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@@ -0,0 +1,286 @@
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+ Static Keys
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+ -----------
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+
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+By: Jason Baron <jbaron@redhat.com>
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+
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+0) 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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+
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+ struct static_key key = STATIC_KEY_INIT_FALSE;
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+
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+ ...
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+
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+ if (static_key_false(&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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+ ...
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+ static_key_slow_inc();
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+ ...
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+ static_key_slow_inc();
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+ ...
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+
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+The static_key_false() 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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+
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+
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+1) Motivation
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+
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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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+
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+
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+2) Solution
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+
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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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+
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+http://gcc.gnu.org/ml/gcc-patches/2009-07/msg01556.html
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+
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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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+
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+For example, if we have a simple branch that is disabled by default:
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+
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+ if (static_key_false(&key))
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+ printk("I am the true branch\n");
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+
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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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+
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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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+
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+3) Static key label API, usage and examples:
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+
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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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+
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+ struct static_key key;
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+
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+Which is initialized as:
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+
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+ struct static_key key = STATIC_KEY_INIT_TRUE;
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+
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+or:
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+
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+ struct static_key key = STATIC_KEY_INIT_FALSE;
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+
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+If the key is not initialized, it is default false. The 'struct static_key',
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+must be a '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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+
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+The key is then used in code as:
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+
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+ if (static_key_false(&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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+Or:
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+
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+ if (static_key_true(&key))
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+ do likely code
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+ else
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+ do unlikely code
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+
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+A key that is initialized via 'STATIC_KEY_INIT_FALSE', must be used in a
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+'static_key_false()' construct. Likewise, a key initialized via
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+'STATIC_KEY_INIT_TRUE' must be used in a 'static_key_true()' construct. A
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+single key can be used in many branches, but all the branches must match the
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+way that the key has been initialized.
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+
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+The branch(es) can then be switched via:
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+
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+ static_key_slow_inc(&key);
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+ ...
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+ static_key_slow_dec(&key);
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+
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+Thus, 'static_key_slow_inc()' means 'make the branch true', and
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+'static_key_slow_dec()' means 'make the 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_key_slow_dec(), will switch the branch to false. And a subsequent
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+static_key_slow_inc(), will change the branch back to true. Likewise, if the
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+key is initialized false, a 'static_key_slow_inc()', will change the branch to
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+true. And then a 'static_key_slow_dec()', will again make the branch false.
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+
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+An example usage in the kernel is the implementation of tracepoints:
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+
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+ static inline void trace_##name(proto) \
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+ { \
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+ if (static_key_false(&__tracepoint_##name.key)) \
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+ __DO_TRACE(&__tracepoint_##name, \
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+ TP_PROTO(data_proto), \
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+ TP_ARGS(data_args), \
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+ TP_CONDITION(cond)); \
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+ }
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+
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+Tracepoints are disabled by default, and can be placed in performance critical
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+pieces of the kernel. Thus, by using a static key, the tracepoints can have
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+absolutely minimal impact when not in use.
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+
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+
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+4) Architecture level code patching interface, 'jump labels'
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+
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+
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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.
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+
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+* select HAVE_ARCH_JUMP_LABEL, see: arch/x86/Kconfig
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+
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+* #define JUMP_LABEL_NOP_SIZE, see: arch/x86/include/asm/jump_label.h
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+
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+* __always_inline bool arch_static_branch(struct static_key *key), see:
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+ arch/x86/include/asm/jump_label.h
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+
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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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+
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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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+
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+
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+* struct jump_entry, see: arch/x86/include/asm/jump_label.h
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+
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+
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+5) Static keys / jump label analysis, results (x86_64):
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+
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+
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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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+
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+SYSCALL_DEFINE0(getppid)
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+{
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+ int pid;
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+
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++ if (static_key_false(&key))
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++ printk("I am the true branch\n");
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+
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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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+
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+ return pid;
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+}
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+
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+The resulting instructions with jump labels generated by GCC is:
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+
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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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+
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+Without the jump label optimization it looks like:
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+
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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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+
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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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+
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+6 (mov) + 2 (test) + 2 (jne) = 10 - 5 (5 byte jump 0) = 5 addition bytes.
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+
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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 fucntion. In this case the non-jump label
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+function is 80 bytes long. Thus, we have 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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+
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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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+
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+jump label disabled:
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+
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+ Performance counter stats for 'bash -c /tmp/pipe-test' (50 runs):
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+
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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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+
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+ 1.601607384 seconds time elapsed ( +- 0.07% )
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+
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+jump label enabled:
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+
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+ Performance counter stats for 'bash -c /tmp/pipe-test' (50 runs):
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+
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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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+
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+ 1.579384366 seconds time elapsed
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+
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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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Jason Baron