Recording information about block IO writes performed by user processes

Question

I am trying to collect some block layer information related to writes performed by some target processes on the block devices. Specifically, I want to find: (1) the starting sector that needs to be written to, (2) the number of sectors written and (3) the time of the write.

I wrote a rudimentary bcc-python script to collect the required information by probing the block_rq_complete event. The script is as follows:

from bcc import BPF

PROG = """
#include <linux/blkdev.h>
#include <linux/sched.h>

struct data_t {
    u32 pid;
    u64 ts;
    u64 sector_start;
    u64 nr_sectors;
    char comm[TASK_COMM_LEN];
};
BPF_PERF_OUTPUT(events);

TRACEPOINT_PROBE(block, block_rq_complete) {
    struct data_t data = {};
    data.pid = bpf_get_current_pid_tgid();
    data.ts = bpf_ktime_get_ns();
    data.sector_start = args->sector;
    data.nr_sectors = args->nr_sector;
    bpf_get_current_comm(&data.comm, sizeof(data.comm));

    events.perf_submit(args, &data, sizeof(data));

    return 0;
}
"""

def print_event(cpu, data, size):
    event = b["events"].event(data)
    print(event.comm, event.ts, event.sector_start, event.nr_sectors)

b = BPF(text=PROG)
b["events"].open_perf_buffer(print_event)
while True:
    b.perf_buffer_poll()

When I run this script I find that majority of the calls to block_rq_complete are actually performed by the swapper. I understand that a lot of these calls are to perhaps write the dirty cache. And that if the process is not explicitly calling fsync, process's pid might never actually show up in the events and the corresponding writes for the process will be written by the swapper.

Is there a way to capture writes performed specifically for (or on behalf of) my desired process? And is there a way to sift through the large number of events from swapper? For e.g. events show up even when I am writing stuff to terminal or using clear on the terminal (surprisingly they take place on the SSD, identified using the major and minor numbers).

I would be extremely grateful for any help. Thank you!

Recording information about block IO writes performed by user processes

0 Answers0