Re: [LSF/MM TOPIC][LSF/MM ATTEND] NAPI polling for block drivers

Sagi Grimberg
2017-01-17T15:47:52Z

[-- Attachment #1: Type: text/plain, Size: 5615 bytes --]

Hey, so I made some initial analysis of whats going on with
irq-poll.

First, I sampled how much time it takes before we
get the interrupt in nvme_irq and the initial visit
to nvme_irqpoll_handler. I ran a single threaded fio
with QD=32 of 4K reads. This is two displays of a
histogram of the latency (ns):
--
[1]
queue = b'nvme0q1'
usecs : count distribution
0 -> 1 : 7310
|****************************************|
2 -> 3 : 11 |
|
4 -> 7 : 10 |
|
8 -> 15 : 20 |
|
16 -> 31 : 0 |
|
32 -> 63 : 0 |
|
64 -> 127 : 1 |
|

[2]
queue = b'nvme0q1'
usecs : count distribution
0 -> 1 : 7309
|****************************************|
2 -> 3 : 14 |
|
4 -> 7 : 7 |
|
8 -> 15 : 17 |
|

We can see that most of the time our latency is pretty good (<1ns) but with
huge tail latencies (some 8-15 ns and even one in 32-63 ns).
**NOTE, in order to reduce the tracing impact on performance I sampled
for every 100 interrupts.

I also sampled for a multiple threads/queues with QD=32 of 4K reads.
This is a collection of histograms for 5 queues (5 fio threads):
queue = b'nvme0q1'
usecs : count distribution
0 -> 1 : 701
|****************************************|
2 -> 3 : 177 |**********
|
4 -> 7 : 56 |***
|
8 -> 15 : 24 |*
|
16 -> 31 : 6 |
|
32 -> 63 : 1 |
|

queue = b'nvme0q2'
usecs : count distribution
0 -> 1 : 412
|****************************************|
2 -> 3 : 52 |*****
|
4 -> 7 : 19 |*
|
8 -> 15 : 13 |*
|
16 -> 31 : 5 |
|

queue = b'nvme0q3'
usecs : count distribution
0 -> 1 : 381
|****************************************|
2 -> 3 : 74 |*******
|
4 -> 7 : 26 |**
|
8 -> 15 : 12 |*
|
16 -> 31 : 3 |
|
32 -> 63 : 0 |
|
64 -> 127 : 0 |
|
128 -> 255 : 1 |
|

queue = b'nvme0q4'
usecs : count distribution
0 -> 1 : 386
|****************************************|
2 -> 3 : 63 |******
|
4 -> 7 : 30 |***
|
8 -> 15 : 11 |*
|
16 -> 31 : 7 |
|
32 -> 63 : 1 |
|

queue = b'nvme0q5'
usecs : count distribution
0 -> 1 : 384
|****************************************|
2 -> 3 : 69 |*******
|
4 -> 7 : 25 |**
|
8 -> 15 : 15 |*
|
16 -> 31 : 3 |
|

Overall looks pretty much the same but some more samples with tails...

Next, I sampled how many completions we are able to consume per interrupt.
Two exaples of histograms of how many completions we take per interrupt.
--
queue = b'nvme0q1'
completed : count distribution
0 : 0 | |
1 : 11690 |****************************************|
2 : 46 | |
3 : 1 | |

queue = b'nvme0q1'
completed : count distribution
0 : 0 | |
1 : 944 |****************************************|
2 : 8 | |
--

So it looks like we are super not efficient because most of the times we
catch 1
completion per interrupt and the whole point is that we need to find
more! This fio
is single threaded with QD=32 so I'd expect that we be somewhere in 8-31
almost all
the time... I also tried QD=1024, histogram is still the same.
**NOTE: Here I also sampled for every 100 interrupts.

I'll try to run the counter on the current nvme driver and see what I get.

I attached the bpf scripts I wrote (nvme-trace-irq, nvme-count-comps)
with hope that someone is interested enough to try and reproduce these
numbers on his/hers setup and maybe suggest some other useful tracing
we can do.

Prerequisites:
1. iovisor is needed for python bpf support.
$ echo "deb [trusted=yes] https://repo.iovisor.org/apt/xenial
xenial-nightly main" | sudo tee /etc/apt/sources.list.d/iovisor.list
$ sudo apt-get update -y
$ sudo apt-get install bcc-tools -y
# Nastty hack .. bcc only available in python2 but copliant with
python3..
$ sudo cp -r /usr/lib/python2.7/dist-packages/bcc
/usr/lib/python3/dist-packages/

2. Because we don't have the nvme-pci symbols exported, The nvme.h file
is needed on the
test machine (where the bpf code is running). I used nfs mount for
the linux source (this
is why I include from /mnt/linux in the scripts).

[-- Attachment #2: nvme-count-comps --]
[-- Type: text/plain, Size: 6398 bytes --]

#!/usr/bin/python3
# @lint-avoid-python-3-compatibility-imports

from __future__ import print_function
from bcc import BPF
from time import sleep, strftime
import argparse

# arguments
examples = """examples:
./nvme_comp_cout # summarize interrupt->irqpoll latency as a histogram
./nvme_comp_cout 1 10 # print 1 second summaries, 10 times
./nvme_comp_cout -mT 1 # 1s summaries, milliseconds, and timestamps
./nvme_comp_cout -Q # show each nvme queue device separately
"""
parser = argparse.ArgumentParser(
description="Summarize block device I/O latency as a histogram",
formatter_class=argparse.RawDescriptionHelpFormatter,
epilog=examples)
parser.add_argument("-T", "--timestamp", action="store_true",
help="include timestamp on output")
parser.add_argument("-m", "--milliseconds", action="store_true",
help="millisecond histogram")
parser.add_argument("-Q", "--queues", action="store_true",
help="print a histogram per queue")
parser.add_argument("--freq", help="Account every N-th request",
type=int, required=False)
parser.add_argument("interval", nargs="?", default=2,
help="output interval, in seconds")
parser.add_argument("count", nargs="?", default=99999999,
help="number of outputs")
args = parser.parse_args()
countdown = int(args.count)
debug = 0

# define BPF program
bpf_text = """
#include <uapi/linux/ptrace.h>
/*****************************************************************
* Nasty hack because we don't have the nvme-pci structs exported
*****************************************************************/
#include <linux/aer.h>
#include <linux/bitops.h>
#include <linux/blkdev.h>
#include <linux/blk-mq.h>
#include <linux/blk-mq-pci.h>
#include <linux/cpu.h>
#include <linux/delay.h>
#include <linux/errno.h>
#include <linux/fs.h>
#include <linux/genhd.h>
#include <linux/hdreg.h>
#include <linux/idr.h>
#include <linux/init.h>
#include <linux/interrupt.h>
#include <linux/io.h>
#include <linux/kdev_t.h>
#include <linux/kernel.h>
#include <linux/mm.h>
#include <linux/module.h>
#include <linux/moduleparam.h>
#include <linux/mutex.h>
#include <linux/pci.h>
#include <linux/poison.h>
#include <linux/ptrace.h>
#include <linux/sched.h>
#include <linux/slab.h>
#include <linux/t10-pi.h>
#include <linux/timer.h>
#include <linux/types.h>
#include <linux/io-64-nonatomic-lo-hi.h>
#include <asm/unaligned.h>
#include <linux/irq_poll.h>
#include "/mnt/linux/drivers/nvme/host/nvme.h"

struct nvme_dev;
struct nvme_queue;
/*
* Represents an NVM Express device. Each nvme_dev is a PCI function.
*/
struct nvme_dev {
struct nvme_queue **queues;
struct blk_mq_tag_set tagset;
struct blk_mq_tag_set admin_tagset;
u32 __iomem *dbs;
struct device *dev;
struct dma_pool *prp_page_pool;
struct dma_pool *prp_small_pool;
unsigned queue_count;
unsigned online_queues;
unsigned max_qid;
int q_depth;
u32 db_stride;
void __iomem *bar;
struct work_struct reset_work;
struct work_struct remove_work;
struct timer_list watchdog_timer;
struct mutex shutdown_lock;
bool subsystem;
void __iomem *cmb;
dma_addr_t cmb_dma_addr;
u64 cmb_size;
u32 cmbsz;
u32 cmbloc;
struct nvme_ctrl ctrl;
struct completion ioq_wait;
};

/*
* An NVM Express queue. Each device has at least two (one for admin
* commands and one for I/O commands).
*/
struct nvme_queue {
struct device *q_dmadev;
struct nvme_dev *dev;
char irqname[24];
spinlock_t sq_lock;
spinlock_t cq_lock;
struct nvme_command *sq_cmds;
struct nvme_command __iomem *sq_cmds_io;
volatile struct nvme_completion *cqes;
struct blk_mq_tags **tags;
dma_addr_t sq_dma_addr;
dma_addr_t cq_dma_addr;
u32 __iomem *q_db;
u16 q_depth;
s16 cq_vector;
u16 sq_tail;
u16 cq_head;
u16 qid;
u8 cq_phase;
struct irq_poll iop;
};

typedef struct queue_key {
char queue[24];
u64 slot;
} queue_key_t;

/* Completion counter context */
struct nvmeq {
struct nvme_queue *q;
u64 completed;
};

BPF_TABLE("percpu_array", int, struct nvmeq, qarr, 1);
BPF_TABLE("percpu_array", int, int, call_count, 1);
STORAGE

/* trace nvme interrupt */
int trace_interrupt_start(struct pt_regs *ctx, int irq, void *data)
{
__CALL__COUNT__FILTER__

struct nvmeq q ={};
int index = 0;

q.q = data;
q.completed = 0; /* reset completions */

qarr.update(&index, &q);
return 0;
}

/* count completed on each irqpoll end */
int trace_irqpoll_end(struct pt_regs *ctx)
{
__CALL__COUNT__FILTER__

struct nvmeq zero = {};
int index = 0;
struct nvmeq *q;
int completed = ctx->ax;

q = qarr.lookup_or_init(&index, &zero);
if (q == NULL) {
goto out;
}

q->completed += completed;
/* No variables in kretprobe :( 64 is our budget */
if (completed < 64) {
/* store as histogram */
STORE
q->completed = 0;
}

out:
return 0;
}
"""

call_count_filter = """
{
int zero = 0;
int index =0;
int *skip;

skip = call_count.lookup_or_init(&index, &zero);

if ((*skip) < %d) {
(*skip)++;
return 0;
}
(*skip) = 0;
}
"""

# code substitutions
if args.queues:
bpf_text = bpf_text.replace('STORAGE',
'BPF_HISTOGRAM(dist, queue_key_t);')
bpf_text = bpf_text.replace('STORE',
'queue_key_t key = {.slot = q->completed}; ' +
'bpf_probe_read(&key.queue, sizeof(key.queue), ' +
'q->q->irqname); dist.increment(key);')
else:
bpf_text = bpf_text.replace('STORAGE', 'BPF_HISTOGRAM(dist);')
bpf_text = bpf_text.replace('STORE',
'dist.increment(q->completed);')

bpf_text = bpf_text.replace("__CALL__COUNT__FILTER__", call_count_filter % (args.freq - 1) if args.freq is not None else "")
if debug:
print(bpf_text)

# load BPF program
b = BPF(text=bpf_text)
b.attach_kprobe(event="nvme_irq", fn_name="trace_interrupt_start")
b.attach_kretprobe(event="nvme_irqpoll_handler", fn_name="trace_irqpoll_end")

print("Tracing nvme I/O interrupt/irqpoll... Hit Ctrl-C to end.")

# output
exiting = 0 if args.interval else 1
dist = b.get_table("dist")
while (1):
try:
sleep(int(args.interval))
except KeyboardInterrupt:
exiting = 1

print()
if args.timestamp:
print("%-8s\n" % strftime("%H:%M:%S"), end="")

dist.print_linear_hist("completed", "queue")
dist.clear()

countdown -= 1
if exiting or countdown == 0:
exit()

[-- Attachment #3: nvme-trace-irq --]
[-- Type: text/plain, Size: 6397 bytes --]

#!/usr/bin/python3
# @lint-avoid-python-3-compatibility-imports

from __future__ import print_function
from bcc import BPF
from time import sleep, strftime
import argparse

# arguments
examples = """examples:
./nvmetrace # summarize interrupt->irqpoll latency as a histogram
./nvmetrace 1 10 # print 1 second summaries, 10 times
./nvmetrace -mT 1 # 1s summaries, milliseconds, and timestamps
./nvmetrace -Q # show each nvme queue device separately
"""
parser = argparse.ArgumentParser(
description="Summarize interrupt to softirq latency as a histogram",
formatter_class=argparse.RawDescriptionHelpFormatter,
epilog=examples)
parser.add_argument("-T", "--timestamp", action="store_true",
help="include timestamp on output")
parser.add_argument("-m", "--milliseconds", action="store_true",
help="millisecond histogram")
parser.add_argument("-Q", "--queues", action="store_true",
help="print a histogram per queue")
parser.add_argument("--freq", help="Account every N-th request", type=int, required=False)
parser.add_argument("interval", nargs="?", default=2,
help="output interval, in seconds")
parser.add_argument("count", nargs="?", default=99999999,
help="number of outputs")
args = parser.parse_args()
countdown = int(args.count)
debug = 0

# define BPF program
bpf_text = """
#include <uapi/linux/ptrace.h>
/*****************************************************************
* Nasty hack because we don't have the nvme-pci structs exported
*****************************************************************/
#include <linux/aer.h>
#include <linux/bitops.h>
#include <linux/blkdev.h>
#include <linux/blk-mq.h>
#include <linux/blk-mq-pci.h>
#include <linux/cpu.h>
#include <linux/delay.h>
#include <linux/errno.h>
#include <linux/fs.h>
#include <linux/genhd.h>
#include <linux/hdreg.h>
#include <linux/idr.h>
#include <linux/init.h>
#include <linux/interrupt.h>
#include <linux/io.h>
#include <linux/kdev_t.h>
#include <linux/kernel.h>
#include <linux/mm.h>
#include <linux/module.h>
#include <linux/moduleparam.h>
#include <linux/mutex.h>
#include <linux/pci.h>
#include <linux/poison.h>
#include <linux/ptrace.h>
#include <linux/sched.h>
#include <linux/slab.h>
#include <linux/t10-pi.h>
#include <linux/timer.h>
#include <linux/types.h>
#include <linux/io-64-nonatomic-lo-hi.h>
#include <asm/unaligned.h>
#include <linux/irq_poll.h>

/* location of nvme.h */
#include "/mnt/linux/drivers/nvme/host/nvme.h"

struct nvme_dev;
struct nvme_queue;
/*
* Represents an NVM Express device. Each nvme_dev is a PCI function.
*/
struct nvme_dev {
struct nvme_queue **queues;
struct blk_mq_tag_set tagset;
struct blk_mq_tag_set admin_tagset;
u32 __iomem *dbs;
struct device *dev;
struct dma_pool *prp_page_pool;
struct dma_pool *prp_small_pool;
unsigned queue_count;
unsigned online_queues;
unsigned max_qid;
int q_depth;
u32 db_stride;
void __iomem *bar;
struct work_struct reset_work;
struct work_struct remove_work;
struct timer_list watchdog_timer;
struct mutex shutdown_lock;
bool subsystem;
void __iomem *cmb;
dma_addr_t cmb_dma_addr;
u64 cmb_size;
u32 cmbsz;
u32 cmbloc;
struct nvme_ctrl ctrl;
struct completion ioq_wait;
};

/*
* An NVM Express queue. Each device has at least two (one for admin
* commands and one for I/O commands).
*/
struct nvme_queue {
struct device *q_dmadev;
struct nvme_dev *dev;
char irqname[24];
spinlock_t sq_lock;
spinlock_t cq_lock;
struct nvme_command *sq_cmds;
struct nvme_command __iomem *sq_cmds_io;
volatile struct nvme_completion *cqes;
struct blk_mq_tags **tags;
dma_addr_t sq_dma_addr;
dma_addr_t cq_dma_addr;
u32 __iomem *q_db;
u16 q_depth;
s16 cq_vector;
u16 sq_tail;
u16 cq_head;
u16 qid;
u8 cq_phase;
struct irq_poll iop;
};

typedef struct queue_key {
char queue[24];
u64 slot;
} queue_key_t;

BPF_HASH(start, struct nvme_queue *);
BPF_TABLE("percpu_array", int, int, call_count, 1);
STORAGE

/* timestamp nvme interrupt */
int trace_interrupt_start(struct pt_regs *ctx, int irq, void *data)
{
__CALL__COUNT__FILTER__

struct nvme_queue *q = data;
u64 ts = bpf_ktime_get_ns();
start.update(&q, &ts);
return 0;
}

/* timestamp nvme irqpoll */
int trace_irqpoll_start(struct pt_regs *ctx, struct irq_poll *iop, int budget)
{
struct nvme_queue *q = container_of(iop, struct nvme_queue, iop);
u64 *tsp, delta;

/* fetch timestamp and calculate delta */
tsp = start.lookup(&q);
if (tsp == 0) {
return 0; /* missed issue */
}
delta = bpf_ktime_get_ns() - *tsp;
FACTOR

/* store as histogram */
STORE
start.delete(&q);

return 0;
}
"""

# code substitutions
call_count_filter = """
{
int zero = 0;
int index =0;
int *skip;

skip = call_count.lookup_or_init(&index, &zero);

if ((*skip) < %d) {
(*skip)++;
return 0;
}
(*skip) = 0;
}
"""

if args.milliseconds:
bpf_text = bpf_text.replace('FACTOR', 'delta /= 1000000;')
label = "msecs"
else:
bpf_text = bpf_text.replace('FACTOR', 'delta /= 1000;')
label = "usecs"
if args.queues:
bpf_text = bpf_text.replace('STORAGE',
'BPF_HISTOGRAM(dist, queue_key_t);')
bpf_text = bpf_text.replace('STORE',
'queue_key_t key = {.slot = bpf_log2l(delta)}; ' +
'bpf_probe_read(&key.queue, sizeof(key.queue), ' +
'q->irqname); dist.increment(key);')
else:
bpf_text = bpf_text.replace('STORAGE', 'BPF_HISTOGRAM(dist);')
bpf_text = bpf_text.replace('STORE',
'dist.increment(bpf_log2l(delta));')

bpf_text = bpf_text.replace("__CALL__COUNT__FILTER__", call_count_filter % (args.freq - 1) if args.freq is not None else "")
if debug:
print(bpf_text)

# load BPF program
b = BPF(text=bpf_text)
b.attach_kprobe(event="nvme_irq", fn_name="trace_interrupt_start")
b.attach_kprobe(event="nvme_irqpoll_handler", fn_name="trace_irqpoll_start")

print("Tracing nvme I/O interrupt/irqpoll... Hit Ctrl-C to end.")

# output
exiting = 0 if args.interval else 1
dist = b.get_table("dist")
while (1):
try:
sleep(int(args.interval))
except KeyboardInterrupt:
exiting = 1

print()
if args.timestamp:
print("%-8s\n" % strftime("%H:%M:%S"), end="")

dist.print_log2_hist(label, "queue")
dist.clear()

countdown -= 1
if exiting or countdown == 0:
exit()