Hi Mathieu, On 12/10/2015 04:39 PM, Mathieu Desnoyers wrote: > 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: Is there a man page for this system call? Thanks, Michael > - 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.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: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > CC: linux-api@xxxxxxxxxxxxxxx > --- > 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.desnoyers@xxxxxxxxxxxx> > + * > + * 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.desnoyers@xxxxxxxxxxxx> > + * > + * 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; > +} > -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- 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
