that, we insert two more long words to pad it up to 32 bytes. IOW, only
>>  * two of them can fit in a cacheline in this case. That is OK as it is rare
>>  * to have more than 2 levels of slowpath nesting in actual use. We don't
>> @@ -79,7 +80,7 @@
>>  */
>> struct qnode {
>> 	struct mcs_spinlock mcs;
>> -#ifdef CONFIG_PARAVIRT_SPINLOCKS
>> +#if defined(CONFIG_PARAVIRT_SPINLOCKS) || defined(CONFIG_NUMA_AWARE_SPINLOCKS)
>> 	long reserved[2];
>> #endif
>> };
>> @@ -103,6 +104,8 @@ struct qnode {
>>  * Exactly fits one 64-byte cacheline on a 64-bit architecture.
>>  *
>>  * PV doubles the storage and uses the second cacheline for PV state.
>> + * CNA also doubles the storage and uses the second cacheline for
>> + * CNA-specific state.
>>  */
>> static DEFINE_PER_CPU_ALIGNED(struct qnode, qnodes[MAX_NODES]);
>> 
>> @@ -316,7 +319,7 @@ static __always_inline void __mcs_pass_lock(struct mcs_spinlock *node,
>> #define try_clear_tail	__try_clear_tail
>> #define mcs_pass_lock		__mcs_pass_lock
>> 
>> -#endif /* _GEN_PV_LOCK_SLOWPATH */
>> +#endif /* _GEN_PV_LOCK_SLOWPATH && _GEN_CNA_LOCK_SLOWPATH */
>> 
>> /**
>>  * queued_spin_lock_slowpath - acquire the queued spinlock
>> @@ -588,6 +591,29 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>> }
>> EXPORT_SYMBOL(queued_spin_lock_slowpath);
>> 
>> +/*
>> + * Generate the code for NUMA-aware spinlocks
>> + */
>> +#if !defined(_GEN_CNA_LOCK_SLOWPATH) && defined(CONFIG_NUMA_AWARE_SPINLOCKS)
>> +#define _GEN_CNA_LOCK_SLOWPATH
>> +
>> +#undef pv_wait_head_or_lock
>> +#define pv_wait_head_or_lock		cna_pre_scan
>> +
>> +#undef try_clear_tail
>> +#define try_clear_tail			cna_try_change_tail
>> +
>> +#undef mcs_pass_lock
>> +#define mcs_pass_lock			cna_pass_lock
>> +
>> +#undef  queued_spin_lock_slowpath
>> +#define queued_spin_lock_slowpath	__cna_queued_spin_lock_slowpath
>> +
>> +#include "qspinlock_cna.h"
>> +#include "qspinlock.c"
>> +
>> +#endif
>> +
>> /*
>>  * Generate the paravirt code for queued_spin_unlock_slowpath().
>>  */
>> diff --git a/kernel/locking/qspinlock_cna.h b/kernel/locking/qspinlock_cna.h
>> new file mode 100644
>> index 000000000000..a638336f9560
>> --- /dev/null
>> +++ b/kernel/locking/qspinlock_cna.h
>> @@ -0,0 +1,264 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +#ifndef _GEN_CNA_LOCK_SLOWPATH
>> +#error "do not include this file"
>> +#endif
>> +
>> +#include <linux/topology.h>
>> +
>> +/*
>> + * Implement a NUMA-aware version of MCS (aka CNA, or compact NUMA-aware lock).
>> + *
>> + * In CNA, spinning threads are organized in two queues, a main queue for
>> + * threads running on the same NUMA node as the current lock holder, and a
>> + * secondary queue for threads running on other nodes. Schematically, it
>> + * looks like this:
>> + *
>> + *    cna_node
>> + *   +----------+    +--------+        +--------+
>> + *   |mcs:next  | -> |mcs:next| -> ... |mcs:next| -> NULL      [Main queue]
>> + *   |mcs:locked| -+ +--------+        +--------+
>> + *   +----------+  |
>> + *                 +----------------------+
>> + *                                        \/
>> + *                 +--------+         +--------+
>> + *                 |mcs:next| -> ...  |mcs:next|          [Secondary queue]
>> + *                 +--------+         +--------+
>> + *                     ^                    |
>> + *                     +--------------------+
>> + *
>> + * N.B. locked = 1 if secondary queue is absent. Othewrise, it contains the
>> + * encoded pointer to the tail of the secondary queue, which is organized as a
>> + * circular list.
>> + *
>> + * After acquiring the MCS lock and before acquiring the spinlock, the lock
>> + * holder scans the main queue looking for a thread running on the same node
>> + * (pre-scan). If found (call it thread T), all threads in the main queue
>> + * between the current lock holder and T are moved to the end of the secondary
>> + * queue.  If such T is not found, we make another scan of the main queue when
>> + * unlocking the MCS lock (post-scan), starting at the node where pre-scan
>> + * stopped. If both scans fail to find such T, the MCS lock is passed to the
>> + * first thread in the secondary queue. If the secondary queue is empty, the
>> + * lock is passed to the next thread in the main queue.
>> + *
>> + * For more details, see https://urldefense.proofpoint.com/v2/url?u=https-3A__arxiv.org_abs_1810.05600&d=DwIFaQ&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=Hvhk3F4omdCk-GE1PTOm3Kn0A7ApWOZ2aZLTuVxFK4k&m=FcJA-hpAHs8MxL3rksMfq_GYsA7RIciHEF3AE4ZATOI&s=SR-p0LtEzUCrpwWY2ShRIF9lXod5Wc_NkN9zIOVvfxM&e= .
>> + *
>> + * Authors: Alex Kogan <alex.kogan@...cle.com>
>> + *          Dave Dice <dave.dice@...cle.com>
>> + */
>> +
>> +struct cna_node {
>> +	struct mcs_spinlock	mcs;
>> +	int			numa_node;
>> +	u32			encoded_tail;
>> +	u32			pre_scan_result; /* 0 or encoded tail */
>> +};
>> +
>> +static void __init cna_init_nodes_per_cpu(unsigned int cpu)
>> +{
>> +	struct mcs_spinlock *base = per_cpu_ptr(&qnodes[0].mcs, cpu);
>> +	int numa_node = cpu_to_node(cpu);
>> +	int i;
>> +
>> +	for (i = 0; i < MAX_NODES; i++) {
>> +		struct cna_node *cn = (struct cna_node *)grab_mcs_node(base, i);
>> +
>> +		cn->numa_node = numa_node;
>> +		cn->encoded_tail = encode_tail(cpu, i);
>> +		/*
>> +		 * @encoded_tail has to be larger than 1, so we do not confuse
>> +		 * it with other valid values for @locked or @pre_scan_result
>> +		 * (0 or 1)
>> +		 */
>> +		WARN_ON(cn->encoded_tail <= 1);
>> +	}
>> +}
>> +
>> +static int __init cna_init_nodes(void)
>> +{
>> +	unsigned int cpu;
>> +
>> +	/*
>> +	 * this will break on 32bit architectures, so we restrict
>> +	 * the use of CNA to 64bit only (see arch/x86/Kconfig)
>> +	 */
>> +	BUILD_BUG_ON(sizeof(struct cna_node) > sizeof(struct qnode));
>> +	/* we store an ecoded tail word in the node's @locked field */
>> +	BUILD_BUG_ON(sizeof(u32) > sizeof(unsigned int));
>> +
>> +	for_each_possible_cpu(cpu)
>> +		cna_init_nodes_per_cpu(cpu);
>> +
>> +	return 0;
>> +}
>> +early_initcall(cna_init_nodes);
>> +
> 
> Include a comment here saying that the cna_try_change_tail() function is
> only called when the primary queue is empty. That will make the code
> easier to read.
> 
> 
>> +static inline bool cna_try_change_tail(struct qspinlock *lock, u32 val,
>> +				       struct mcs_spinlock *node)
>> +{
>> +	struct mcs_spinlock *head_2nd, *tail_2nd;
>> +	u32 new;
>> +
>> +	/* If the secondary queue is empty, do what MCS does. */
>> +	if (node->locked <= 1)
>> +		return __try_clear_tail(lock, val, node);
>> +
>> +	/*
>> +	 * Try to update the tail value to the last node in the secondary queue.
>> +	 * If successful, pass the lock to the first thread in the secondary
>> +	 * queue. Doing those two actions effectively moves all nodes from the
>> +	 * secondary queue into the main one.
>> +	 */
>> +	tail_2nd = decode_tail(node->locked);
>> +	head_2nd = tail_2nd->next;
>> +	new = ((struct cna_node *)tail_2nd)->encoded_tail + _Q_LOCKED_VAL;
>> +
>> +	if (atomic_try_cmpxchg_relaxed(&lock->val, &val, new)) {
>> +		/*
>> +		 * Try to reset @next in tail_2nd to NULL, but no need to check
>> +		 * the result - if failed, a new successor has updated it.
>> +		 */
>> +		cmpxchg_relaxed(&tail_2nd->next, head_2nd, NULL);
>> +		arch_mcs_pass_lock(&head_2nd->locked, 1);
>> +		return true;
>> +	}
>> +
>> +	return false;
>> +}
> Cheers,
> Longman
> 

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