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DTRACE_FBT(4) Kernel Interfaces Manual DTRACE_FBT(4) NAME dtrace_fbt -- a DTrace provider for dynamic kernel tracing based on function boundaries SYNOPSIS fbt:module:function:entry fbt:module:function:return DESCRIPTION The Function Boundary Tracing (fbt) provider instruments the entry and return of almost every kernel function corresponding to an elf(5) sym- bol in the kernel and loaded kernel modules. fbt:module:function:entry fires whenever the function is called. fbt:module:function:return fires when the function returns. The module in the probe description is either the name of the loaded kernel module or `kernel' for functions compiled into the kernel. Function Boundary Instrumentation The fbt will always instrument a function's entry, but its return will be intsrumented so long as it can find a `ret' instruction. In some cases, fbt cannot instrument a function's entry and/or return. Refer to subsection "Frame Pointer" for more details. Probe Arguments The arguments of the entry probe (fbt:module:function:entry) are the arguments of the traced function call. Entry Probe Argument Definition args[0] Function's first argument, typed (e.g., malloc(9)'s size_t size) args[1] Function's second argument, typed (e.g., malloc(9)'s struct malloc_type *type) args[2] Function's third argument, typed (e.g., malloc(9)'s int flags) ... ... The arguments of the return probe (fbt:module:function:return) are args[0] (the offset of the firing return instruction within the function; useful to tell apart two different return statements in a single function) and args[1] (the return value, if any). Return Probe Argument Definition args[0] Offset of the traced return instruction args[1] Function's return value (e.g., a kernel virtual address if returning from a successful malloc(9)) Subsection "Example 2: Getting Details About Probe's Arguments" shows how to get probe's argument count and types directly with dtrace(1) without having to resort to the reading function's source code or docu- mentation. EXAMPLES Example 1: Listing Available FBT Probes The following example shows how to list all the available fbt probes. # dtrace -l -P fbt ID PROVIDER MODULE FUNCTION NAME [...] 31868 fbt kernel hammer_time entry 31869 fbt kernel hammer_time return [...] Since hammer_time() is a part of the kernel and not a separate loaded module, the module column displays `kernel'. Example 2: Getting Details About Probe's Arguments The following example shows how to generate a program stability report of malloc(9)'s entry and return probes. Those reports are useful to view the probe's number of arguments and their types. # dtrace -l -v -n fbt::malloc:entry [...] Argument Types args[0]: size_t args[1]: struct malloc_type * args[2]: int The count and types of fbt::malloc:entry arguments match the function signature of malloc(9): args[0] is size_t, args[1] is struct malloc_type *, and args[2] is int. # dtrace -l -v -n fbt::malloc:return [...] Argument Types args[0]: int args[1]: void * The return probe reports two arguments and their types: the return in- struction offset (the usual int) and the function's return value, which in this case is void *, as malloc(9) returns a kernel virtual address. Example 3: Counting Kernel Slab Memory Allocation by Function # dtrace -n 'fbt::kmem*:entry { @[probefunc] = count(); }' dtrace: description 'fbt::kmem*:entry ' matched 47 probes ^C kmem_alloc_contig 1 kmem_alloc_contig_domainset 1 kmem_cache_reap_active 1 kmem_alloc_contig_pages 2 kmem_free 2 kmem_std_destructor 19 kmem_std_constructor 26 kmem_cache_free 151 kmem_cache_alloc 181 Example 4: Counting Kernel Slab Memory Allocation by Calling Function # dtrace -q -n 'fbt::kmem*:entry { @[caller] = count(); } END { printa("%40a %@16d\n", @); }' ^C kernel`contigmalloc+0x33 1 kernel`free+0xd3 1 kernel`kmem_alloc_contig+0x29 1 kernel`kmem_alloc_contig_domainset+0x19a 1 zfs.ko`arc_reap_cb_check+0x16 1 Example 5: Counting Kernel malloc()'s by Calling Function # dtrace -q -n 'fbt::malloc:entry { @[caller] = count(); } END { printa("%45a %@16d\n", @); }' ^C kernel`devclass_get_devices+0xa8 1 kernel`sys_ioctl+0xb7 1 dtrace.ko`dtrace_ioctl+0x15c1 1 dtrace.ko`dtrace_ioctl+0x972 2 dtrace.ko`dtrace_dof_create+0x35 2 kernel`kern_poll_kfds+0x2f0 4 kernel`kern_poll_kfds+0x28a 19 Example 6: Counting Kernel malloc()'s by Kernel Stack Trace # dtrace -q -n 'fbt::malloc:entry { @[stack()] = count(); }' ^C dtrace.ko`dtrace_dof_create+0x35 dtrace.ko`dtrace_ioctl+0x827 kernel`devfs_ioctl+0xd1 kernel`VOP_IOCTL_APV+0x2a kernel`vn_ioctl+0xb6 kernel`devfs_ioctl_f+0x1e kernel`kern_ioctl+0x286 kernel`sys_ioctl+0x12f kernel`amd64_syscall+0x169 kernel`0xffffffff81092b0b 2 Example 7: Summarizing vmem_alloc()'s by Arena Name and Size Distribution # dtrace -q -n 'fbt::vmem_alloc:entry { @[args[0]->vm_name] = quantize(arg1); }' ^C kernel arena dom value ------------- Distribution ------------- count 2048 | 0 4096 |@@@@@@@@@@@@@@@@@@@@@@@@@@@ 4 8192 |@@@@@@@@@@@@@ 2 16384 | 0 Example 8: Measuring Total Time Spent Executing a Function This DTrace script measures the total time spent in vm_page*() kernel functions. The quantize() aggregation organizes the measurements into power-of-two buckets, providing a time distribution in nanoseconds for each function. fbt::vm_page*:entry { self->start = timestamp; } fbt::vm_page*:return /self->start/ { @[probefunc] = quantize(timestamp - self->start); self->start = 0; } SEE ALSO dtrace(1), dtrace_kinst(4), tracing(7) Brendan Gregg and Jim Mauro, DTrace: Dynamic Tracing in Oracle Solaris, Mac OS X and FreeBSD, Prentice Hall, https://www.brendangregg.com/dtracebook/, pp. 898-903, 2011. The illumos Dynamic Tracing Guide, https://illumos.org/books/dtrace/chp-fbt.html#chp-fbt, 2008, Chapter fbt Provider. AUTHORS This manual page was written by Mateusz Piotrowski <0mp@FreeBSD.org>. CAVEATS Stability and Portability fbt probes are by definition tightly coupled to kernel code; if the code underlying a script changes, the script may fail to run or may produce incorrect results. Scripts written for one version of FreeBSD might not work on others, and almost certainly will not work on other operating systems. Individual fbt probes often do not correspond nicely to logical system events. For example, consider a DTrace script which prints the desti- nation address of every IP packet as the kernel hands them over to the network card driver (NIC). An fbt-based implementation of such a script is a discouragingly difficult task: it involves instrumenting at least four different functions in different parts of the IPv4 and IPv6 code. At the same time, with the dtrace_ip(4) provider the script is a simple one-liner: dtrace -n 'ip:::send {printf("%s", args[2]->ip_daddr);}' Make sure to review available dtrace(1) providers first before imple- menting a custom script with the fbt provider. If none of the DTrace providers offer the desired probes, consider adding new statically-de- fined tracing probes (SDT(9)). Frame Pointer Inline functions are not instrumentable by fbt as they lack a frame pointer. A developer might explicitly disable inlining by adding the `__noinline' attribute to a function definition, but of course this re- quires a recompilation of the kernel. Building the kernel with -fno-omit-frame-pointer is another way of preserving frame pointers. Note, that sometimes compilers will omit the frame pointer in leaf functions, even when configured with -fno-omit-frame-pointer. Function returns via a tail call are also not instrumentable by fbt. As a result, a function might have an entry probe and a mix of instru- mented and uninstrumentable returns. Use dtrace_kinst(4) to trace arbitrary instructions inside kernel func- tions and work around some of the limitations of fbt. Tracing DTrace The fbt provider cannot attach to functions inside DTrace provider ker- nel modules. FreeBSD 15.0 July 16, 2025 DTRACE_FBT(4)
NAME | SYNOPSIS | DESCRIPTION | EXAMPLES | SEE ALSO | AUTHORS | CAVEATS
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