Expose a new system call allowing threads to register one userspace memory area where to store the CPU number on which the calling thread is running. Scheduler migration sets the TIF_NOTIFY_RESUME flag on the current thread. Upon return to user-space, a notify-resume handler updates the current CPU value within each registered user-space memory area. User-space can then read the current CPU number directly from memory. This getcpu cache is an improvement over current mechanisms available to read the current CPU number, which has the following benefits: - 44x speedup on ARM vs system call through glibc, - 14x speedup on x86 compared to calling glibc, which calls vdso executing a "lsl" instruction, - 11x speedup on x86 compared to inlined "lsl" instruction, - Unlike vdso approaches, this cached value can be read from an inline assembly, which makes it a useful building block for restartable sequences. - The getcpu cache approach is portable (e.g. ARM), which is not the case for the lsl-based x86 vdso. On x86, yet another possible approach would be to use the gs segment selector to point to user-space per-cpu data. This approach performs similarly to the getcpu cache, but it has two disadvantages: it is not portable, and it is incompatible with existing applications already using the gs segment selector for other purposes. This approach is inspired by Paul Turner and Andrew Hunter's work on percpu atomics, which lets the kernel handle restart of critical sections: Ref.: * https://lkml.org/lkml/2015/10/27/1095 * https://lkml.org/lkml/2015/6/24/665 * https://lwn.net/Articles/650333/ * http://www.linuxplumbersconf.org/2013/ocw/system/presentations/1695/original/LPC%20-%20PerCpu%20Atomics.pdf Benchmarking various approaches for reading the current CPU number: ARMv7 Processor rev 10 (v7l) Machine model: Wandboard i.MX6 Quad Board - Baseline (empty loop): 10.1 ns - Read CPU from getcpu cache: 10.1 ns - glibc 2.19-0ubuntu6.6 getcpu: 445.6 ns - getcpu system call: 322.2 ns x86-64 Intel(R) Xeon(R) CPU E5-2630 v3 @ 2.40GHz: - Baseline (empty loop): 1.0 ns - Read CPU from getcpu cache: 1.0 ns - Read using gs segment selector: 1.0 ns - "lsl" inline assembly: 11.2 ns - glibc 2.19-0ubuntu6.6 getcpu: 14.3 ns - getcpu system call: 51.0 ns Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx> CC: Paul Turner <pjt@xxxxxxxxxx> CC: Andrew Hunter <ahh@xxxxxxxxxx> CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx> CC: Andy Lutomirski <luto@xxxxxxxxxxxxxx> CC: Andi Kleen <andi@xxxxxxxxxxxxxx> CC: Dave Watson <davejwatson@xxxxxx> CC: Chris Lameter <cl@xxxxxxxxx> CC: Ingo Molnar <mingo@xxxxxxxxxx> CC: Ben Maurer <bmaurer@xxxxxx> CC: Steven Rostedt <rostedt@xxxxxxxxxxx> CC: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> CC: Josh Triplett <josh@xxxxxxxxxxxxxxxx> CC: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> CC: Russell King <linux@xxxxxxxxxxxxxxxx> CC: Catalin Marinas <catalin.marinas@xxxxxxx> CC: Will Deacon <will.deacon@xxxxxxx> CC: Michael Kerrisk <mtk.manpages@xxxxxxxxx> CC: linux-api@xxxxxxxxxxxxxxx --- Changes since v1: - Return -1, errno=EINVAL if cpu_cache pointer is not aligned on sizeof(int32_t). - Update man page to describe the pointer alignement requirements and update atomicity guarantees. - Add MAINTAINERS file GETCPU_CACHE entry. - Remove dynamic memory allocation: go back to having a single getcpu_cache entry per thread. Update documentation accordingly. - Rebased on Linux 4.4. Rationale for the getcpu_cache system call rather than the thread-local ABI system call proposed earlier: Rather than doing a "generic" thread-local ABI, specialize this system call for a cpu number cache only. Anyway, the thread-local ABI approach would have required that we introduce "feature" flags, which would have ended up reimplementing multiplexing of features on top of a system call. It seems better to introduce one system call per feature instead. Man page associated: GETCPU_CACHE(2) Linux Programmer's Manual GETCPU_CACHE(2) NAME getcpu_cache - cache CPU number on which the calling thread is running SYNOPSIS #include <stdint.h> int getcpu_cache(int32_t **cpu_cachep, int flags); DESCRIPTION The getcpu_cache() helps speeding up reading the current CPU number by ensuring that the memory location registered by each user-space thread is always updated with the CPU number on which the thread is running when reading that memory loca‐ tion. The cpu_cachep argument is a pointer to a int32_t pointer. It is used as both an input argument and output argument. As input, it expects the target location to contain a pointer to a possible cpu number cache to use for this thread (this pointer must be naturally aligned on 4-byte multiples), or contain NULL. As output, on success, it populates the target location with a pointer to the location of the cpu number cache for this thread. This cpu number cache address can be either the one provided as input, or one already registered by this thread previously. The flags argument is currently unused and must be specified as 0. Typically, a library or application will put the cpu number cache in a thread-local storage variable, or other memory areas belonging to each thread. It is recommended to perform a volatile read of the cpu number cache to prevent the com‐ piler from doing load tearing. An alternative approach is to read the cpu number cache from inline assembly in a single instruction. Each thread is responsible for registering its own cpu number cache. Only one cpu_cache address can be registered per thread. Following registration will return the previously registered address in the cpu_cachep target location. The symbol __getcpu_cache_tls is recommended to be used across libraries and applications wishing to register a thread-local getcpu_cache. The attribute "weak" is recom‐ mended when declaring this variable in libraries. Applica‐ tions can choose to define their own version of this symbol without the weak attribute as a performance improvement. In a typical usage scenario, the thread registering the cpu number cache will be performing reads from that cache. It is however also allowed to read the cpu number cache from other threads. The cpu number cache updates performed by the kernel provide single-copy atomicity semantics, which guarantee that other threads performing single-copy atomic reads of the cpu number cache will always observe a consistent value. Memory registered as cpu number cache should never be deallo‐ cated before the thread which registered it exits: specifi‐ cally, it should not be freed, and the library containing the registered thread-local storage should not be dlclose'd. Unregistration of associated cpu_cache are implicitly per‐ formed when a thread or process exit. RETURN VALUE A return value of 0 indicates success. On success, the memory location pointed to by cpu_cachep contains the address used as cpu number cache for this thread, which may differ from the address provided as input. On error, -1 is returned, and errno is set appropriately. ERRORS EINVAL cpu_cachep points to a location containing an invalid address, cpu_cachep points to a location containing an address which is not aligned on 4-byte multiples, or flags is non-zero. ENOSYS The getcpu_cache() system call is not implemented by this kernel. EFAULT cpu_cachep is an invalid address, or cpu_cachep points to a location containing an invalid address. VERSIONS The getcpu_cache() system call was added in Linux 4.X (TODO). CONFORMING TO getcpu_cache() is Linux-specific. EXAMPLE The following code uses the getcpu_cache() system call to keep a thread local storage variable up to date with the cur‐ rent CPU number. For example simplicity, it is done in main(), but multithreaded programs would need to invoke getcpu_cache() from each program thread. #define _GNU_SOURCE #include <stdlib.h> #include <stdio.h> #include <unistd.h> #include <stdint.h> #include <sys/syscall.h> static inline int getcpu_cache(volatile int32_t **cpu_cachep, int flags) { return syscall(__NR_getcpu_cache, cpu_cachep, flags); } /* * __getcpu_cache_tls is recommended as symbol name. Weak * attribute is recommended when declaring this variable in * libraries. Applications can choose to define their own * version of this symbol without the weak attribute as a * performance improvement. */ __thread __attribute__((weak)) volatile int32_t __getcpu_cache_tls; int main(int argc, char **argv) { volatile int32_t *cpu_cache = &__getcpu_cache_tls; if (getcpu_cache(&cpu_cache, 0) < 0) { perror("getcpu_cache"); exit(EXIT_FAILURE); } if (cpu_cache != &__getcpu_cache_tls) { fprintf(stderr, "Unexpected CPU cache pointer %p\n", cpu_cache); exit(EXIT_FAILURE); } printf("Current CPU number: %d\n", __getcpu_cache_tls); exit(EXIT_SUCCESS); } Linux 2016-01-27 GETCPU_CACHE(2) --- MAINTAINERS | 6 +++ fs/exec.c | 1 + include/linux/sched.h | 36 +++++++++++++++++ init/Kconfig | 10 +++++ kernel/Makefile | 1 + kernel/fork.c | 4 ++ kernel/getcpu_cache.c | 106 ++++++++++++++++++++++++++++++++++++++++++++++++++ kernel/sched/sched.h | 1 + kernel/sys_ni.c | 3 ++ 9 files changed, 168 insertions(+) create mode 100644 kernel/getcpu_cache.c diff --git a/MAINTAINERS b/MAINTAINERS index 233f834..e9106b7 100644 --- a/MAINTAINERS +++ b/MAINTAINERS @@ -4712,6 +4712,12 @@ M: Joe Perches <joe@xxxxxxxxxxx> S: Maintained F: scripts/get_maintainer.pl +GETCPU_CACHE SUPPORT +M: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> +L: linux-kernel@xxxxxxxxxxxxxxx +S: Supported +F: kernel/getcpu_cache.c + GFS2 FILE SYSTEM M: Steven Whitehouse <swhiteho@xxxxxxxxxx> M: Bob Peterson <rpeterso@xxxxxxxxxx> diff --git a/fs/exec.c b/fs/exec.c index b06623a..1d66af6 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -1594,6 +1594,7 @@ static int do_execveat_common(int fd, struct filename *filename, /* execve succeeded */ current->fs->in_exec = 0; current->in_execve = 0; + getcpu_cache_execve(current); acct_update_integrals(current); task_numa_free(current); free_bprm(bprm); diff --git a/include/linux/sched.h b/include/linux/sched.h index fa39434..2fa2db8 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1813,6 +1813,9 @@ struct task_struct { unsigned long task_state_change; #endif int pagefault_disabled; +#ifdef CONFIG_GETCPU_CACHE + int32_t __user *cpu_cache; +#endif /* CPU-specific state of this task */ struct thread_struct thread; /* @@ -3190,4 +3193,37 @@ static inline unsigned long rlimit_max(unsigned int limit) return task_rlimit_max(current, limit); } +#ifdef CONFIG_GETCPU_CACHE +void getcpu_cache_fork(struct task_struct *t); +void getcpu_cache_execve(struct task_struct *t); +void getcpu_cache_exit(struct task_struct *t); +void __getcpu_cache_handle_notify_resume(struct task_struct *t); +static inline void getcpu_cache_set_notify_resume(struct task_struct *t) +{ + if (t->cpu_cache) + set_tsk_thread_flag(t, TIF_NOTIFY_RESUME); +} +static inline void getcpu_cache_handle_notify_resume(struct task_struct *t) +{ + if (t->cpu_cache) + __getcpu_cache_handle_notify_resume(t); +} +#else +static inline void getcpu_cache_fork(struct task_struct *t) +{ +} +static inline void getcpu_cache_execve(struct task_struct *t) +{ +} +static inline void getcpu_cache_exit(struct task_struct *t) +{ +} +static inline void getcpu_cache_set_notify_resume(struct task_struct *t) +{ +} +static inline void getcpu_cache_handle_notify_resume(struct task_struct *t) +{ +} +#endif + #endif diff --git a/init/Kconfig b/init/Kconfig index 235c7a2..fee2fa1 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -1614,6 +1614,16 @@ config MEMBARRIER If unsure, say Y. +config GETCPU_CACHE + bool "Enable getcpu cache" if EXPERT + default y + help + Enable the getcpu cache system call. It provides a user-space + cache for the current CPU number value, which speeds up + getting the current CPU number from user-space. + + If unsure, say Y. + config EMBEDDED bool "Embedded system" option allnoconfig_y diff --git a/kernel/Makefile b/kernel/Makefile index 53abf00..b630247 100644 --- a/kernel/Makefile +++ b/kernel/Makefile @@ -103,6 +103,7 @@ obj-$(CONFIG_TORTURE_TEST) += torture.o obj-$(CONFIG_MEMBARRIER) += membarrier.o obj-$(CONFIG_HAS_IOMEM) += memremap.o +obj-$(CONFIG_GETCPU_CACHE) += getcpu_cache.o $(obj)/configs.o: $(obj)/config_data.h diff --git a/kernel/fork.c b/kernel/fork.c index 1155eac..37d0645 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -252,6 +252,7 @@ void __put_task_struct(struct task_struct *tsk) WARN_ON(tsk == current); cgroup_free(tsk); + getcpu_cache_exit(tsk); task_numa_free(tsk); security_task_free(tsk); exit_creds(tsk); @@ -1554,6 +1555,9 @@ static struct task_struct *copy_process(unsigned long clone_flags, */ copy_seccomp(p); + if (!(clone_flags & CLONE_THREAD)) + getcpu_cache_fork(p); + /* * Process group and session signals need to be delivered to just the * parent before the fork or both the parent and the child after the diff --git a/kernel/getcpu_cache.c b/kernel/getcpu_cache.c new file mode 100644 index 0000000..4a1bda5 --- /dev/null +++ b/kernel/getcpu_cache.c @@ -0,0 +1,106 @@ +/* + * Copyright (C) 2015 Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> + * + * getcpu cache system call + * + * This program is free software; you can redistribute it and/or modify + * it under the terms of the GNU General Public License as published by + * the Free Software Foundation; either version 2 of the License, or + * (at your option) any later version. + * + * This program is distributed in the hope that it will be useful, + * but WITHOUT ANY WARRANTY; without even the implied warranty of + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + * GNU General Public License for more details. + */ + +#include <linux/sched.h> +#include <linux/uaccess.h> +#include <linux/syscalls.h> + +static int getcpu_cache_update(int32_t __user *cpu_cache) +{ + if (put_user(raw_smp_processor_id(), cpu_cache)) + return -1; + return 0; +} + +/* + * This resume handler should always be executed between a migration + * triggered by preemption and return to user-space. + */ +void __getcpu_cache_handle_notify_resume(struct task_struct *t) +{ + if (unlikely(t->flags & PF_EXITING)) + return; + if (getcpu_cache_update(t->cpu_cache)) + force_sig(SIGSEGV, t); +} + +/* + * If parent process has a thread-local ABI, the child inherits. Only applies + * when forking a process, not a thread. + */ +void getcpu_cache_fork(struct task_struct *t) +{ + t->cpu_cache = current->cpu_cache; +} + +void getcpu_cache_execve(struct task_struct *t) +{ + t->cpu_cache = NULL; +} + +void getcpu_cache_exit(struct task_struct *t) +{ + t->cpu_cache = NULL; +} + +/* + * sys_getcpu_cache - setup getcpu cache for caller thread + */ +SYSCALL_DEFINE2(getcpu_cache, int32_t __user **, cpu_cachep, int, flags) +{ + int32_t __user *cpu_cache; + + if (unlikely(flags)) + return -EINVAL; + /* Check if cpu_cache is already registered. */ + if (current->cpu_cache) { + if (put_user(current->cpu_cache, cpu_cachep)) + return -EFAULT; + return 0; + } + if (get_user(cpu_cache, cpu_cachep)) + return -EFAULT; + if (unlikely(!IS_ALIGNED((unsigned long)cpu_cache, sizeof(int32_t)) + || !cpu_cache)) + return -EINVAL; + /* + * Do an initial cpu cache update to ensure we won't hit + * SIGSEGV if put_user() fails in the resume notifier. + */ + if (getcpu_cache_update(cpu_cache)) { + return -EFAULT; + } + current->cpu_cache = cpu_cache; + /* + * Migration checks the getcpu cache to see whether the + * notify_resume flag should be set. + * Therefore, we need to ensure that the scheduler sees + * the getcpu cache pointer update before we update the getcpu + * cache content with the current CPU number. + * + * Set cpu_cache pointer before updating content. + */ + barrier(); + /* + * Set the resume notifier to ensure we update the current CPU + * number before returning to userspace if needed. This handles + * migration happening between the initial + * get_cpu_cache_update() call and setting the current + * cpu_cache pointer. + */ + getcpu_cache_set_notify_resume(current); + return 0; +} diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index b242775..3edcd13 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -957,6 +957,7 @@ static inline void __set_task_cpu(struct task_struct *p, unsigned int cpu) { set_task_rq(p, cpu); #ifdef CONFIG_SMP + getcpu_cache_set_notify_resume(p); /* * After ->cpu is set up to a new value, task_rq_lock(p, ...) can be * successfuly executed on another CPU. We must ensure that updates of diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c index 0623787..1e1c299 100644 --- a/kernel/sys_ni.c +++ b/kernel/sys_ni.c @@ -249,3 +249,6 @@ cond_syscall(sys_execveat); /* membarrier */ cond_syscall(sys_membarrier); + +/* thread-local ABI */ +cond_syscall(sys_getcpu_cache); -- 2.1.4 -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
