Re: [PATCH V2 2/4] RDMA/core: Introduce shared CQ pool API

[Yamin Friedman <yaminf@mellanox.com>]

On 5/18/2020 11:30 AM, Leon Romanovsky wrote:
> On Wed, May 13, 2020 at 02:52:41PM +0300, Yamin Friedman wrote:
>> Allow a ULP to ask the core to provide a completion queue based on a
>> least-used search on a per-device CQ pools. The device CQ pools grow in a
>> lazy fashion when more CQs are requested.
>>
>> This feature reduces the amount of interrupts when using many QPs.
>> Using shared CQs allows for more effcient completion handling. It also
>> reduces the amount of overhead needed for CQ contexts.
>>
>> Test setup:
>> Intel(R) Xeon(R) Platinum 8176M CPU @ 2.10GHz servers.
>> Running NVMeoF 4KB read IOs over ConnectX-5EX across Spectrum switch.
>> TX-depth = 32. The patch was applied in the nvme driver on both the target
>> and initiator. Four controllers are accessed from each core. In the
>> current test case we have exposed sixteen NVMe namespaces using four
>> different subsystems (four namespaces per subsystem) from one NVM port.
>> Each controller allocated X queues (RDMA QPs) and attached to Y CQs.
>> Before this series we had X == Y, i.e for four controllers we've created
>> total of 4X QPs and 4X CQs. In the shared case, we've created 4X QPs and
>> only X CQs which means that we have four controllers that share a
>> completion queue per core. Until fourteen cores there is no significant
>> change in performance and the number of interrupts per second is less than
>> a million in the current case.
>> ==================================================
>> |Cores|Current KIOPs  |Shared KIOPs  |improvement|
>> |-----|---------------|--------------|-----------|
>> |14   |2332           |2723          |16.7%      |
>> |-----|---------------|--------------|-----------|
>> |20   |2086           |2712          |30%        |
>> |-----|---------------|--------------|-----------|
>> |28   |1971           |2669          |35.4%      |
>> |=================================================
>> |Cores|Current avg lat|Shared avg lat|improvement|
>> |-----|---------------|--------------|-----------|
>> |14   |767us          |657us         |14.3%      |
>> |-----|---------------|--------------|-----------|
>> |20   |1225us         |943us         |23%        |
>> |-----|---------------|--------------|-----------|
>> |28   |1816us         |1341us        |26.1%      |
>> ========================================================
>> |Cores|Current interrupts|Shared interrupts|improvement|
>> |-----|------------------|-----------------|-----------|
>> |14   |1.6M/sec          |0.4M/sec         |72%        |
>> |-----|------------------|-----------------|-----------|
>> |20   |2.8M/sec          |0.6M/sec         |72.4%      |
>> |-----|------------------|-----------------|-----------|
>> |28   |2.9M/sec          |0.8M/sec         |63.4%      |
>> ====================================================================
>> |Cores|Current 99.99th PCTL lat|Shared 99.99th PCTL lat|improvement|
>> |-----|------------------------|-----------------------|-----------|
>> |14   |67ms                    |6ms                    |90.9%      |
>> |-----|------------------------|-----------------------|-----------|
>> |20   |5ms                     |6ms                    |-10%       |
>> |-----|------------------------|-----------------------|-----------|
>> |28   |8.7ms                   |6ms                    |25.9%      |
>> |===================================================================
>>
>> Performance improvement with sixteen disks (sixteen CQs per core) is
>> comparable.
>>
>> Signed-off-by: Yamin Friedman <yaminf@mellanox.com>
>> Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
>> Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
>> ---
>>   drivers/infiniband/core/core_priv.h |   4 ++
>>   drivers/infiniband/core/cq.c        | 137 ++++++++++++++++++++++++++++++++++++
>>   drivers/infiniband/core/device.c    |   2 +
>>   include/rdma/ib_verbs.h             |  35 +++++++++
>>   4 files changed, 178 insertions(+)
>>
>> diff --git a/drivers/infiniband/core/core_priv.h b/drivers/infiniband/core/core_priv.h
>> index cf42acc..fa3151b 100644
>> --- a/drivers/infiniband/core/core_priv.h
>> +++ b/drivers/infiniband/core/core_priv.h
>> @@ -414,4 +414,8 @@ void rdma_umap_priv_init(struct rdma_umap_priv *priv,
>>   			 struct vm_area_struct *vma,
>>   			 struct rdma_user_mmap_entry *entry);
>>
>> +void ib_cq_pool_init(struct ib_device *dev);
>> +
>> +void ib_cq_pool_destroy(struct ib_device *dev);
>> +
>>   #endif /* _CORE_PRIV_H */
>> diff --git a/drivers/infiniband/core/cq.c b/drivers/infiniband/core/cq.c
>> index 04046eb..5319c14 100644
>> --- a/drivers/infiniband/core/cq.c
>> +++ b/drivers/infiniband/core/cq.c
>> @@ -7,7 +7,11 @@
>>   #include <linux/slab.h>
>>   #include <rdma/ib_verbs.h>
>>
>> +#include "core_priv.h"
>> +
>>   #include <trace/events/rdma_core.h>
>> +/* Max size for shared CQ, may require tuning */
>> +#define IB_MAX_SHARED_CQ_SZ		4096
>>
>>   /* # of WCs to poll for with a single call to ib_poll_cq */
>>   #define IB_POLL_BATCH			16
>> @@ -218,6 +222,7 @@ struct ib_cq *__ib_alloc_cq_user(struct ib_device *dev, void *private,
>>   	cq->cq_context = private;
>>   	cq->poll_ctx = poll_ctx;
>>   	atomic_set(&cq->usecnt, 0);
>> +	cq->comp_vector = comp_vector;
>>
>>   	cq->wc = kmalloc_array(IB_POLL_BATCH, sizeof(*cq->wc), GFP_KERNEL);
>>   	if (!cq->wc)
>> @@ -304,6 +309,8 @@ static void _ib_free_cq_user(struct ib_cq *cq, struct ib_udata *udata)
>>   {
>>   	if (WARN_ON_ONCE(atomic_read(&cq->usecnt)))
>>   		return;
>> +	if (WARN_ON_ONCE(cq->cqe_used != 0))
> Let's do WARN_ON_ONCE(cq->cqe_used)
>
>> +		return;
>>
>>   	switch (cq->poll_ctx) {
>>   	case IB_POLL_DIRECT:
>> @@ -340,3 +347,133 @@ void ib_free_cq_user(struct ib_cq *cq, struct ib_udata *udata)
>>   		_ib_free_cq_user(cq, udata);
>>   }
>>   EXPORT_SYMBOL(ib_free_cq_user);
>> +
>> +void ib_cq_pool_init(struct ib_device *dev)
>> +{
>> +	int i;
>> +
>> +	spin_lock_init(&dev->cq_pools_lock);
>> +	for (i = 0; i < ARRAY_SIZE(dev->cq_pools); i++)
>> +		INIT_LIST_HEAD(&dev->cq_pools[i]);
>> +}
>> +
>> +void ib_cq_pool_destroy(struct ib_device *dev)
>> +{
>> +	struct ib_cq *cq, *n;
>> +	int i;
>> +
>> +	for (i = 0; i < ARRAY_SIZE(dev->cq_pools); i++) {
>> +		list_for_each_entry_safe(cq, n, &dev->cq_pools[i], pool_entry)
>> +			_ib_free_cq_user(cq, NULL);
>> +	}
>> +
>> +}
>> +
>> +static int ib_alloc_cqs(struct ib_device *dev, int nr_cqes,
>> +			enum ib_poll_context poll_ctx)
>> +{
>> +	LIST_HEAD(tmp_list);
>> +	struct ib_cq *cq;
>> +	unsigned long flags;
>> +	int nr_cqs, ret, i;
>> +
>> +	/*
>> +	 * Allocated at least as many CQEs as requested, and otherwise
>> +	 * a reasonable batch size so that we can share CQs between
>> +	 * multiple users instead of allocating a larger number of CQs.
>> +	 */
>> +	nr_cqes = min(dev->attrs.max_cqe, max(nr_cqes, IB_MAX_SHARED_CQ_SZ));
>> +	nr_cqs = min_t(int, dev->num_comp_vectors, num_online_cpus());
>> +	for (i = 0; i < nr_cqs; i++) {
>> +		cq = ib_alloc_cq(dev, NULL, nr_cqes, i, poll_ctx);
>> +		if (IS_ERR(cq)) {
>> +			ret = PTR_ERR(cq);
>> +			goto out_free_cqs;
>> +		}
>> +		cq->shared = true;
>> +		list_add_tail(&cq->pool_entry, &tmp_list);
>> +	}
>> +
>> +	spin_lock_irqsave(&dev->cq_pools_lock, flags);
>> +	list_splice(&tmp_list, &dev->cq_pools[poll_ctx - 1]);
>> +	spin_unlock_irqrestore(&dev->cq_pools_lock, flags);
>> +
>> +	return 0;
>> +
>> +out_free_cqs:
>> +	list_for_each_entry(cq, &tmp_list, pool_entry)
>> +		ib_free_cq(cq);
>> +	return ret;
>> +}
>> +
>> +struct ib_cq *ib_cq_pool_get(struct ib_device *dev, unsigned int nr_cqe,
>> +			     int comp_vector_hint,
>> +			     enum ib_poll_context poll_ctx)
>> +{
>> +	static unsigned int default_comp_vector;
>> +	int vector, ret, num_comp_vectors;
>> +	struct ib_cq *cq, *found = NULL;
>> +	unsigned long flags;
>> +
>> +	if (poll_ctx > ARRAY_SIZE(dev->cq_pools) || poll_ctx == IB_POLL_DIRECT)
>> +		return ERR_PTR(-EINVAL);
>> +
>> +	num_comp_vectors = min_t(int, dev->num_comp_vectors,
>> +				 num_online_cpus());
>> +	/* Project the affinty to the device completion vector range */
>> +	if (comp_vector_hint < 0)
>> +		vector = default_comp_vector++ % num_comp_vectors;
>> +	else
>> +		vector = comp_vector_hint % num_comp_vectors;
>> +
>> +	/*
>> +	 * Find the least used CQ with correct affinity and
>> +	 * enough free CQ entries
>> +	 */
>> +	while (!found) {
>> +		spin_lock_irqsave(&dev->cq_pools_lock, flags);
>> +		list_for_each_entry(cq, &dev->cq_pools[poll_ctx - 1],
>> +				    pool_entry) {
>> +			if (vector != cq->comp_vector)
> I think that this check worth to have a comment.
> At least for me, it is not clear if it will work correctly if
> comp_vector == 0.
>
>> +				continue;
>> +			if (cq->cqe_used + nr_cqe > cq->cqe)
>> +				continue;
>> +			if (found && cq->cqe_used >= found->cqe_used)
>> +				continue;
>> +			found = cq;
>> +			break;
>> +		}
>> +
>> +		if (found) {
>> +			found->cqe_used += nr_cqe;
>> +			spin_unlock_irqrestore(&dev->cq_pools_lock, flags);
>> +
>> +			return found;
>> +		}
>> +		spin_unlock_irqrestore(&dev->cq_pools_lock, flags);
>> +
>> +		/*
>> +		 * Didn't find a match or ran out of CQs in the device
>> +		 * pool, allocate a new array of CQs.
>> +		 */
>> +		ret = ib_alloc_cqs(dev, nr_cqe, poll_ctx);
>> +		if (ret)
>> +			return ERR_PTR(ret);
>> +	}
>> +
>> +	return found;
>> +}
>> +EXPORT_SYMBOL(ib_cq_pool_get);
>> +
>> +void ib_cq_pool_put(struct ib_cq *cq, unsigned int nr_cqe)
>> +{
>> +	unsigned long flags;
>> +
>> +	if (nr_cqe > cq->cqe_used)
>> +		return;
> Is it possible?
> 1. It is racy
> 2. It is a bug in the ib_cq_pool_put() caller.

It is possible, the pool doesn't save the amount of cqes used per user.

I think to make it really secure I would have to never reduce the cqes 
used, save the number of active users, and have some form of garbage 
collection for used up CQs but that seems to me a lot for something that 
should not occur during proper use.

Would it be better to just have a WARN for this case?

>
>> +
>> +	spin_lock_irqsave(&cq->device->cq_pools_lock, flags);
>> +	cq->cqe_used -= nr_cqe;
>> +	spin_unlock_irqrestore(&cq->device->cq_pools_lock, flags);
>> +}
>> +EXPORT_SYMBOL(ib_cq_pool_put);
>> diff --git a/drivers/infiniband/core/device.c b/drivers/infiniband/core/device.c
>> index d9f565a..abd7cd0 100644
>> --- a/drivers/infiniband/core/device.c
>> +++ b/drivers/infiniband/core/device.c
>> @@ -600,6 +600,7 @@ struct ib_device *_ib_alloc_device(size_t size)
>>   	mutex_init(&device->compat_devs_mutex);
>>   	init_completion(&device->unreg_completion);
>>   	INIT_WORK(&device->unregistration_work, ib_unregister_work);
>> +	ib_cq_pool_init(device);
> Why did you add this function in _ib_alloc_device() and not
> to the ib_register_device()?
I will move it to the register device.
>
>>   	return device;
>>   }
>> @@ -1455,6 +1456,7 @@ static void __ib_unregister_device(struct ib_device *ib_dev)
>>   	device_del(&ib_dev->dev);
>>   	ib_device_unregister_rdmacg(ib_dev);
>>   	ib_cache_cleanup_one(ib_dev);
>> +	ib_cq_pool_destroy(ib_dev);
> It is not symmetric to the registration flow.
>
>>   	/*
>>   	 * Drivers using the new flow may not call ib_dealloc_device except
>> diff --git a/include/rdma/ib_verbs.h b/include/rdma/ib_verbs.h
>> index b79737b..0ca6617 100644
>> --- a/include/rdma/ib_verbs.h
>> +++ b/include/rdma/ib_verbs.h
>> @@ -1555,6 +1555,7 @@ enum ib_poll_context {
>>   	IB_POLL_SOFTIRQ,	   /* poll from softirq context */
>>   	IB_POLL_WORKQUEUE,	   /* poll from workqueue */
>>   	IB_POLL_UNBOUND_WORKQUEUE, /* poll from unbound workqueue */
>> +	IB_POLL_LAST,
>>   };
>>
>>   struct ib_cq {
>> @@ -1564,9 +1565,12 @@ struct ib_cq {
>>   	void                  (*event_handler)(struct ib_event *, void *);
>>   	void                   *cq_context;
>>   	int               	cqe;
>> +	int			cqe_used;
>>   	atomic_t          	usecnt; /* count number of work queues */
>>   	enum ib_poll_context	poll_ctx;
>> +	int                     comp_vector;
>>   	struct ib_wc		*wc;
>> +	struct list_head        pool_entry;
>>   	union {
>>   		struct irq_poll		iop;
>>   		struct work_struct	work;
>> @@ -2695,6 +2699,10 @@ struct ib_device {
>>   #endif
>>
>>   	u32                          index;
>> +
>> +	spinlock_t                   cq_pools_lock;
>> +	struct list_head             cq_pools[IB_POLL_LAST - 1];
>> +
>>   	struct rdma_restrack_root *res;
>>
>>   	const struct uapi_definition   *driver_def;
>> @@ -3957,6 +3965,33 @@ static inline int ib_req_notify_cq(struct ib_cq *cq,
>>   	return cq->device->ops.req_notify_cq(cq, flags);
>>   }
>>
>> +/*
>> + * ib_cq_pool_get() - Find the least used completion queue that matches
>> + *     a given cpu hint (or least used for wild card affinity)
>> + *     and fits nr_cqe
>> + * @dev: rdma device
>> + * @nr_cqe: number of needed cqe entries
>> + * @comp_vector_hint: completion vector hint (-1) for the driver to assign
>> + *   a comp vector based on internal counter
>> + * @poll_ctx: cq polling context
>> + *
>> + * Finds a cq that satisfies @comp_vector_hint and @nr_cqe requirements and
>> + * claim entries in it for us. In case there is no available cq, allocate
>> + * a new cq with the requirements and add it to the device pool.
>> + * IB_POLL_DIRECT cannot be used for shared cqs so it is not a valid value
>> + * for @poll_ctx.
>> + */
>> +struct ib_cq *ib_cq_pool_get(struct ib_device *dev, unsigned int nr_cqe,
>> +			     int comp_vector_hint,
>> +			     enum ib_poll_context poll_ctx);
>> +
>> +/**
>> + * ib_cq_pool_put - Return a CQ taken from a shared pool.
>> + * @cq: The CQ to return.
>> + * @nr_cqe: The max number of cqes that the user had requested.
>> + */
>> +void ib_cq_pool_put(struct ib_cq *cq, unsigned int nr_cqe);
>> +
>>   /**
>>    * ib_req_ncomp_notif - Request completion notification when there are
>>    *   at least the specified number of unreaped completions on the CQ.
>> --
>> 1.8.3.1
>>