----- On Dec 11, 2015, at 1:56 PM, Michael Kerrisk mtk.manpages@xxxxxxxxx wrote: > 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? Hi Michael, Not yet. I first want check whether the overall technical approach is deemed acceptable before adding documentation. Adding a manpage is going to be one of the first steps after we agree on the syscall interface. Or perhaps you are suggesting that adding a manpage at this RFC stage could help the interface discussion ? Thanks, Mathieu > > 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/ -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com -- 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
