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Filename : kprobe-latency.lua
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#!/usr/bin/env bcc-lua
--[[
Copyright 2016 Marek Vavrusa <mvavrusa@cloudflare.com>

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
]]
-- This example program measures latency of block device operations and plots it
-- in a histogram. It is similar to BPF example:
-- https://github.com/torvalds/linux/blob/master/samples/bpf/tracex3_kern.c
local ffi = require('ffi')
local bpf = require('bpf')
local S = require('syscall')

-- Shared part of the program
local bins = 100
local map = bpf.map('hash', 512, ffi.typeof('uint64_t'), ffi.typeof('uint64_t'))
local lat_map = bpf.map('array', bins)

-- Kernel-space part of the program
local trace_start = bpf.kprobe('myprobe:blk_start_request', function (ptregs)
	map[ptregs.parm1] = time()
end, false, -1, 0)
local trace_end = bpf.kprobe('myprobe2:blk_account_io_done', function (ptregs)
	-- The lines below are computing index
	-- using log10(x)*10 = log2(x)*10/log2(10) = log2(x)*3
	-- index = 29 ~ 1 usec
	-- index = 59 ~ 1 msec
	-- index = 89 ~ 1 sec
	-- index = 99 ~ 10sec or more
	local delta = time() - map[ptregs.parm1]
	local index = 3 * math.log2(delta)
	if index >= bins then
		index = bins-1
	end
	xadd(lat_map[index], 1)
	return true
end, false, -1, 0)
-- User-space part of the program
pcall(function()
	local counter = 0
	local sym = {' ',' ','.','.','*','*','o','o','O','O','#','#'}
	while true do
		-- Print header once in a while
		if counter % 50 == 0 then
			print('|1us      |10us     |100us    |1ms      |10ms     |100ms    |1s       |10s')
			counter = 0
		end
		counter = counter + 1
		-- Collect all events
		local hist, events = {}, 0
		for i=29,bins-1 do
			local v = tonumber(lat_map[i] or 0)
			if v > 0 then
				hist[i] = hist[i] or 0 + v
				events = events + v
			end
		end
		-- Print histogram symbols based on relative frequency
		local s = ''
		for i=29,bins-1 do
			if hist[i] then
				local c = math.ceil((hist[i] / (events + 1)) * #sym)
				s = s .. sym[c]
			else s = s .. ' ' end
		end
		print(s .. string.format('  ; %d events', events))
		S.sleep(1)
	end
end)