本文的例子 可用于判断程序性能问题是否由于futex锁冲突引起的.
This section describes how to identify contended user-space locks throughout the system within a specific time period. The ability to identify contended user-space locks can help you investigate poor program performance that you suspect may be caused by futex contentions.
Simply put, futex contention occurs when multiple processes are trying to access the same lock variable at the same time. This can result in a poor performance because the lock serializes execution; one process obtains the lock while the other processes must wait for the lock variable to become available again.
修改成间隔输出后的脚本及注解如下 :
[root@db-172-16-3-150 process]# cd /opt/systemtap/share/doc/systemtap/examples/process
[root@db-172-16-3-150 process]# cat futexes.stp
#! /usr/bin/env stap
# This script tries to identify contended user-space locks by hooking
# into the futex system call.
global FUTEX_WAIT = 0 /*, FUTEX_WAKE = 1 */
global FUTEX_PRIVATE_FLAG = 128 /* linux 2.6.22+ */
global FUTEX_CLOCK_REALTIME = 256 /* linux 2.6.29+ */
global lock_waits # long-lived stats on (tid,lock) blockage elapsed time
global process_names # long-lived pid-to-execname mapping
probe syscall.futex.return {
elapsed = gettimeofday_us() - @entry(gettimeofday_us())
// elapsed 调到第一条, 增加计算时间精度.
if (($op & ~(FUTEX_PRIVATE_FLAG|FUTEX_CLOCK_REALTIME)) != FUTEX_WAIT) next
// next 表示跳过这次probe handler.
process_names[pid()] = execname()
lock_waits[pid(), $uaddr] <<< elapsed
}
// $op存储一个比特位变量, 通过($op & ~(FUTEX_PRIVATE_FLAG|FUTEX_CLOCK_REALTIME)) != FUTEX_WAIT操作.
// 判断进程是否处于等待状态.
// ~ 比特位翻转
// | 比特或
// & 比特与
probe timer.s(5) {
foreach ([pid+, lock] in lock_waits)
printf ("%s[%d] lock %p contended %d times, %d avg us\n",
process_names[pid], pid, lock, @count(lock_waits[pid,lock]),
@avg(lock_waits[pid,lock]))
delete process_names
delete process_names
}
// 输出: 进程名, 锁内存地址, 锁冲突次数, 平均等待时间.
输出如下 :
[root@db-172-16-3-150 process]# stap futexes.stp
auditd[1599] lock 0x7f08ffd1f294 contended 1 times, 3291 avg us
rs:main Q:Reg[1624] lock 0x7fbbc9d87e64 contended 1 times, 9619461 avg us
[1599] lock 0x7f08ffd1f294 contended 1 times, 3291 avg us
[1624] lock 0x7fbbc9d87e64 contended 1 times, 9619461 avg us
pgbench[23916] lock 0x7f1440f07360 contended 4 times, 134 avg us
[1599] lock 0x7f08ffd1f294 contended 1 times, 3291 avg us
[1624] lock 0x7fbbc9d87e64 contended 1 times, 9619461 avg us
automount[1973] lock 0x7faeb99e7224 contended 1 times, 3596 avg us
automount[1973] lock 0x7faea00008ec contended 1 times, 2014 avg us
automount[1973] lock 0x7faea00008c0 contended 1 times, 523 avg us
[23916] lock 0x7f1440f07360 contended 4 times, 134 avg us
[1599] lock 0x7f08ffd1f294 contended 1 times, 3291 avg us
[1624] lock 0x7fbbc9d87e64 contended 1 times, 9619461 avg us
[1973] lock 0x7faeb99e7224 contended 1 times, 3596 avg us
[1973] lock 0x7faea00008ec contended 1 times, 2014 avg us
[1973] lock 0x7faea00008c0 contended 1 times, 523 avg us
[23916] lock 0x7f1440f07360 contended 4 times, 134 avg us
本文涉及的probe alias原型 :
# futex ______________________________________________________
# long sys_futex(u32 __user *uaddr,
# int op,
# int val,
# struct timespec __user *utime,
# u32 __user *uaddr2,
# int val3)
# long compat_sys_futex(u32 __user *uaddr, int op, u32 val,
# struct compat_timespec __user *utime, u32 __user *uaddr2,
# u32 val3)
#
probe syscall.futex = kernel.function("sys_futex").call ?
{
name = "futex"
futex_uaddr = $uaddr
op = $op
val = $val
utime_uaddr = $utime
uaddr2_uaddr = $uaddr2
val3 = $val3
if ($op == 0)
argstr = sprintf("%p, %s, %d, %s", $uaddr, _futex_op_str($op),
$val, _struct_timespec_u($utime, 1))
else
argstr = sprintf("%p, %s, %d", $uaddr, _futex_op_str($op),
$val)
}
probe syscall.futex.return = kernel.function("sys_futex").return ?
{
name = "futex"
retstr = return_str(1, $return)
}
源代码 :
[root@db-172-16-3-150 linux]# stap -L 'kernel.function("sys_futex").return'
kernel.function("sys_futex@kernel/futex.c:2692").return $return:long int $uaddr:u32* $op:int $val:u32 $utime:struct timespec* $uaddr2:u32* $val3:u32 $ts:struct timespec $t:ktime_t
/usr/src/debug/kernel-2.6.32-358.el6/linux-2.6.32-358.el6.x86_64/kernel/futex.c
SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
struct timespec __user *, utime, u32 __user *, uaddr2,
u32, val3)
{
struct timespec ts;
ktime_t t, *tp = NULL;
u32 val2 = 0;
int cmd = op & FUTEX_CMD_MASK;
if (utime && (cmd == FUTEX_WAIT || cmd == FUTEX_LOCK_PI ||
cmd == FUTEX_WAIT_BITSET ||
cmd == FUTEX_WAIT_REQUEUE_PI)) {
if (copy_from_user(&ts, utime, sizeof(ts)) != 0)
return -EFAULT;
if (!timespec_valid(&ts))
return -EINVAL;
t = timespec_to_ktime(ts);
if (cmd == FUTEX_WAIT)
t = ktime_add_safe(ktime_get(), t);
tp = &t;
}
/*
* requeue parameter in 'utime' if cmd == FUTEX_*_REQUEUE_*.
* number of waiters to wake in 'utime' if cmd == FUTEX_WAKE_OP.
*/
if (cmd == FUTEX_REQUEUE || cmd == FUTEX_CMP_REQUEUE ||
cmd == FUTEX_CMP_REQUEUE_PI || cmd == FUTEX_WAKE_OP)
val2 = (u32) (unsigned long) utime;
return do_futex(uaddr, op, val, tp, uaddr2, val2, val3);
}
[参考]
1. man 2 futex
FUTEX(2) Linux Programmer’s Manual FUTEX(2)
NAME
futex - Fast Userspace Locking system call
SYNOPSIS
#include <linux/futex.h>
#include <sys/time.h>
int futex(int *uaddr, int op, int val, const struct timespec *timeout,
int *uaddr2, int val3);
DESCRIPTION
The futex() system call provides a method for a program to wait for a value at a given address to change, and a
method to wake up anyone waiting on a particular address (while the addresses for the same memory in separate
processes may not be equal, the kernel maps them internally so the same memory mapped in different locations
will correspond for futex() calls). It is typically used to implement the contended case of a lock in shared
memory, as described in futex(7).
When a futex(7) operation did not finish uncontended in userspace, a call needs to be made to the kernel to
arbitrate. Arbitration can either mean putting the calling process to sleep or, conversely, waking a waiting
process.
Callers of this function are expected to adhere to the semantics as set out in futex(7). As these semantics
involve writing non-portable assembly instructions, this in turn probably means that most users will in fact be
library authors and not general application developers.
The uaddr argument needs to point to an aligned integer which stores the counter. The operation to execute is
passed via the op argument, along with a value val.
Five operations are currently defined:
FUTEX_WAIT
This operation atomically verifies that the futex address uaddr still contains the value val, and sleeps
awaiting FUTEX_WAKE on this futex address. If the timeout argument is non-NULL, its contents describe
the maximum duration of the wait, which is infinite otherwise. The arguments uaddr2 and val3 are
ignored.
For futex(7), this call is executed if decrementing the count gave a negative value (indicating con-
tention), and will sleep until another process releases the futex and executes the FUTEX_WAKE operation.
FUTEX_WAKE
This operation wakes at most val processes waiting on this futex address (i.e., inside FUTEX_WAIT). The
arguments timeout, uaddr2 and val3 are ignored.
For futex(7), this is executed if incrementing the count showed that there were waiters, once the futex
value has been set to 1 (indicating that it is available).
FUTEX_FD (present up to and including Linux 2.6.25)
To support asynchronous wakeups, this operation associates a file descriptor with a futex. If another
process executes a FUTEX_WAKE, the process will receive the signal number that was passed in val. The
calling process must close the returned file descriptor after use. The arguments timeout, uaddr2 and
val3 are ignored.
To prevent race conditions, the caller should test if the futex has been upped after FUTEX_FD returns.
Because it was inherently racy, FUTEX_FD has been removed from Linux 2.6.26 onwards.
FUTEX_REQUEUE (since Linux 2.5.70)
This operation was introduced in order to avoid a "thundering herd" effect when FUTEX_WAKE is used and
all processes woken up need to acquire another futex. This call wakes up val processes, and requeues
all other waiters on the futex at address uaddr2. The arguments timeout and val3 are ignored.
FUTEX_CMP_REQUEUE (since Linux 2.6.7)
There was a race in the intended use of FUTEX_REQUEUE, so FUTEX_CMP_REQUEUE was introduced. This is
similar to FUTEX_REQUEUE, but first checks whether the location uaddr still contains the value val3. If
not, the operation fails with the error EAGAIN. The argument timeout is ignored.
RETURN VALUE
Depending on which operation was executed, the returned value for a successful call can have differing mean-
ings.
FUTEX_WAIT
Returns 0 if the process was woken by a FUTEX_WAKE call. In case of timeout, the operation fails with
the error ETIMEDOUT. If the futex was not equal to the expected value, the operation fails with the
error EWOULDBLOCK. Signals (see signal(7)) or other spurious wakeups cause FUTEX_WAIT to fail with the
error EINTR.
FUTEX_WAKE
Returns the number of processes woken up.
FUTEX_FD
Returns the new file descriptor associated with the futex.
FUTEX_REQUEUE
Returns the number of processes woken up.
FUTEX_CMP_REQUEUE
Returns the number of processes woken up.
In the event of an error, all operations return -1, and set errno to indicate the error.
ERRORS
EACCES No read access to futex memory.
EAGAIN FUTEX_CMP_REQUEUE found an unexpected futex value. (This probably indicates a race; use the safe
FUTEX_WAKE now.)
EFAULT Error in getting timeout information from userspace.
EINVAL An operation was not defined or error in page alignment.
ENFILE The system limit on the total number of open files has been reached.
ENOSYS Invalid operation specified in op.
VERSIONS
Initial futex support was merged in Linux 2.5.7 but with different semantics from what was described above. A
4-argument system call with the semantics given here was introduced in Linux 2.5.40. In Linux 2.5.70 one argu-
ment was added. In Linux 2.6.7 a sixth argument was added — messy, especially on the s390 architecture.
CONFORMING TO
This system call is Linux-specific.
NOTES
To reiterate, bare futexes are not intended as an easy-to-use abstraction for end-users. (There is no wrapper
function for this system call in glibc.) Implementors are expected to be assembly literate and to have read
the sources of the futex userspace library referenced below.
SEE ALSO
futex(7)
Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux (proceedings of the Ottawa Linux Symposium 2002),
futex example library, futex-*.tar.bz2 <URL:ftp://ftp.nl.kernel.org/pub/linux/kernel/people/rusty/>.
COLOPHON
This page is part of release 3.22 of the Linux man-pages project. A description of the project, and informa-
tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.
Linux 2008-11-27 FUTEX(2)
2. man 7 futex
FUTEX(7) Linux Programmer’s Manual FUTEX(7)
NAME
futex - Fast Userspace Locking
SYNOPSIS
#include <linux/futex.h>
DESCRIPTION
The Linux kernel provides futexes ("Fast Userspace muTexes") as a building block for fast userspace locking and
semaphores. Futexes are very basic and lend themselves well for building higher level locking abstractions
such as POSIX mutexes.
This page does not set out to document all design decisions but restricts itself to issues relevant for appli-
cation and library development. Most programmers will in fact not be using futexes directly but instead rely
on system libraries built on them, such as the NPTL pthreads implementation.
A futex is identified by a piece of memory which can be shared between different processes. In these different
processes, it need not have identical addresses. In its bare form, a futex has semaphore semantics; it is a
counter that can be incremented and decremented atomically; processes can wait for the value to become posi-
tive.
Futex operation is entirely userspace for the non-contended case. The kernel is only involved to arbitrate the
contended case. As any sane design will strive for non-contention, futexes are also optimized for this situa-
tion.
In its bare form, a futex is an aligned integer which is only touched by atomic assembler instructions. Pro-
cesses can share this integer using mmap(2), via shared memory segments or because they share memory space, in
which case the application is commonly called multithreaded.
Semantics
Any futex operation starts in userspace, but it may necessary to communicate with the kernel using the futex(2)
system call.
To "up" a futex, execute the proper assembler instructions that will cause the host CPU to atomically increment
the integer. Afterwards, check if it has in fact changed from 0 to 1, in which case there were no waiters and
the operation is done. This is the non-contended case which is fast and should be common.
In the contended case, the atomic increment changed the counter from -1 (or some other negative number). If
this is detected, there are waiters. Userspace should now set the counter to 1 and instruct the kernel to wake
up any waiters using the FUTEX_WAKE operation.
Waiting on a futex, to "down" it, is the reverse operation. Atomically decrement the counter and check if it
changed to 0, in which case the operation is done and the futex was uncontended. In all other circumstances,
the process should set the counter to -1 and request that the kernel wait for another process to up the futex.
This is done using the FUTEX_WAIT operation.
The futex(2) system call can optionally be passed a timeout specifying how long the kernel should wait for the
futex to be upped. In this case, semantics are more complex and the programmer is referred to futex(2) for
more details. The same holds for asynchronous futex waiting.
VERSIONS
Initial futex support was merged in Linux 2.5.7 but with different semantics from those described above. Cur-
rent semantics are available from Linux 2.5.40 onwards.
NOTES
To reiterate, bare futexes are not intended as an easy to use abstraction for end-users. Implementors are
expected to be assembly literate and to have read the sources of the futex userspace library referenced below.
This man page illustrates the most common use of the futex(2) primitives: it is by no means the only one.
SEE ALSO
futex(2)
Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux (proceedings of the Ottawa Linux Symposium 2002),
futex example library, futex-*.tar.bz2 <URL:ftp://ftp.kernel.org/pub/linux/kernel/people/rusty/>.
COLOPHON
This page is part of release 3.22 of the Linux man-pages project. A description of the project, and informa-
tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.
Linux 2002-12-31 FUTEX(7)
