writeback.bt

#!/usr/bin/env bpftrace
/*
 * writeback	Trace file system writeback events with details.
 * 		For Linux, uses bpftrace and eBPF.
 *
 * This traces when file system dirtied pages are flushed to disk by kernel
 * writeback, and prints details including when the event occurred, and the
 * duration of the event. This can be useful for correlating these times with
 * other performance problems, and if there is a match, it would be a clue
 * that the problem may be caused by writeback. How quickly the kernel does
 * writeback can be tuned: see the kernel docs, eg,
 * vm.dirty_writeback_centisecs.
 *
 * USAGE: writeback.bt
 *
 * Copyright 2018 Netflix, Inc.
 * Licensed under the Apache License, Version 2.0 (the "License")
 *
 * 14-Sep-2018	Brendan Gregg	Created this.
 */

BEGIN
{
	printf("Tracing writeback... Hit Ctrl-C to end.\n");
	printf("%-9s %-8s %-8s %-16s %s\n", "TIME", "DEVICE", "PAGES",
	    "REASON", "ms");

	// see /sys/kernel/debug/tracing/events/writeback/writeback_start/format
	@reason[0] = "background";
	@reason[1] = "vmscan";
	@reason[2] = "sync";
	@reason[3] = "periodic";
	@reason[4] = "laptop_timer";
	@reason[5] = "free_more_memory";
	@reason[6] = "fs_free_space";
	@reason[7] = "forker_thread";
}

tracepoint:writeback:writeback_start
{
	@start[args->sb_dev] = nsecs;
}

tracepoint:writeback:writeback_written
{
	$sb_dev = args->sb_dev;
	$s = @start[$sb_dev];
	delete(@start[$sb_dev]);
	$lat = $s ? (nsecs - $s) / 1000 : 0;

	time("%H:%M:%S  ");
	printf("%-8s %-8d %-16s %d.%03d\n", args->name,
	    args->nr_pages & 0xffff,	// TODO: explain these bitmasks
	    @reason[args->reason & 0xffffffff],
	    $lat / 1000, $lat % 1000);
}

END
{
	clear(@reason);
	clear(@start);
}