Re: [PATCH v3 1/3] block: track per-node I/O latency

[Hannes Reinecke <hare@suse.de>]

On 5/10/24 09:11, Damien Le Moal wrote:
> On 5/10/24 05:43, John Meneghini wrote:
>> From: Hannes Reinecke <hare@kernel.org>
>>
>> Add a new option 'BLK_NODE_LATENCY' to track per-node I/O latency.
>> This can be used by I/O schedulers to determine the 'best' queue
>> to send I/O to.
>>
>> Signed-off-by: Hannes Reinecke <hare@kernel.org>
>>
>> Cleaned up checkpatch warnings and updated MAINTAINERS.
> 
> This note should be before Hannes SoB. E.g:
> 
> [John] Fixed checkpatch warnings and updated MAINTAINERS.
> 
>>
>> Signed-off-by: John Meneghini <jmeneghi@redhat.com>
>> ---
>>   MAINTAINERS            |   1 +
>>   block/Kconfig          |   9 +
>>   block/Makefile         |   1 +
>>   block/blk-mq-debugfs.c |   2 +
>>   block/blk-nlatency.c   | 389 +++++++++++++++++++++++++++++++++++++++++
>>   block/blk-rq-qos.h     |   6 +
>>   include/linux/blk-mq.h |  11 ++
>>   7 files changed, 419 insertions(+)
>>   create mode 100644 block/blk-nlatency.c
>>
>> diff --git a/MAINTAINERS b/MAINTAINERS
>> index 7c121493f43d..a4634365c82f 100644
>> --- a/MAINTAINERS
>> +++ b/MAINTAINERS
>> @@ -5405,6 +5405,7 @@ F:	block/bfq-cgroup.c
>>   F:	block/blk-cgroup.c
>>   F:	block/blk-iocost.c
>>   F:	block/blk-iolatency.c
>> +F:	block/blk-nlatency.c
>>   F:	block/blk-throttle.c
>>   F:	include/linux/blk-cgroup.h
>>   
>> diff --git a/block/Kconfig b/block/Kconfig
>> index 1de4682d48cc..641ed39d609c 100644
>> --- a/block/Kconfig
>> +++ b/block/Kconfig
>> @@ -186,6 +186,15 @@ config BLK_CGROUP_IOPRIO
>>   	scheduler and block devices process requests. Only some I/O schedulers
>>   	and some block devices support I/O priorities.
>>   
>> +config BLK_NODE_LATENCY
>> +	bool "Track per-node I/O latency"
>> +	help
>> +	Enable per-node I/O latency tracking for multipathing. This uses the
>> +	blk-nodelat latency tracker to provide latencies for each node, and schedules
>> +	I/O on the path with the least latency for the submitting node. This can be
>> +	used by I/O schedulers to determine the node with the least latency. Currently
>> +	only supports nvme over fabrics devices.
>> +
>>   config BLK_DEBUG_FS
>>   	bool "Block layer debugging information in debugfs"
>>   	default y
>> diff --git a/block/Makefile b/block/Makefile
>> index 46ada9dc8bbf..9d2e71a3e36f 100644
>> --- a/block/Makefile
>> +++ b/block/Makefile
>> @@ -21,6 +21,7 @@ obj-$(CONFIG_BLK_DEV_THROTTLING)	+= blk-throttle.o
>>   obj-$(CONFIG_BLK_CGROUP_IOPRIO)	+= blk-ioprio.o
>>   obj-$(CONFIG_BLK_CGROUP_IOLATENCY)	+= blk-iolatency.o
>>   obj-$(CONFIG_BLK_CGROUP_IOCOST)	+= blk-iocost.o
>> +obj-$(CONFIG_BLK_NODE_LATENCY) += blk-nlatency.o
> 
> Let's keep the alignment please.
> 
>>   obj-$(CONFIG_MQ_IOSCHED_DEADLINE)	+= mq-deadline.o
>>   obj-$(CONFIG_MQ_IOSCHED_KYBER)	+= kyber-iosched.o
>>   bfq-y				:= bfq-iosched.o bfq-wf2q.o bfq-cgroup.o
>> diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
>> index 94668e72ab09..cb38228b95d8 100644
>> --- a/block/blk-mq-debugfs.c
>> +++ b/block/blk-mq-debugfs.c
>> @@ -762,6 +762,8 @@ static const char *rq_qos_id_to_name(enum rq_qos_id id)
>>   		return "latency";
>>   	case RQ_QOS_COST:
>>   		return "cost";
>> +	case RQ_QOS_NLAT:
>> +		return "node-latency";
>>   	}
>>   	return "unknown";
>>   }
>> diff --git a/block/blk-nlatency.c b/block/blk-nlatency.c
>> new file mode 100644
>> index 000000000000..219c3f636d76
>> --- /dev/null
>> +++ b/block/blk-nlatency.c
>> @@ -0,0 +1,389 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +/*
>> + * Per-node request latency tracking.
>> + *
>> + * Copyright (C) 2023 Hannes Reinecke
>> + *
>> + * A simple per-node latency tracker for use by I/O scheduler.
>> + * Latencies are measures over 'win_usec' microseconds and stored per node.
>> + * If the number of measurements falls below 'lowat' the measurement is
>> + * assumed to be unreliable and will become 'stale'.
>> + * These 'stale' latencies can be 'decayed', where during each measurement
>> + * interval the 'stale' latency value is decreased by 'decay' percent.
>> + * Once the 'stale' latency reaches zero it will be updated by the
>> + * measured latency.
>> + */
>> +#include <linux/kernel.h>
>> +#include <linux/blk_types.h>
>> +#include <linux/slab.h>
>> +
>> +#include "blk-stat.h"
>> +#include "blk-rq-qos.h"
>> +#include "blk.h"
>> +
>> +#define NLAT_DEFAULT_LOWAT 2
>> +#define NLAT_DEFAULT_DECAY 50
>> +
>> +struct rq_nlat {
>> +	struct rq_qos rqos;
>> +
>> +	u64 win_usec;		/* latency measurement window in microseconds */
> 
> Using microseconds forces you to do costly multiplications and divisions by
> 1000. Why not keep things in nanoseconds ?
> 
I wanted to keep the user interface simple; entering nanoseconds values
is tedious. But sure, I can change it.

>> +	unsigned int lowat;	/* Low Watermark latency measurement */
>> +	unsigned int decay;	/* Percentage for 'decaying' latencies */
>> +	bool enabled;
>> +
>> +	struct blk_stat_callback *cb;
>> +
>> +	unsigned int num;
>> +	u64 *latency;
>> +	unsigned int *samples;
>> +};
>> +
>> +static inline struct rq_nlat *RQNLAT(struct rq_qos *rqos)
>> +{
>> +	return container_of(rqos, struct rq_nlat, rqos);
>> +}
>> +
>> +static u64 nlat_default_latency_usec(struct request_queue *q)
>> +{
>> +	/*
>> +	 * We default to 2msec for non-rotational storage, and 75msec
>> +	 * for rotational storage.
>> +	 */
>> +	if (blk_queue_nonrot(q))
>> +		return 2000ULL;
>> +	else
> 
> No need for this else.
> 
OK.

>> +		return 75000ULL;
>> +}
>> +
>> +static void nlat_timer_fn(struct blk_stat_callback *cb)
>> +{
>> +	struct rq_nlat *nlat = cb->data;
>> +	int n;
>> +
>> +	for (n = 0; n < cb->buckets; n++) {
>> +		if (cb->stat[n].nr_samples < nlat->lowat) {
>> +			/*
>> +			 * 'decay' the latency by the specified
>> +			 * percentage to ensure the queues are
>> +			 * being tested to balance out temporary
>> +			 * latency spikes.
>> +			 */
>> +			nlat->latency[n] =
>> +				div64_u64(nlat->latency[n] * nlat->decay, 100);
>> +		} else
>> +			nlat->latency[n] = cb->stat[n].mean;
> 
> Missing the curly brackets around the else block.
> Nit: n is a rather unusual name for a loop index. Why not the usual "i" ? Does
> notmatter much though.
> 
There was a reason once ... but yeah, let's move to 'i'.

>> +		nlat->samples[n] = cb->stat[n].nr_samples;
>> +	}
>> +	if (nlat->enabled)
>> +		blk_stat_activate_nsecs(nlat->cb, nlat->win_usec * 1000);
>> +}
>> +
>> +static int nlat_bucket_node(const struct request *rq)
>> +{
>> +	if (!rq->mq_ctx)
>> +		return -1;
>> +	return cpu_to_node(blk_mq_rq_cpu((struct request *)rq));
>> +}
>> +
>> +static void nlat_exit(struct rq_qos *rqos)
>> +{
>> +	struct rq_nlat *nlat = RQNLAT(rqos);
>> +
>> +	blk_stat_remove_callback(nlat->rqos.disk->queue, nlat->cb);
>> +	blk_stat_free_callback(nlat->cb);
>> +	kfree(nlat->samples);
>> +	kfree(nlat->latency);
>> +	kfree(nlat);
>> +}
>> +
>> +#ifdef CONFIG_BLK_DEBUG_FS
>> +static int nlat_win_usec_show(void *data, struct seq_file *m)
>> +{
>> +	struct rq_qos *rqos = data;
>> +	struct rq_nlat *nlat = RQNLAT(rqos);
>> +
>> +	seq_printf(m, "%llu\n", nlat->win_usec);
>> +	return 0;
>> +}
>> +
>> +static ssize_t nlat_win_usec_write(void *data, const char __user *buf,
>> +			size_t count, loff_t *ppos)
>> +{
>> +	struct rq_qos *rqos = data;
>> +	struct rq_nlat *nlat = RQNLAT(rqos);
>> +	char val[16] = { };
>> +	u64 usec;
>> +	int err;
>> +
>> +	if (blk_queue_dying(nlat->rqos.disk->queue))
>> +		return -ENOENT;
>> +
>> +	if (count >= sizeof(val))
>> +		return -EINVAL;
>> +
>> +	if (copy_from_user(val, buf, count))
>> +		return -EFAULT;
>> +
>> +	err = kstrtoull(val, 10, &usec);
>> +	if (err)
>> +		return err;
>> +	blk_stat_deactivate(nlat->cb);
>> +	nlat->win_usec = usec;
>> +	blk_stat_activate_nsecs(nlat->cb, nlat->win_usec * 1000);
>> +
>> +	return count;
>> +}
>> +
>> +static int nlat_lowat_show(void *data, struct seq_file *m)
>> +{
>> +	struct rq_qos *rqos = data;
>> +	struct rq_nlat *nlat = RQNLAT(rqos);
>> +
>> +	seq_printf(m, "%u\n", nlat->lowat);
>> +	return 0;
>> +}
>> +
>> +static ssize_t nlat_lowat_write(void *data, const char __user *buf,
>> +			size_t count, loff_t *ppos)
>> +{
>> +	struct rq_qos *rqos = data;
>> +	struct rq_nlat *nlat = RQNLAT(rqos);
>> +	char val[16] = { };
>> +	unsigned int lowat;
>> +	int err;
>> +
>> +	if (blk_queue_dying(nlat->rqos.disk->queue))
>> +		return -ENOENT;
>> +
>> +	if (count >= sizeof(val))
>> +		return -EINVAL;
>> +
>> +	if (copy_from_user(val, buf, count))
>> +		return -EFAULT;
>> +
>> +	err = kstrtouint(val, 10, &lowat);
>> +	if (err)
>> +		return err;
>> +	blk_stat_deactivate(nlat->cb);
>> +	nlat->lowat = lowat;
>> +	blk_stat_activate_nsecs(nlat->cb, nlat->win_usec * 1000);
>> +
>> +	return count;
>> +}
>> +
>> +static int nlat_decay_show(void *data, struct seq_file *m)
>> +{
>> +	struct rq_qos *rqos = data;
>> +	struct rq_nlat *nlat = RQNLAT(rqos);
>> +
>> +	seq_printf(m, "%u\n", nlat->decay);
>> +	return 0;
>> +}
>> +
>> +static ssize_t nlat_decay_write(void *data, const char __user *buf,
>> +			size_t count, loff_t *ppos)
>> +{
>> +	struct rq_qos *rqos = data;
>> +	struct rq_nlat *nlat = RQNLAT(rqos);
>> +	char val[16] = { };
>> +	unsigned int decay;
>> +	int err;
>> +
>> +	if (blk_queue_dying(nlat->rqos.disk->queue))
>> +		return -ENOENT;
>> +
>> +	if (count >= sizeof(val))
>> +		return -EINVAL;
>> +
>> +	if (copy_from_user(val, buf, count))
>> +		return -EFAULT;
>> +
>> +	err = kstrtouint(val, 10, &decay);
>> +	if (err)
>> +		return err;
>> +	if (decay > 100)
>> +		return -EINVAL;
>> +	blk_stat_deactivate(nlat->cb);
>> +	nlat->decay = decay;
>> +	blk_stat_activate_nsecs(nlat->cb, nlat->win_usec * 1000);
>> +
>> +	return count;
>> +}
>> +
>> +static int nlat_enabled_show(void *data, struct seq_file *m)
>> +{
>> +	struct rq_qos *rqos = data;
>> +	struct rq_nlat *nlat = RQNLAT(rqos);
>> +
>> +	seq_printf(m, "%d\n", nlat->enabled);
>> +	return 0;
>> +}
>> +
>> +static int nlat_id_show(void *data, struct seq_file *m)
>> +{
>> +	struct rq_qos *rqos = data;
>> +
>> +	seq_printf(m, "%u\n", rqos->id);
>> +	return 0;
>> +}
>> +
>> +static int nlat_latency_show(void *data, struct seq_file *m)
>> +{
>> +	struct rq_qos *rqos = data;
>> +	struct rq_nlat *nlat = RQNLAT(rqos);
>> +	int n;
>> +
>> +	if (!nlat->enabled)
>> +		return 0;
>> +
>> +	for (n = 0; n < nlat->num; n++) {
>> +		if (n > 0)
>> +			seq_puts(m, " ");
>> +		seq_printf(m, "%llu", nlat->latency[n]);
>> +	}
>> +	seq_puts(m, "\n");
>> +	return 0;
>> +}
>> +
>> +static int nlat_samples_show(void *data, struct seq_file *m)
>> +{
>> +	struct rq_qos *rqos = data;
>> +	struct rq_nlat *nlat = RQNLAT(rqos);
>> +	int n;
>> +
>> +	if (!nlat->enabled)
>> +		return 0;
>> +
>> +	for (n = 0; n < nlat->num; n++) {
>> +		if (n > 0)
>> +			seq_puts(m, " ");
>> +		seq_printf(m, "%u", nlat->samples[n]);
>> +	}
>> +	seq_puts(m, "\n");
>> +	return 0;
>> +}
>> +
>> +static const struct blk_mq_debugfs_attr nlat_debugfs_attrs[] = {
>> +	{"win_usec", 0600, nlat_win_usec_show, nlat_win_usec_write},
>> +	{"lowat", 0600, nlat_lowat_show, nlat_lowat_write},
>> +	{"decay", 0600, nlat_decay_show, nlat_decay_write},
>> +	{"enabled", 0400, nlat_enabled_show},
>> +	{"id", 0400, nlat_id_show},
>> +	{"latency", 0400, nlat_latency_show},
>> +	{"samples", 0400, nlat_samples_show},
>> +	{},
>> +};
>> +#endif
>> +
>> +static const struct rq_qos_ops nlat_rqos_ops = {
>> +	.exit = nlat_exit,
>> +#ifdef CONFIG_BLK_DEBUG_FS
>> +	.debugfs_attrs = nlat_debugfs_attrs,
>> +#endif
>> +};
>> +
>> +u64 blk_nlat_latency(struct gendisk *disk, int node)
>> +{
>> +	struct rq_qos *rqos;
>> +	struct rq_nlat *nlat;
>> +
>> +	rqos = nlat_rq_qos(disk->queue);
>> +	if (!rqos)
>> +		return 0;
>> +	nlat = RQNLAT(rqos);
>> +	if (node > nlat->num)
>> +		return 0;
>> +
>> +	return div64_u64(nlat->latency[node], 1000);
> 
> See comment at the top. Why not keep everything in nanoseconds to avoid this
> costly division ?
> 
Agreed.

>> +}
>> +EXPORT_SYMBOL_GPL(blk_nlat_latency);
>> +
>> +int blk_nlat_enable(struct gendisk *disk)
>> +{
>> +	struct rq_qos *rqos;
>> +	struct rq_nlat *nlat;
>> +
>> +	/* Latency tracking not enabled? */
>> +	rqos = nlat_rq_qos(disk->queue);
>> +	if (!rqos)
>> +		return -EINVAL;
>> +	nlat = RQNLAT(rqos);
>> +	if (nlat->enabled)
>> +		return 0;
>> +
>> +	/* Queue not registered? Maybe shutting down... */
>> +	if (!blk_queue_registered(disk->queue))
>> +		return -EAGAIN;
>> +
>> +	nlat->enabled = true;
>> +	memset(nlat->latency, 0, sizeof(u64) * nlat->num);
>> +	memset(nlat->samples, 0, sizeof(unsigned int) * nlat->num);
>> +	blk_stat_activate_nsecs(nlat->cb, nlat->win_usec * 1000);
>> +
>> +	return 0;
>> +}
>> +EXPORT_SYMBOL_GPL(blk_nlat_enable);
>> +
>> +void blk_nlat_disable(struct gendisk *disk)
>> +{
>> +	struct rq_qos *rqos = nlat_rq_qos(disk->queue);
>> +	struct rq_nlat *nlat;
>> +
>> +	if (!rqos)
>> +		return;
>> +	nlat = RQNLAT(rqos);
>> +	if (nlat->enabled) {
>> +		blk_stat_deactivate(nlat->cb);
>> +		nlat->enabled = false;
>> +	}
>> +}
>> +EXPORT_SYMBOL_GPL(blk_nlat_disable);
>> +
>> +int blk_nlat_init(struct gendisk *disk)
>> +{
>> +	struct rq_nlat *nlat;
>> +	int ret = -ENOMEM;
>> +
>> +	nlat = kzalloc(sizeof(*nlat), GFP_KERNEL);
>> +	if (!nlat)
>> +		return -ENOMEM;
>> +
>> +	nlat->num = num_possible_nodes();
>> +	nlat->lowat = NLAT_DEFAULT_LOWAT;
>> +	nlat->decay = NLAT_DEFAULT_DECAY;
>> +	nlat->win_usec = nlat_default_latency_usec(disk->queue);
>> +
>> +	nlat->latency = kcalloc(nlat->num, sizeof(u64), GFP_KERNEL);
>> +	if (!nlat->latency)
>> +		goto err_free;
>> +	nlat->samples = kcalloc(nlat->num, sizeof(unsigned int), GFP_KERNEL);
>> +	if (!nlat->samples)
>> +		goto err_free;
>> +	nlat->cb = blk_stat_alloc_callback(nlat_timer_fn, nlat_bucket_node,
>> +					   nlat->num, nlat);
>> +	if (!nlat->cb)
>> +		goto err_free;
>> +
>> +	/*
>> +	 * Assign rwb and add the stats callback.
>> +	 */
> 
> This can be a single line comment.
> 
Ok.

>> +	mutex_lock(&disk->queue->rq_qos_mutex);
>> +	ret = rq_qos_add(&nlat->rqos, disk, RQ_QOS_NLAT, &nlat_rqos_ops);
>> +	mutex_unlock(&disk->queue->rq_qos_mutex);
>> +	if (ret)
>> +		goto err_free_cb;
>> +
>> +	blk_stat_add_callback(disk->queue, nlat->cb);
>> +
>> +	return 0;
>> +
>> +err_free_cb:
>> +	blk_stat_free_callback(nlat->cb);
>> +err_free:
>> +	kfree(nlat->samples);
>> +	kfree(nlat->latency);
>> +	kfree(nlat);
>> +	return ret;
>> +}
>> +EXPORT_SYMBOL_GPL(blk_nlat_init);
>> diff --git a/block/blk-rq-qos.h b/block/blk-rq-qos.h
>> index 37245c97ee61..2fc11ced0c00 100644
>> --- a/block/blk-rq-qos.h
>> +++ b/block/blk-rq-qos.h
>> @@ -17,6 +17,7 @@ enum rq_qos_id {
>>   	RQ_QOS_WBT,
>>   	RQ_QOS_LATENCY,
>>   	RQ_QOS_COST,
>> +	RQ_QOS_NLAT,
>>   };
>>   
>>   struct rq_wait {
>> @@ -79,6 +80,11 @@ static inline struct rq_qos *iolat_rq_qos(struct request_queue *q)
>>   	return rq_qos_id(q, RQ_QOS_LATENCY);
>>   }
>>   
>> +static inline struct rq_qos *nlat_rq_qos(struct request_queue *q)
>> +{
>> +	return rq_qos_id(q, RQ_QOS_NLAT);
>> +}
>> +
>>   static inline void rq_wait_init(struct rq_wait *rq_wait)
>>   {
>>   	atomic_set(&rq_wait->inflight, 0);
>> diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
>> index d3d8fd8e229b..1f3829627f1b 100644
>> --- a/include/linux/blk-mq.h
>> +++ b/include/linux/blk-mq.h
>> @@ -1231,4 +1231,15 @@ static inline bool blk_req_can_dispatch_to_zone(struct request *rq)
>>   }
>>   #endif /* CONFIG_BLK_DEV_ZONED */
>>   
>> +#ifdef CONFIG_BLK_NODE_LATENCY
>> +int blk_nlat_enable(struct gendisk *disk);
>> +void blk_nlat_disable(struct gendisk *disk);
>> +u64 blk_nlat_latency(struct gendisk *disk, int node);
>> +int blk_nlat_init(struct gendisk *disk);
>> +#else
>> +static inline int blk_nlat_enable(struct gendisk *disk) { return 0; }
>> +static inline void blk_nlat_disable(struct gendisk *disk) {}
>> +static inline u64 blk_nlat_latency(struct gendisk *disk, int node) { return 0; }
>> +static inline int blk_nlat_init(struct gendisk *disk) { return -EOPNOTSUPP; }
>> +#endif
>>   #endif /* BLK_MQ_H */
> 

Thanks for the review!

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@suse.de                              +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), GF: Ivo Totev, Andrew McDonald,
Werner Knoblich