mdflush.bt

#!/usr/bin/env bpftrace
// mdflush	Trace md flush events.
// 		For Linux, uses bpftrace and eBPF.
//
// Example of usage:
//
// # ./mdflush.bt
// Tracing md flush requests... Hit Ctrl-C to end.
// TIME     PID    COMM             DEVICE
// 03:13:49 16770  sync             md0
// 03:14:08 16864  sync             md0
// 03:14:49 496    kworker/1:0H     md0
// 03:14:49 488    xfsaild/md0      md0
// 03:14:54 488    xfsaild/md0      md0
// [...]
//
// This can be useful for correlation with latency outliers or spikes in disk
// latency, as measured using another tool (eg, system monitoring). If spikes in
// disk latency often coincide with md flush events, then it would make flushing
// a target for tuning.
//
// Note that the flush events are likely to originate from higher in the I/O
// stack, such as from file systems. This traces md processing them, and the
// timestamp corresponds with when md began to issue the flush to disks.
//
// This is a bpftrace version of the bcc tool of the same name.
//
// For Linux 5.12+ (see tools/old for script for lower versions).
//
// Copyright 2018 Netflix, Inc.
//
// 08-Sep-2018	Brendan Gregg	Created this.

#ifndef BPFTRACE_HAVE_BTF
#include <linux/blkdev.h>
#include <linux/bio.h>
#endif

BEGIN
{
  printf("Tracing md flush events... Hit Ctrl-C to end.\n");
  printf("%-8s %-6s %-16s %s\n", "TIME", "PID", "COMM", "DEVICE");
}

kprobe:md_flush_request
{
  time("%H:%M:%S ");
  printf("%-6d %-16s %s\n", pid, comm, ((struct bio *)arg1).bi_bdev.bd_disk.disk_name);
}