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Manual Pages — LOCKSTAT

NAME

lockstat – report kernel lock and profiling statistics

CONTENTS

SYNOPSIS
DESCRIPTION
OPTIONS

Event Selection
Data Gathering
Data Gathering (Mutually Exclusive)
Data Filtering
Data Reporting

DISPLAY FORMATS
EXAMPLES
SEE ALSO
HISTORY
NOTES

SYNOPSIS

lockstat [-ACEHIV] [-e event-list] [-i rate] [-b | -t | -h | -s depth] [-n num-records] [-l lock [,size ]] [-d duration] [-f function [,size ]] [-T] [-kgwWRpP] [-D count] [-o -filename] [-x opt [=val ]] command [[args ]]

DESCRIPTION

The lockstat utility gathers and displays kernel locking and profiling statistics. lockstat allows you to specify which events to watch (for example, spin on adaptive mutex, block on read access to rwlock due to waiting writers, and so forth), how much data to gather for each event, and how to display the data. By default, lockstat monitors all lock contention events, gathers frequency and timing data about those events, and displays the data in decreasing frequency order, so that the most common events appear first.

lockstat gathers data until the specified command completes. For example, to gather statistics for a fixed-time interval, use sleep(1) as the command, as follows:

# lockstat sleep 5

When the -I option is specified, lockstat establishes a per-processor high-level periodic interrupt source to gather profiling data. The interrupt handler simply generates a lockstat event whose caller is the interrupted PC (program counter). The profiling event is just like any other lockstat event, so all of the normal lockstat options are applicable.

lockstat relies on DTrace to modify the running kernel's text to intercept events of interest. This imposes a small but measurable overhead on all system activity, so access to lockstat is restricted to super-user by default.

OPTIONS

The following options are supported:

-V
	Print the D program used to gather the requested data.

Event Selection

If no event selection options are specified, the default is -C.

-A
	Watch all lock events. -A is equivalent to -CH.
-C
	Watch contention events.
-E
	Watch error events.
-e event-list
	Only watch the specified events. event-list is a comma-separated list of events or ranges of events such as 1,4-7,35. Run lockstat with no arguments to get a brief description of all events.
-H
	Watch hold events.
-I
	Watch profiling interrupt events.
-i rate
	Interrupt rate (per second) for -I. The default is 97 Hz, so that profiling doesn't run in lockstep with the clock interrupt (which runs at 100 Hz).

Data Gathering

-x arg [=val ]
	Enable or modify a dtrace(1) runtime option or D compiler option. Boolean options are enabled by specifying their name. Options with values are set by separating the option name and value with an equals sign.

Data Gathering (Mutually Exclusive)

-b
	Basic statistics: lock, caller, number of events.
-h
	Histogram: timing plus time-distribution histograms.
-s depth
	Stack trace: histogram plus stack traces up to depth frames deep.
-t
	Timing: Basic plus timing for all events (default).

Data Filtering

-d duration
	Only watch events longer than duration.
-f func[,size ]
	Only watch events generated by func, which can be specified as a symbolic name or hex address. size defaults to the ELF symbol size if available, or 1 if not.
-l lock[,size ]
	Only watch lock, which can be specified as a symbolic name or hex address. size defaults to the ELF symbol size or 1 if the symbol size is not available.
-n num-records
	Maximum number of data records.
-T
	Trace (rather than sample) events. This is off by default.

Data Reporting

-D count
	Only display the top count events of each type.
-g
	Show total events generated by function. For example, if `foo()` calls `bar()` in a loop, the work done by `bar()` counts as work generated by `foo()` (along with any work done by `foo()` itself). The -g option works by counting the total number of stack frames in which each function appears. This implies two things: (1) the data reported by -g can be misleading if the stack traces are not deep enough, and (2) functions that are called recursively might show greater than 100% activity. In light of issue (1), the default data gathering mode when using -g is -s -50.
-k
	Coalesce PCs within functions.
-o filename
	Direct output to filename.
-P
	Sort data by (count time*) product.
-p
	Parsable output format.
-R
	Display rates (events per second) rather than counts.
-W
	Whichever: distinguish events only by caller, not by lock.
-w
	Wherever: distinguish events only by lock, not by caller.

DISPLAY FORMATS

The following headers appear over various columns of data.

Count or ops/s
	Number of times this event occurred, or the rate (times per second) if -R was specified.
indv	Percentage of all events represented by this individual event.
genr	Percentage of all events generated by this function.
cuml	Cumulative percentage; a running total of the individuals.
rcnt	Average reference count. This will always be 1 for exclusive locks (mutexes, spin locks, rwlocks held as writer) but can be greater than 1 for shared locks (rwlocks held as reader).
nsec	Average duration of the events in nanoseconds, as appropriate for the event. For the profiling event, duration means interrupt latency.
Lock	Address of the lock; displayed symbolically if possible.
CPU+Pri_Class
	CPU plus the priority class of the interrupted thread. For example, if CPU 4 is interrupted while running a timeshare thread, this will be reported as ‘`cpu[4]+TShar`’.
Caller
	Address of the caller; displayed symbolically if possible.

EXAMPLES

Example 1 Measuring Kernel Lock Contention
	`# lockstat sleep 5` Adaptive mutex spin: 41411 events in 5.011 seconds (8263 events/sec) Count indv cuml rcnt nsec Lock Caller ------------------------------------------------------------------------------- 13750 33% 33% 0.00 72 vm_page_queue_free_mtx vm_page_free_toq+0x12e 13648 33% 66% 0.00 66 vm_page_queue_free_mtx vm_page_alloc+0x138 4023 10% 76% 0.00 51 vm_dom+0x80 vm_page_dequeue+0x68 2672 6% 82% 0.00 186 vm_dom+0x80 vm_page_enqueue+0x63 618 1% 84% 0.00 31 0xfffff8000cd83a88 qsyncvp+0x37 506 1% 85% 0.00 164 0xfffff8000cb3f098 vputx+0x5a 477 1% 86% 0.00 69 0xfffff8000c7eb180 uma_dbg_getslab+0x5b 288 1% 87% 0.00 77 0xfffff8000cd8b000 vn_finished_write+0x29 263 1% 88% 0.00 103 0xfffff8000cbad448 vinactive+0xdc 259 1% 88% 0.00 53 0xfffff8000cd8b000 vfs_ref+0x24 237 1% 89% 0.00 20 0xfffff8000cbad448 vfs_hash_get+0xcc 233 1% 89% 0.00 22 0xfffff8000bfd9480 uma_dbg_getslab+0x5b 223 1% 90% 0.00 20 0xfffff8000cb3f098 cache_lookup+0x561 193 0% 90% 0.00 16 0xfffff8000cb40ba8 vref+0x27 175 0% 91% 0.00 34 0xfffff8000cbad448 vputx+0x5a 169 0% 91% 0.00 51 0xfffff8000cd8b000 vfs_unbusy+0x27 164 0% 92% 0.00 31 0xfffff8000cb40ba8 vputx+0x5a [...] Adaptive mutex block: 10 events in 5.011 seconds (2 events/sec) Count indv cuml rcnt nsec Lock Caller ------------------------------------------------------------------------------- 3 30% 30% 0.00 17592 vm_page_queue_free_mtx vm_page_alloc+0x138 2 20% 50% 0.00 20528 vm_dom+0x80 vm_page_enqueue+0x63 2 20% 70% 0.00 55502 0xfffff8000cb40ba8 vputx+0x5a 1 10% 80% 0.00 12007 vm_page_queue_free_mtx vm_page_free_toq+0x12e 1 10% 90% 0.00 9125 0xfffff8000cbad448 vfs_hash_get+0xcc 1 10% 100% 0.00 7864 0xfffff8000cd83a88 qsyncvp+0x37 ------------------------------------------------------------------------------- [...]
Example 2 Measuring Hold Times
	`# lockstat -H -D 10 sleep 1` Adaptive mutex hold: 109589 events in 1.039 seconds (105526 events/sec) Count indv cuml rcnt nsec Lock Caller ------------------------------------------------------------------------------- 8998 8% 8% 0.00 617 0xfffff8000c7eb180 uma_dbg_getslab+0xd4 5901 5% 14% 0.00 917 vm_page_queue_free_mtx vm_object_terminate+0x16a 5040 5% 18% 0.00 902 vm_dom+0x80 vm_page_free_toq+0x88 4884 4% 23% 0.00 1056 vm_page_queue_free_mtx vm_page_alloc+0x44e 4664 4% 27% 0.00 759 vm_dom+0x80 vm_fault_hold+0x1a13 4011 4% 31% 0.00 888 vm_dom vm_page_advise+0x11b 4010 4% 34% 0.00 957 vm_dom+0x80 _vm_page_deactivate+0x5c 3743 3% 38% 0.00 582 0xfffff8000cf04838 pmap_is_prefaultable+0x158 2254 2% 40% 0.00 952 vm_dom vm_page_free_toq+0x88 1639 1% 41% 0.00 591 0xfffff800d60065b8 trap_pfault+0x1f7 ------------------------------------------------------------------------------- [...] R/W writer hold: 64314 events in 1.039 seconds (61929 events/sec) Count indv cuml rcnt nsec Lock Caller ------------------------------------------------------------------------------- 7421 12% 12% 0.00 2994 pvh_global_lock pmap_page_is_mapped+0xb6 4668 7% 19% 0.00 3313 pvh_global_lock pmap_enter+0x9ae 1639 3% 21% 0.00 733 0xfffff80168d10200 vm_object_deallocate+0x683 1639 3% 24% 0.00 3061 0xfffff80168d10200 unlock_and_deallocate+0x2b 1639 3% 26% 0.00 2966 0xfffff80168d10200 vm_fault_hold+0x16ee 1567 2% 29% 0.00 733 0xfffff80168d10200 vm_fault_hold+0x19bc 821 1% 30% 0.00 786 0xfffff801eb0cc000 vm_object_madvise+0x32d 649 1% 31% 0.00 4918 0xfffff80191105300 vm_fault_hold+0x16ee 648 1% 32% 0.00 8112 0xfffff80191105300 unlock_and_deallocate+0x2b 647 1% 33% 0.00 1261 0xfffff80191105300 vm_object_deallocate+0x683 -------------------------------------------------------------------------------
Example 3 Measuring Hold Times for Stack Traces Containing a Specific Function
	`# lockstat -H -f tcp_input -s 50 -D 10 sleep 1` Adaptive mutex hold: 68 events in 1.026 seconds (66 events/sec) ------------------------------------------------------------------------------- Count indv cuml rcnt nsec Lock Caller 32 47% 47% 0.00 1631 0xfffff800686f50d8 tcp_do_segment+0x284b nsec ------ Time Distribution ------ count Stack 1024 \|@@@@@@@@@@ 11 tcp_input+0xf54 2048 \|@@@@@@@@@@@@@ 14 ip_input+0xc8 4096 \|@@@@@ 6 swi_net+0x192 8192 \| 1 intr_event_execute_handlers+0x93 ithread_loop+0xa6 fork_exit+0x84 0xffffffff808cf9ee ------------------------------------------------------------------------------- Count indv cuml rcnt nsec Lock Caller 29 43% 90% 0.00 4851 0xfffff800686f50d8 sowakeup+0xf8 nsec ------ Time Distribution ------ count Stack 4096 \|@@@@@@@@@@@@@@@ 15 tcp_do_segment+0x2423 8192 \|@@@@@@@@@@@@ 12 tcp_input+0xf54 16384 \|@@ 2 ip_input+0xc8 swi_net+0x192 intr_event_execute_handlers+0x93 ithread_loop+0xa6 fork_exit+0x84 0xffffffff808cf9ee ------------------------------------------------------------------------------- [...]

SEE ALSO

dtrace(1), ksyms(4), locking(9)

HISTORY

The lockstat utility first appeared in FreeBSD 7.1 .

NOTES

Tail-call elimination can affect call sites. For example, if foo()+0x50 calls bar() and the last thing bar() does is call mtx_unlock(), the compiler can arrange for bar() to branch to mtx_unlock() with a return address of foo()+0x58. Thus, the mtx_unlock() in bar() will appear as though it occurred at foo()+0x58.

The PC in the stack frame in which an interrupt occurs can be bogus because, between function calls, the compiler is free to use the return address register for local storage.

When using the -I and -s options together, the interrupted PC will usually not appear anywhere in the stack since the interrupt handler is entered asynchronously, not by a function call from that PC.

February 25, 2020

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