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