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Uprobe is a tracing mechanism for userspace programs. Typical uprobe will incur overhead of two traps. First trap is caused by replaced trap insn, and the second trap is to execute the original displaced insn in user space. To reduce the overhead, kernel provides hooks for architectures to emulate the original insn and skip the second trap. In x86, emulation is done for certain branch insns. This patch extends the emulation to "push <reg>" insns. These insns are typical in the beginning of the function. For example, bcc in https://github.com/iovisor/bcc repo provides tools to measure funclantency, detect memleak, etc. The tools will place uprobes in the beginning of function and possibly uretprobes at the end of function. This patch is able to reduce the trap overhead for uprobe from 2 to 1. Without this patch, uretprobe will typically incur three traps. With this patch, if the function starts with "push" insn, the number of traps can be reduced from 3 to 2. An experiment was conducted on two local VMs, fedora 26 64-bit VM and 32-bit VM, both 4 processors and 4GB memory, booted with latest tip repo (and this patch). The host is MacBook with intel i7 processor. The test program looks like: #include <stdio.h> #include <stdlib.h> #include <time.h> #include <sys/time.h> static void test() __attribute__((noinline)); void test() {} int main() { struct timeval start, end; gettimeofday(&start, NULL); for (int i = 0; i < 1000000; i++) { test(); } gettimeofday(&end, NULL); printf("%ld\n", ((end.tv_sec * 1000000 + end.tv_usec) - (start.tv_sec * 1000000 + start.tv_usec))); return 0; } The program is compiled without optimization, and the first insn for function "test" is "push %rbp". The host is relatively idle. Before the test run, the uprobe is inserted as below for uprobe: echo 'p <binary>:<test_func_offset>' > /sys/kernel/debug/tracing/uprobe_events echo 1 > /sys/kernel/debug/tracing/events/uprobes/enable and for uretprobe: echo 'r <binary>:<test_func_offset>' > /sys/kernel/debug/tracing/uprobe_events echo 1 > /sys/kernel/debug/tracing/events/uprobes/enable Unit: microsecond(usec) per loop iteration x86_64 W/ this patch W/O this patch uprobe 1.55 3.1 uretprobe 2.0 3.6 x86_32 W/ this patch W/O this patch uprobe 1.41 3.5 uretprobe 1.75 4.0 You can see that this patch significantly reduced the overhead, 50% for uprobe and 44% for uretprobe on x86_64, and even more on x86_32. Signed-off-by: Yonghong Song <yhs@fb.com> Reviewed-by: Oleg Nesterov <oleg@redhat.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: kernel-team@fb.com Link: http://lkml.kernel.org/r/20171201001202.3706564-1-yhs@fb.com Signed-off-by: Ingo Molnar <mingo@kernel.org> |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.