Re: [RFC PATCH 1/2] thread_local_abi system call: caching current CPU number (x86)

Mathieu Desnoyers
2015-12-12T12:40:57Z

----- On Dec 11, 2015, at 1:56 PM, Michael Kerrisk mtk.manpages@gmail.com 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@efficios.com>
>> CC: Thomas Gleixner <tglx@linutronix.de>
>> CC: Paul Turner <pjt@google.com>
>> CC: Andrew Hunter <ahh@google.com>
>> CC: Peter Zijlstra <peterz@infradead.org>
>> CC: Andy Lutomirski <luto@amacapital.net>
>> CC: Andi Kleen <andi@firstfloor.org>
>> CC: Dave Watson <davejwatson@fb.com>
>> CC: Chris Lameter <cl@linux.com>
>> CC: Ingo Molnar <mingo@redhat.com>
>> CC: Ben Maurer <bmaurer@fb.com>
>> CC: Steven Rostedt <rostedt@goodmis.org>
>> CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
>> CC: Josh Triplett <josh@joshtriplett.org>
>> CC: Linus Torvalds <torvalds@linux-foundation.org>
>> CC: Andrew Morton <akpm@linux-foundation.org>
>> CC: Thomas Gleixner <tglx@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.desnoyers@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.desnoyers@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;
>> +}
>>
>
>
> --
> 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.