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3 Analysis

Pages of generic tracing text may give you enough information for exploring a system. With systemtap, it is possible to analyze that data, to filter, aggregate, transform, and summarize it. Different probes can work together to share data. Probe handlers can use a rich set of control constructs to describe algorithms, with a syntax taken roughly from awk. With these tools, systemtap scripts can focus on a specific question and provide a compact response: no grep needed.

3.1 Basic constructs

Most systemtap scripts include conditionals, to limit tracing or other logic to those processes or users or whatever of interest. The syntax is simple:

if (EXPR) STATEMENT [else STATEMENT] if/else statement
while (EXPR) STATEMENT while loop
for (A; B; C) STATEMENT for loop

Scripts may use break/continue as in C. Probe handlers can return early using next as in awk. Blocks of statements are enclosed in { and }. In systemtap, the semicolon (;) is accepted as a null statement rather than as a statement terminator, so is only rarely2necessary. Shell-style (#), C-style (/* */), and C++-style (//) comments are all accepted.

Expressions look like C or awk, and support the usual operators, precedences, and numeric literals. Strings are treated as atomic values rather than arrays of characters. String concatenation is done with the dot ("a" . "b"). Some examples:

(uid() > 100) probably an ordinary user
(execname() == "sed") current process is sed
(cpu() == 0 && gettimeofday_s() > 1140498000) after Feb. 21, 2006, on CPU 0
"hello" . " " . "world" a string in three easy pieces

Variables may be used as well. Just pick a name, assign to it, and use it in expressions. They are automatically initialized and declared. The type of each identifier - string vs. number - is automatically inferred by systemtap from the kinds of operators and literals used on it. Any inconsistencies will be reported as errors. Conversion between string and number types is done through explicit function calls.

foo = gettimeofday_s() foo is a number
bar = "/usr/bin/" . execname() bar is a string
c++ c is a number
s = sprint(2345) s becomes the string "2345"

By default, variables are local to the probe they are used in. That is, they are initialized, used, and disposed of at each probe handler invocation. To share variables between probes, declare them global anywhere in the script. Because of possible concurrency (multiple probe handlers running on different CPUs), each global variable used by a probe is automatically read- or write-locked while the handler is running.

Figure: Experimentally measuring CONFIG_HZ.

3.2 Target variables

A class of special ``target variables'' allow access to the probe point context. In a symbolic debugger, when you're stopped at a breakpoint, you can print values from the program's context. In systemtap scripts, for those probe points that match with specific executable point (rather than an asynchronous event like a timer), you can do the same.

In addition, you can take their address (the & operator), pretty-print structures (the $ and $$ suffix), pretty-print multiple variables in scope (the $$vars and related variables), or cast pointers to their types (the @cast operator), or test their existence / resolvability (the @defined operator). Read about these in the manual pages.

To know which variables are likely to be available, you will need to be familiar with the kernel source you are probing. In addition, you will need to check that the compiler has not optimized those values into unreachable nonexistence. You can use stap -L PROBEPOINT to enumerate the variables available there.

Let's say that you are trying to trace filesystem reads/writes to a particular device/inode. From your knowledge of the kernel, you know that two functions of interest could be vfs_read and vfs_write. Each takes a struct file * argument, inside there is either a struct dentry * or struct path * which has a struct dentry *. The struct dentry * contains a struct inode *, and so on. Systemtap allows limited dereferencing of such pointer chains. Two functions, user_string and kernel_string, can copy char * target variables into systemtap strings. Figure [*] demonstrates one way to monitor a particular file (identifed by device number and inode number). The script selects the appropriate variants of dev_nr andinode_nr based on the kernel version. This example also demonstrates passing numeric command-line arguments ($1 etc.) into scripts.

Figure: Watching for reads/writes to a particular file.

3.3 Functions

Functions are conveniently packaged reusable software: it would be a shame to have to duplicate a complex condition expression or logging directive in every placed it's used. So, systemtap lets you define functions of your own. Like global variables, systemtap functions may be defined anywhere in the script. They may take any number of string or numeric arguments (by value), and may return a single string or number. The parameter types are inferred as for ordinary variables, and must be consistent throughout the program. Local and global script variables are available, but target variables are not. That's because there is no specific debugging-level context associated with a function.

A function is defined with the keyword function followed by a name. Then comes a comma-separated formal argument list (just a list of variable names). The { }-enclosed body consists of any list of statements, including expressions that call functions. Recursion is possible, up to a nesting depth limit. Figure [*] displays function syntax.

Figure: Some functions of dubious utility.

3.4 Arrays

Often, probes will want to share data that cannot be represented as a simple scalar value. Much data is naturally tabular in nature, indexed by some tuple of thread numbers, processor ids, names, time, and so on. Systemtap offers associative arrays for this purpose. These arrays are implemented as hash tables with a maximum size that is fixed at startup. Because they are too large to be created dynamically for inidividual probes handler runs, they must be declared as global.

global a declare global scalar or array variable
global b[400] declare array, reserving space for up to 400 tuples

The basic operations for arrays are setting and looking up elements. These are expressed in awk syntax: the array name followed by an opening [ bracket, a comma-separated list of index expressions, and a closing ] bracket. Each index expression may be string or numeric, as long as it is consistently typed throughout the script.

foo [4,"hello"] ++ increment the named array slot
processusage [uid(),execname()] ++ update a statistic
times [tid()] = get_cycles() set a timestamp reference point
delta = get_cycles() - times [tid()] compute a timestamp delta

Array elements that have not been set may be fetched, and return a dummy null value (zero or an empty string) as appropriate. However, assigning a null value does not delete the element: an explicit delete statement is required. Systemtap provides syntactic sugar for these operations, in the form of explicit membership testing and deletion.

if ([4,"hello"] in foo) { } membership test
delete times[tid()] deletion of a single element
delete times deletion of all elements

One final and important operation is iteration over arrays. This uses the keyword foreach. Like awk, this creates a loop that iterates over key tuples of an array, not just values. In addition, the iteration may be sorted by any single key or the value by adding an extra + or - code.

The break and continue statements work inside foreach loops, too. Since arrays can be large but probe handlers must not run for long, it is a good idea to exit iteration early if possible. The limit option in the foreach expression is one way. For simplicity, systemtap forbids any modification of an array while it is being iterated using a foreach.

foreach (x = [a,b] in foo) { fuss_with(x) } simple loop in arbitrary sequence
foreach ([a,b] in foo+ limit 5) { } loop in increasing sequence of value, stop after 5
foreach ([a-,b] in foo) { } loop in decreasing sequence of first key

3.5 Aggregates

When we said above that values can only be strings or numbers, we lied a little. There is a third type: statistics aggregates, or aggregates for short. Instances of this type are used to collect statistics on numerical values, where it is important to accumulate new data quickly (without exclusive locks) and in large volume (storing only aggregated stream statistics). This type only makes sense for global variables, and may be stored individually or as elements of an array.

To add a value to a statistics aggregate, systemtap uses the special operator <<<. Think of it like C++'s << output streamer: the left hand side object accumulates the data sample given on the right hand side. This operation is efficient (taking a shared lock) because the aggregate values are kept separately on each processor, and are only aggregated across processors on request.

a <<< delta_timestamp
writes[execname()] <<< count

To read the aggregate value, special functions are available to extract a selected statistical function. The aggregate value cannot be read by simply naming it as if it were an ordinary variable. These operations take an exclusive lock on the respective globals, and should therefore be relatively rare. The simple ones are: @min, @max, @count, @avg, and @sum, and evaluate to a single number. In addition, histograms of the data stream may be extracted using the @hist_log and @hist_linear. These evaluate to a special sort of array that may at present3 only be printed.

@avg(a) the average of all the values accumulated into a
print(@hist_linear(a,0,100,10)) print an ``ascii art'' linear histogram of the same data stream, bounds $0 \ldots 100$, bucket width is $10$
@count(writes["zsh"]) the number of times ``zsh'' ran the probe handler
print(@hist_log(writes["zsh"])) print an ``ascii art'' logarithmic histogram of the same data stream

3.6 Safety

The full expressivity of the scripting language raises good questions of safety. Here is a set of Q&A:

What about infinite loops? recursion? A probe handler is bounded in time. The C code generated by systemtap includes explicit checks that limit the total number of statements executed to a small number. A similar limit is imposed on the nesting depth of function calls. When either limit is exceeded, that probe handler cleanly aborts and signals an error. The systemtap session is normally configured to abort as a whole at that time.

What about running out of memory? No dynamic memory allocation whatsoever takes place during the execution of probe handlers. Arrays, function contexts, and buffers are allocated during initialization. These resources may run out during a session, and generally result in errors.

What about locking? If multiple probes seek conflicting locks on the same global variables, one or more of them will time out, and be aborted. Such events are tallied as ``skipped'' probes, and a count is displayed at session end. A configurable number of skipped probes can trigger an abort of the session.

What about null pointers? division by zero? The C code generated by systemtap translates potentially dangerous operations to routines that check their arguments at run time. These signal errors if they are invalid. Many arithmetic and string operations silently overflow if the results exceed representation limits.

What about bugs in the translator? compiler? While bugs in the translator, or the runtime layer certainly exist4, our test suite gives some assurance. Plus, the entire generated C code may be inspected (try the -p3 option). Compiler bugs are unlikely to be of any greater concern for systemtap than for the kernel as a whole. In other words, if it was reliable enough to build the kernel, it will build the systemtap modules properly too.

Is that the whole truth? In practice, there are several weak points in systemtap and the underlying kprobes system at the time of writing. Putting probes indiscriminately into unusually sensitive parts of the kernel (low level context switching, interrupt dispatching) has reportedly caused crashes in the past. We are fixing these bugs as they are found, and constructing a probe point ``blacklist'', but it is not complete.

3.7 Exercises

  1. Alter the last probe in timer-jiffies.stp to reset the counters and continue reporting instead of exiting.

  2. Write a script that, every ten seconds, displays the top five most frequent users of open system call during that interval.

  3. Write a script that experimentally measures the speed of the get_cycles() counter on each processor.

  4. Use any suitable probe point to get an approximate profile of process CPU usage: which processes/users use how much of each CPU.

next up previous contents
Next: 4 Tapsets Up: tutorial Previous: 2 Tracing   Contents