Expose a new system call allowing threads to register a userspace memory area where to store the current CPU number. 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 that user-space memory area.
This getcpu cache is an alternative to the sched_getcpu() vdso which has a few benefits: - It is faster to do a memory read that to call a vDSO, - This cached value can be read from within an inline assembly, which makes it a useful building block for restartable sequences. 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/2015/6/24/665 * https://lwn.net/Articles/650333/ * http://www.linuxplumbersconf.org/2013/ocw/system/presentations/1695/original/LPC%20-%20PerCpu%20Atomics.pdf Benchmarking sched_getcpu() vs tls cache approach. Getting the current CPU number: - With Linux vdso: 12.7 ns - With TLS-cached cpu number: 0.3 ns The system call can be extended by registering a larger structure in the future. Signed-off-by: Mathieu Desnoyers <mathieu.desnoy...@efficios.com> CC: Thomas Gleixner <t...@linutronix.de> CC: Paul Turner <p...@google.com> CC: Andrew Hunter <a...@google.com> CC: Peter Zijlstra <pet...@infradead.org> CC: Andy Lutomirski <l...@amacapital.net> CC: Andi Kleen <a...@firstfloor.org> CC: Dave Watson <davejwat...@fb.com> CC: Chris Lameter <c...@linux.com> CC: Ingo Molnar <mi...@redhat.com> CC: Ben Maurer <bmau...@fb.com> CC: Steven Rostedt <rost...@goodmis.org> CC: "Paul E. McKenney" <paul...@linux.vnet.ibm.com> CC: Josh Triplett <j...@joshtriplett.org> CC: Linus Torvalds <torva...@linux-foundation.org> CC: Andrew Morton <a...@linux-foundation.org> CC: Thomas Gleixner <t...@linutronix.de> CC: linux-api@vger.kernel.org --- arch/x86/entry/common.c | 2 + arch/x86/entry/syscalls/syscall_64.tbl | 1 + fs/exec.c | 1 + include/linux/sched.h | 32 ++++++++++++ include/uapi/asm-generic/unistd.h | 4 +- include/uapi/linux/Kbuild | 1 + include/uapi/linux/thread_local_abi.h | 37 ++++++++++++++ init/Kconfig | 7 +++ kernel/Makefile | 1 + kernel/fork.c | 2 + kernel/sched/core.c | 4 ++ kernel/sched/sched.h | 2 + kernel/sys_ni.c | 3 ++ kernel/thread_local_abi.c | 92 ++++++++++++++++++++++++++++++++++ 14 files changed, 188 insertions(+), 1 deletion(-) create mode 100644 include/uapi/linux/thread_local_abi.h create mode 100644 kernel/thread_local_abi.c diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c index a89fdbc..fdfdb14 100644 --- a/arch/x86/entry/common.c +++ b/arch/x86/entry/common.c @@ -249,6 +249,8 @@ static void exit_to_usermode_loop(struct pt_regs *regs, u32 cached_flags) if (cached_flags & _TIF_NOTIFY_RESUME) { clear_thread_flag(TIF_NOTIFY_RESUME); tracehook_notify_resume(regs); + if (getcpu_cache_active(current)) + getcpu_cache_handle_notify_resume(current); } if (cached_flags & _TIF_USER_RETURN_NOTIFY) diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl index 314a90b..748aee3 100644 --- a/arch/x86/entry/syscalls/syscall_64.tbl +++ b/arch/x86/entry/syscalls/syscall_64.tbl @@ -332,6 +332,7 @@ 323 common userfaultfd sys_userfaultfd 324 common membarrier sys_membarrier 325 common mlock2 sys_mlock2 +326 common thread_local_abi sys_thread_local_abi # # x32-specific system call numbers start at 512 to avoid cache impact diff --git a/fs/exec.c b/fs/exec.c index b06623a..88490cc 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; + thread_local_abi_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 edad7a4..b39d9a3 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -2,6 +2,7 @@ #define _LINUX_SCHED_H #include <uapi/linux/sched.h> +#include <uapi/linux/thread_local_abi.h> #include <linux/sched/prio.h> @@ -1812,6 +1813,10 @@ struct task_struct { unsigned long task_state_change; #endif int pagefault_disabled; +#ifdef CONFIG_THREAD_LOCAL_ABI + size_t thread_local_abi_len; + struct thread_local_abi __user *thread_local_abi; +#endif /* CPU-specific state of this task */ struct thread_struct thread; /* @@ -3188,4 +3193,31 @@ static inline unsigned long rlimit_max(unsigned int limit) return task_rlimit_max(current, limit); } +#ifdef CONFIG_THREAD_LOCAL_ABI +void thread_local_abi_fork(struct task_struct *t); +void thread_local_abi_execve(struct task_struct *t); +void getcpu_cache_handle_notify_resume(struct task_struct *t); +static inline bool getcpu_cache_active(struct task_struct *t) +{ + if (t->thread_local_abi_len < offsetof(struct thread_local_abi, cpu) + + sizeof(t->thread_local_abi->cpu)) + return false; + return true; +} +#else +static inline void thread_local_abi_fork(struct task_struct *t) +{ +} +static inline void thread_local_abi_execve(struct task_struct *t) +{ +} +static inline void getcpu_cache_handle_notify_resume(struct task_struct *t) +{ +} +static inline bool getcpu_cache_active(struct task_struct *t) +{ + return false; +} +#endif + #endif diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h index 1324b02..89a107a 100644 --- a/include/uapi/asm-generic/unistd.h +++ b/include/uapi/asm-generic/unistd.h @@ -715,9 +715,11 @@ __SYSCALL(__NR_userfaultfd, sys_userfaultfd) __SYSCALL(__NR_membarrier, sys_membarrier) #define __NR_mlock2 284 __SYSCALL(__NR_mlock2, sys_mlock2) +#define __NR_thread_local_abi 285 +__SYSCALL(__NR_thread_local_abi, sys_thread_local_abi) #undef __NR_syscalls -#define __NR_syscalls 285 +#define __NR_syscalls 286 /* * All syscalls below here should go away really, diff --git a/include/uapi/linux/Kbuild b/include/uapi/linux/Kbuild index 628e6e6..5df5460 100644 --- a/include/uapi/linux/Kbuild +++ b/include/uapi/linux/Kbuild @@ -397,6 +397,7 @@ header-y += tcp_metrics.h header-y += telephony.h header-y += termios.h header-y += thermal.h +header-y += thread_local_abi.h header-y += time.h header-y += times.h header-y += timex.h diff --git a/include/uapi/linux/thread_local_abi.h b/include/uapi/linux/thread_local_abi.h new file mode 100644 index 0000000..6487c92 --- /dev/null +++ b/include/uapi/linux/thread_local_abi.h @@ -0,0 +1,37 @@ +#ifndef _UAPI_LINUX_THREAD_LOCAL_ABI_H +#define _UAPI_LINUX_THREAD_LOCAL_ABI_H + +/* + * linux/thread_local_abi.h + * + * thread_local_abi system call API + * + * Copyright (c) 2015 Mathieu Desnoyers <mathieu.desnoy...@efficios.com> + * + * Permission is hereby granted, free of charge, to any person obtaining a copy + * of this software and associated documentation files (the "Software"), to deal + * in the Software without restriction, including without limitation the rights + * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell + * copies of the Software, and to permit persons to whom the Software is + * furnished to do so, subject to the following conditions: + * + * The above copyright notice and this permission notice shall be included in + * all copies or substantial portions of the Software. + * + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR + * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, + * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE + * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER + * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, + * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE + * SOFTWARE. + */ + +#include <linux/types.h> + +/* This structure is an ABI that can only be extended. */ +struct thread_local_abi { + int32_t cpu; +}; + +#endif /* _UAPI_LINUX_THREAD_LOCAL_ABI_H */ diff --git a/init/Kconfig b/init/Kconfig index c24b6f7..df29803 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -1612,6 +1612,13 @@ config MEMBARRIER pairs of memory barriers into pairs consisting of membarrier() and a compiler barrier. +config THREAD_LOCAL_ABI + bool "Enable thread-local ABI" if EXPERT + default y + help + Enable the thread-local ABI system call. It provides a user-space + cache for the current CPU number value. + If unsure, say Y. config EMBEDDED diff --git a/kernel/Makefile b/kernel/Makefile index 53abf00..327fbd9 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_THREAD_LOCAL_ABI) += thread_local_abi.o $(obj)/configs.o: $(obj)/config_data.h diff --git a/kernel/fork.c b/kernel/fork.c index f97f2c4..42dd565 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -1612,6 +1612,8 @@ static struct task_struct *copy_process(unsigned long clone_flags, cgroup_post_fork(p, cgrp_ss_priv); if (clone_flags & CLONE_THREAD) threadgroup_change_end(current); + if (!(clone_flags & CLONE_THREAD)) + thread_local_abi_fork(p); perf_event_fork(p); trace_task_newtask(p, clone_flags); diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 4d568ac..b78f92f 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2120,6 +2120,10 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p) p->numa_group = NULL; #endif /* CONFIG_NUMA_BALANCING */ +#ifdef CONFIG_THREAD_LOCAL_ABI + p->thread_local_abi_len = 0; + p->thread_local_abi = NULL; +#endif } DEFINE_STATIC_KEY_FALSE(sched_numa_balancing); diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index efd3bfc..d828b97 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -957,6 +957,8 @@ static inline void __set_task_cpu(struct task_struct *p, unsigned int cpu) { set_task_rq(p, cpu); #ifdef CONFIG_SMP + if (getcpu_cache_active(p)) + set_tsk_thread_flag(p, TIF_NOTIFY_RESUME); /* * 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..e803824 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_thread_local_abi); diff --git a/kernel/thread_local_abi.c b/kernel/thread_local_abi.c new file mode 100644 index 0000000..f05505a --- /dev/null +++ b/kernel/thread_local_abi.c @@ -0,0 +1,92 @@ +/* + * Copyright (C) 2015 Mathieu Desnoyers <mathieu.desnoy...@efficios.com> + * + * thread_local_abi 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/init.h> +#include <linux/sched.h> +#include <linux/uaccess.h> +#include <linux/syscalls.h> + +static int getcpu_cache_update(struct task_struct *t) +{ + if (put_user(raw_smp_processor_id(), &t->thread_local_abi->cpu)) { + t->thread_local_abi_len = 0; + t->thread_local_abi = NULL; + 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) +{ + BUG_ON(!getcpu_cache_active(t)); + if (unlikely(t->flags & PF_EXITING)) + return; + if (getcpu_cache_update(t)) + 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 thread_local_abi_fork(struct task_struct *t) +{ + t->thread_local_abi_len = current->thread_local_abi_len; + t->thread_local_abi = current->thread_local_abi; +} + +void thread_local_abi_execve(struct task_struct *t) +{ + t->thread_local_abi_len = 0; + t->thread_local_abi = NULL; +} + +/* + * sys_thread_local_abi - setup thread-local ABI for caller thread + */ +SYSCALL_DEFINE3(thread_local_abi, struct thread_local_abi __user *, tlap, + size_t, len, int, flags) +{ + size_t minlen; + + if (flags) + return -EINVAL; + if (current->thread_local_abi && tlap) + return -EBUSY; + /* Agree on the intersection of userspace and kernel features */ + if (!tlap) + minlen = 0; + else + minlen = min_t(size_t, len, sizeof(struct thread_local_abi)); + current->thread_local_abi_len = minlen; + current->thread_local_abi = tlap; + /* + * Migration checks ->thread_local_abi_len to see if notify_resume + * flag should be set. Therefore, we need to ensure that + * the scheduler sees ->thread_local_abi_len before we update + * the getcpu cache content with the current CPU number. + */ + barrier(); /* Store thread_local_abi_len before update content */ + if (getcpu_cache_active(current)) { + if (getcpu_cache_update(current)) + return -EFAULT; + } + return minlen; +} -- 2.1.4 -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
