332 lines
13 KiB
Plaintext
332 lines
13 KiB
Plaintext
|
===========
|
||
|
|
Static Keys
|
||
|
|
===========
|
||
|
|
|
||
|
|
.. warning::
|
||
|
|
|
||
|
|
DEPRECATED API:
|
||
|
|
|
||
|
|
The use of 'struct static_key' directly, is now DEPRECATED. In addition
|
||
|
|
static_key_{true,false}() is also DEPRECATED. IE DO NOT use the following::
|
||
|
|
|
||
|
|
struct static_key false = STATIC_KEY_INIT_FALSE;
|
||
|
|
struct static_key true = STATIC_KEY_INIT_TRUE;
|
||
|
|
static_key_true()
|
||
|
|
static_key_false()
|
||
|
|
|
||
|
|
The updated API replacements are::
|
||
|
|
|
||
|
|
DEFINE_STATIC_KEY_TRUE(key);
|
||
|
|
DEFINE_STATIC_KEY_FALSE(key);
|
||
|
|
DEFINE_STATIC_KEY_ARRAY_TRUE(keys, count);
|
||
|
|
DEFINE_STATIC_KEY_ARRAY_FALSE(keys, count);
|
||
|
|
static_branch_likely()
|
||
|
|
static_branch_unlikely()
|
||
|
|
|
||
|
|
Abstract
|
||
|
|
========
|
||
|
|
|
||
|
|
Static keys allows the inclusion of seldom used features in
|
||
|
|
performance-sensitive fast-path kernel code, via a GCC feature and a code
|
||
|
|
patching technique. A quick example::
|
||
|
|
|
||
|
|
DEFINE_STATIC_KEY_FALSE(key);
|
||
|
|
|
||
|
|
...
|
||
|
|
|
||
|
|
if (static_branch_unlikely(&key))
|
||
|
|
do unlikely code
|
||
|
|
else
|
||
|
|
do likely code
|
||
|
|
|
||
|
|
...
|
||
|
|
static_branch_enable(&key);
|
||
|
|
...
|
||
|
|
static_branch_disable(&key);
|
||
|
|
...
|
||
|
|
|
||
|
|
The static_branch_unlikely() branch will be generated into the code with as little
|
||
|
|
impact to the likely code path as possible.
|
||
|
|
|
||
|
|
|
||
|
|
Motivation
|
||
|
|
==========
|
||
|
|
|
||
|
|
|
||
|
|
Currently, tracepoints are implemented using a conditional branch. The
|
||
|
|
conditional check requires checking a global variable for each tracepoint.
|
||
|
|
Although the overhead of this check is small, it increases when the memory
|
||
|
|
cache comes under pressure (memory cache lines for these global variables may
|
||
|
|
be shared with other memory accesses). As we increase the number of tracepoints
|
||
|
|
in the kernel this overhead may become more of an issue. In addition,
|
||
|
|
tracepoints are often dormant (disabled) and provide no direct kernel
|
||
|
|
functionality. Thus, it is highly desirable to reduce their impact as much as
|
||
|
|
possible. Although tracepoints are the original motivation for this work, other
|
||
|
|
kernel code paths should be able to make use of the static keys facility.
|
||
|
|
|
||
|
|
|
||
|
|
Solution
|
||
|
|
========
|
||
|
|
|
||
|
|
|
||
|
|
gcc (v4.5) adds a new 'asm goto' statement that allows branching to a label:
|
||
|
|
|
||
|
|
http://gcc.gnu.org/ml/gcc-patches/2009-07/msg01556.html
|
||
|
|
|
||
|
|
Using the 'asm goto', we can create branches that are either taken or not taken
|
||
|
|
by default, without the need to check memory. Then, at run-time, we can patch
|
||
|
|
the branch site to change the branch direction.
|
||
|
|
|
||
|
|
For example, if we have a simple branch that is disabled by default::
|
||
|
|
|
||
|
|
if (static_branch_unlikely(&key))
|
||
|
|
printk("I am the true branch\n");
|
||
|
|
|
||
|
|
Thus, by default the 'printk' will not be emitted. And the code generated will
|
||
|
|
consist of a single atomic 'no-op' instruction (5 bytes on x86), in the
|
||
|
|
straight-line code path. When the branch is 'flipped', we will patch the
|
||
|
|
'no-op' in the straight-line codepath with a 'jump' instruction to the
|
||
|
|
out-of-line true branch. Thus, changing branch direction is expensive but
|
||
|
|
branch selection is basically 'free'. That is the basic tradeoff of this
|
||
|
|
optimization.
|
||
|
|
|
||
|
|
This lowlevel patching mechanism is called 'jump label patching', and it gives
|
||
|
|
the basis for the static keys facility.
|
||
|
|
|
||
|
|
Static key label API, usage and examples
|
||
|
|
========================================
|
||
|
|
|
||
|
|
|
||
|
|
In order to make use of this optimization you must first define a key::
|
||
|
|
|
||
|
|
DEFINE_STATIC_KEY_TRUE(key);
|
||
|
|
|
||
|
|
or::
|
||
|
|
|
||
|
|
DEFINE_STATIC_KEY_FALSE(key);
|
||
|
|
|
||
|
|
|
||
|
|
The key must be global, that is, it can't be allocated on the stack or dynamically
|
||
|
|
allocated at run-time.
|
||
|
|
|
||
|
|
The key is then used in code as::
|
||
|
|
|
||
|
|
if (static_branch_unlikely(&key))
|
||
|
|
do unlikely code
|
||
|
|
else
|
||
|
|
do likely code
|
||
|
|
|
||
|
|
Or::
|
||
|
|
|
||
|
|
if (static_branch_likely(&key))
|
||
|
|
do likely code
|
||
|
|
else
|
||
|
|
do unlikely code
|
||
|
|
|
||
|
|
Keys defined via DEFINE_STATIC_KEY_TRUE(), or DEFINE_STATIC_KEY_FALSE, may
|
||
|
|
be used in either static_branch_likely() or static_branch_unlikely()
|
||
|
|
statements.
|
||
|
|
|
||
|
|
Branch(es) can be set true via::
|
||
|
|
|
||
|
|
static_branch_enable(&key);
|
||
|
|
|
||
|
|
or false via::
|
||
|
|
|
||
|
|
static_branch_disable(&key);
|
||
|
|
|
||
|
|
The branch(es) can then be switched via reference counts::
|
||
|
|
|
||
|
|
static_branch_inc(&key);
|
||
|
|
...
|
||
|
|
static_branch_dec(&key);
|
||
|
|
|
||
|
|
Thus, 'static_branch_inc()' means 'make the branch true', and
|
||
|
|
'static_branch_dec()' means 'make the branch false' with appropriate
|
||
|
|
reference counting. For example, if the key is initialized true, a
|
||
|
|
static_branch_dec(), will switch the branch to false. And a subsequent
|
||
|
|
static_branch_inc(), will change the branch back to true. Likewise, if the
|
||
|
|
key is initialized false, a 'static_branch_inc()', will change the branch to
|
||
|
|
true. And then a 'static_branch_dec()', will again make the branch false.
|
||
|
|
|
||
|
|
The state and the reference count can be retrieved with 'static_key_enabled()'
|
||
|
|
and 'static_key_count()'. In general, if you use these functions, they
|
||
|
|
should be protected with the same mutex used around the enable/disable
|
||
|
|
or increment/decrement function.
|
||
|
|
|
||
|
|
Note that switching branches results in some locks being taken,
|
||
|
|
particularly the CPU hotplug lock (in order to avoid races against
|
||
|
|
CPUs being brought in the kernel whilst the kernel is getting
|
||
|
|
patched). Calling the static key API from within a hotplug notifier is
|
||
|
|
thus a sure deadlock recipe. In order to still allow use of the
|
||
|
|
functionnality, the following functions are provided:
|
||
|
|
|
||
|
|
static_key_enable_cpuslocked()
|
||
|
|
static_key_disable_cpuslocked()
|
||
|
|
static_branch_enable_cpuslocked()
|
||
|
|
static_branch_disable_cpuslocked()
|
||
|
|
|
||
|
|
These functions are *not* general purpose, and must only be used when
|
||
|
|
you really know that you're in the above context, and no other.
|
||
|
|
|
||
|
|
Where an array of keys is required, it can be defined as::
|
||
|
|
|
||
|
|
DEFINE_STATIC_KEY_ARRAY_TRUE(keys, count);
|
||
|
|
|
||
|
|
or::
|
||
|
|
|
||
|
|
DEFINE_STATIC_KEY_ARRAY_FALSE(keys, count);
|
||
|
|
|
||
|
|
4) Architecture level code patching interface, 'jump labels'
|
||
|
|
|
||
|
|
|
||
|
|
There are a few functions and macros that architectures must implement in order
|
||
|
|
to take advantage of this optimization. If there is no architecture support, we
|
||
|
|
simply fall back to a traditional, load, test, and jump sequence. Also, the
|
||
|
|
struct jump_entry table must be at least 4-byte aligned because the
|
||
|
|
static_key->entry field makes use of the two least significant bits.
|
||
|
|
|
||
|
|
* ``select HAVE_ARCH_JUMP_LABEL``,
|
||
|
|
see: arch/x86/Kconfig
|
||
|
|
|
||
|
|
* ``#define JUMP_LABEL_NOP_SIZE``,
|
||
|
|
see: arch/x86/include/asm/jump_label.h
|
||
|
|
|
||
|
|
* ``__always_inline bool arch_static_branch(struct static_key *key, bool branch)``,
|
||
|
|
see: arch/x86/include/asm/jump_label.h
|
||
|
|
|
||
|
|
* ``__always_inline bool arch_static_branch_jump(struct static_key *key, bool branch)``,
|
||
|
|
see: arch/x86/include/asm/jump_label.h
|
||
|
|
|
||
|
|
* ``void arch_jump_label_transform(struct jump_entry *entry, enum jump_label_type type)``,
|
||
|
|
see: arch/x86/kernel/jump_label.c
|
||
|
|
|
||
|
|
* ``__init_or_module void arch_jump_label_transform_static(struct jump_entry *entry, enum jump_label_type type)``,
|
||
|
|
see: arch/x86/kernel/jump_label.c
|
||
|
|
|
||
|
|
* ``struct jump_entry``,
|
||
|
|
see: arch/x86/include/asm/jump_label.h
|
||
|
|
|
||
|
|
|
||
|
|
5) Static keys / jump label analysis, results (x86_64):
|
||
|
|
|
||
|
|
|
||
|
|
As an example, let's add the following branch to 'getppid()', such that the
|
||
|
|
system call now looks like::
|
||
|
|
|
||
|
|
SYSCALL_DEFINE0(getppid)
|
||
|
|
{
|
||
|
|
int pid;
|
||
|
|
|
||
|
|
+ if (static_branch_unlikely(&key))
|
||
|
|
+ printk("I am the true branch\n");
|
||
|
|
|
||
|
|
rcu_read_lock();
|
||
|
|
pid = task_tgid_vnr(rcu_dereference(current->real_parent));
|
||
|
|
rcu_read_unlock();
|
||
|
|
|
||
|
|
return pid;
|
||
|
|
}
|
||
|
|
|
||
|
|
The resulting instructions with jump labels generated by GCC is::
|
||
|
|
|
||
|
|
ffffffff81044290 <sys_getppid>:
|
||
|
|
ffffffff81044290: 55 push %rbp
|
||
|
|
ffffffff81044291: 48 89 e5 mov %rsp,%rbp
|
||
|
|
ffffffff81044294: e9 00 00 00 00 jmpq ffffffff81044299 <sys_getppid+0x9>
|
||
|
|
ffffffff81044299: 65 48 8b 04 25 c0 b6 mov %gs:0xb6c0,%rax
|
||
|
|
ffffffff810442a0: 00 00
|
||
|
|
ffffffff810442a2: 48 8b 80 80 02 00 00 mov 0x280(%rax),%rax
|
||
|
|
ffffffff810442a9: 48 8b 80 b0 02 00 00 mov 0x2b0(%rax),%rax
|
||
|
|
ffffffff810442b0: 48 8b b8 e8 02 00 00 mov 0x2e8(%rax),%rdi
|
||
|
|
ffffffff810442b7: e8 f4 d9 00 00 callq ffffffff81051cb0 <pid_vnr>
|
||
|
|
ffffffff810442bc: 5d pop %rbp
|
||
|
|
ffffffff810442bd: 48 98 cltq
|
||
|
|
ffffffff810442bf: c3 retq
|
||
|
|
ffffffff810442c0: 48 c7 c7 e3 54 98 81 mov $0xffffffff819854e3,%rdi
|
||
|
|
ffffffff810442c7: 31 c0 xor %eax,%eax
|
||
|
|
ffffffff810442c9: e8 71 13 6d 00 callq ffffffff8171563f <printk>
|
||
|
|
ffffffff810442ce: eb c9 jmp ffffffff81044299 <sys_getppid+0x9>
|
||
|
|
|
||
|
|
Without the jump label optimization it looks like::
|
||
|
|
|
||
|
|
ffffffff810441f0 <sys_getppid>:
|
||
|
|
ffffffff810441f0: 8b 05 8a 52 d8 00 mov 0xd8528a(%rip),%eax # ffffffff81dc9480 <key>
|
||
|
|
ffffffff810441f6: 55 push %rbp
|
||
|
|
ffffffff810441f7: 48 89 e5 mov %rsp,%rbp
|
||
|
|
ffffffff810441fa: 85 c0 test %eax,%eax
|
||
|
|
ffffffff810441fc: 75 27 jne ffffffff81044225 <sys_getppid+0x35>
|
||
|
|
ffffffff810441fe: 65 48 8b 04 25 c0 b6 mov %gs:0xb6c0,%rax
|
||
|
|
ffffffff81044205: 00 00
|
||
|
|
ffffffff81044207: 48 8b 80 80 02 00 00 mov 0x280(%rax),%rax
|
||
|
|
ffffffff8104420e: 48 8b 80 b0 02 00 00 mov 0x2b0(%rax),%rax
|
||
|
|
ffffffff81044215: 48 8b b8 e8 02 00 00 mov 0x2e8(%rax),%rdi
|
||
|
|
ffffffff8104421c: e8 2f da 00 00 callq ffffffff81051c50 <pid_vnr>
|
||
|
|
ffffffff81044221: 5d pop %rbp
|
||
|
|
ffffffff81044222: 48 98 cltq
|
||
|
|
ffffffff81044224: c3 retq
|
||
|
|
ffffffff81044225: 48 c7 c7 13 53 98 81 mov $0xffffffff81985313,%rdi
|
||
|
|
ffffffff8104422c: 31 c0 xor %eax,%eax
|
||
|
|
ffffffff8104422e: e8 60 0f 6d 00 callq ffffffff81715193 <printk>
|
||
|
|
ffffffff81044233: eb c9 jmp ffffffff810441fe <sys_getppid+0xe>
|
||
|
|
ffffffff81044235: 66 66 2e 0f 1f 84 00 data32 nopw %cs:0x0(%rax,%rax,1)
|
||
|
|
ffffffff8104423c: 00 00 00 00
|
||
|
|
|
||
|
|
Thus, the disable jump label case adds a 'mov', 'test' and 'jne' instruction
|
||
|
|
vs. the jump label case just has a 'no-op' or 'jmp 0'. (The jmp 0, is patched
|
||
|
|
to a 5 byte atomic no-op instruction at boot-time.) Thus, the disabled jump
|
||
|
|
label case adds::
|
||
|
|
|
||
|
|
6 (mov) + 2 (test) + 2 (jne) = 10 - 5 (5 byte jump 0) = 5 addition bytes.
|
||
|
|
|
||
|
|
If we then include the padding bytes, the jump label code saves, 16 total bytes
|
||
|
|
of instruction memory for this small function. In this case the non-jump label
|
||
|
|
function is 80 bytes long. Thus, we have saved 20% of the instruction
|
||
|
|
footprint. We can in fact improve this even further, since the 5-byte no-op
|
||
|
|
really can be a 2-byte no-op since we can reach the branch with a 2-byte jmp.
|
||
|
|
However, we have not yet implemented optimal no-op sizes (they are currently
|
||
|
|
hard-coded).
|
||
|
|
|
||
|
|
Since there are a number of static key API uses in the scheduler paths,
|
||
|
|
'pipe-test' (also known as 'perf bench sched pipe') can be used to show the
|
||
|
|
performance improvement. Testing done on 3.3.0-rc2:
|
||
|
|
|
||
|
|
jump label disabled::
|
||
|
|
|
||
|
|
Performance counter stats for 'bash -c /tmp/pipe-test' (50 runs):
|
||
|
|
|
||
|
|
855.700314 task-clock # 0.534 CPUs utilized ( +- 0.11% )
|
||
|
|
200,003 context-switches # 0.234 M/sec ( +- 0.00% )
|
||
|
|
0 CPU-migrations # 0.000 M/sec ( +- 39.58% )
|
||
|
|
487 page-faults # 0.001 M/sec ( +- 0.02% )
|
||
|
|
1,474,374,262 cycles # 1.723 GHz ( +- 0.17% )
|
||
|
|
<not supported> stalled-cycles-frontend
|
||
|
|
<not supported> stalled-cycles-backend
|
||
|
|
1,178,049,567 instructions # 0.80 insns per cycle ( +- 0.06% )
|
||
|
|
208,368,926 branches # 243.507 M/sec ( +- 0.06% )
|
||
|
|
5,569,188 branch-misses # 2.67% of all branches ( +- 0.54% )
|
||
|
|
|
||
|
|
1.601607384 seconds time elapsed ( +- 0.07% )
|
||
|
|
|
||
|
|
jump label enabled::
|
||
|
|
|
||
|
|
Performance counter stats for 'bash -c /tmp/pipe-test' (50 runs):
|
||
|
|
|
||
|
|
841.043185 task-clock # 0.533 CPUs utilized ( +- 0.12% )
|
||
|
|
200,004 context-switches # 0.238 M/sec ( +- 0.00% )
|
||
|
|
0 CPU-migrations # 0.000 M/sec ( +- 40.87% )
|
||
|
|
487 page-faults # 0.001 M/sec ( +- 0.05% )
|
||
|
|
1,432,559,428 cycles # 1.703 GHz ( +- 0.18% )
|
||
|
|
<not supported> stalled-cycles-frontend
|
||
|
|
<not supported> stalled-cycles-backend
|
||
|
|
1,175,363,994 instructions # 0.82 insns per cycle ( +- 0.04% )
|
||
|
|
206,859,359 branches # 245.956 M/sec ( +- 0.04% )
|
||
|
|
4,884,119 branch-misses # 2.36% of all branches ( +- 0.85% )
|
||
|
|
|
||
|
|
1.579384366 seconds time elapsed
|
||
|
|
|
||
|
|
The percentage of saved branches is .7%, and we've saved 12% on
|
||
|
|
'branch-misses'. This is where we would expect to get the most savings, since
|
||
|
|
this optimization is about reducing the number of branches. In addition, we've
|
||
|
|
saved .2% on instructions, and 2.8% on cycles and 1.4% on elapsed time.
|