Users of TWA_SIGNAL need to break out of kernel waits loops, or force re-entry into the kernel, to ensure that the queued task_work is run. TWA_SIGNAL currently works like signal delivery in that sense, and uses the same delivery mechanism. This currently works well from a functional standpoint, but it is very heavy handed on a multithreaded application where sighand is shared between all threads and main process. Adding TWA_SIGNAL task_work on such setups need to grab the sighand->lock, which creates a hot spot for otherwise unrelated task_work. This lock grabbing is necessary on both the queue-work and run-work side of things, exacerbating the problem/contention.
This adds TIF_TASKWORK for x86, which if set, will return true on checking for pending signals. That in turn causes tasks to restart the system call, which will run the added task_work. If TIF_TASKWORK is available, we'll use that for notification when TWA_SIGNAL is specified. If it isn't available, the existing TIF_SIGPENDING path is used. Once all archs have added support for TIF_TASKWORK, we can kill the old code completely. That will also allow removal of JOBCTL_TASK_WORK and related code. On my test box, even just using 16 threads shows a nice improvement running an io_uring based echo server. stock kernel: 0.01% <= 0.1 milliseconds 95.86% <= 0.2 milliseconds 98.27% <= 0.3 milliseconds 99.71% <= 0.4 milliseconds 100.00% <= 0.5 milliseconds 100.00% <= 0.6 milliseconds 100.00% <= 0.7 milliseconds 100.00% <= 0.8 milliseconds 100.00% <= 0.9 milliseconds 100.00% <= 1.0 milliseconds 100.00% <= 1.1 milliseconds 100.00% <= 2 milliseconds 100.00% <= 3 milliseconds 100.00% <= 3 milliseconds 1378930.00 requests per second ~1600% CPU 1.38M requests/second, and all 16 CPUs are maxed out. patched kernel: 0.01% <= 0.1 milliseconds 98.24% <= 0.2 milliseconds 99.47% <= 0.3 milliseconds 99.99% <= 0.4 milliseconds 100.00% <= 0.5 milliseconds 100.00% <= 0.6 milliseconds 100.00% <= 0.7 milliseconds 100.00% <= 0.8 milliseconds 100.00% <= 0.9 milliseconds 100.00% <= 1.2 milliseconds 1666111.38 requests per second ~1450% CPU 1.67M requests/second, and we're no longer just hammering on the sighand lock. The original reporter states: "For 5.7.15 my benchmark achieves 1.6M qps and system cpu is at ~80%. for 5.7.16 or later it achieves only 1M qps and the system cpu is is at ~100%" with the only difference there being that TWA_SIGNAL is used unconditionally in 5.7.16, since we need it to be able to solve an inability to run task_work if the application is waiting in the kernel already on an event that needs task_work run to be satisfied. Also see commit 0ba9c9edcd15. Reported-by: Roman Gershman <[email protected]> Signed-off-by: Jens Axboe <[email protected]> --- arch/x86/include/asm/thread_info.h | 2 ++ arch/x86/kernel/signal.c | 32 +++++++++++++++++------------- include/linux/entry-common.h | 20 ++++++++++++++++--- include/linux/sched/signal.h | 19 ++++++++++++++++-- kernel/entry/common.c | 14 ++++++++++--- 5 files changed, 65 insertions(+), 22 deletions(-) diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h index 267701ae3d86..79fe7db3208c 100644 --- a/arch/x86/include/asm/thread_info.h +++ b/arch/x86/include/asm/thread_info.h @@ -93,6 +93,7 @@ struct thread_info { #define TIF_NOTSC 16 /* TSC is not accessible in userland */ #define TIF_IA32 17 /* IA32 compatibility process */ #define TIF_SLD 18 /* Restore split lock detection on context switch */ +#define TIF_TASKWORK 19 /* task_work pending */ #define TIF_MEMDIE 20 /* is terminating due to OOM killer */ #define TIF_POLLING_NRFLAG 21 /* idle is polling for TIF_NEED_RESCHED */ #define TIF_IO_BITMAP 22 /* uses I/O bitmap */ @@ -123,6 +124,7 @@ struct thread_info { #define _TIF_NOTSC (1 << TIF_NOTSC) #define _TIF_IA32 (1 << TIF_IA32) #define _TIF_SLD (1 << TIF_SLD) +#define _TIF_TASKWORK (1 << TIF_TASKWORK) #define _TIF_POLLING_NRFLAG (1 << TIF_POLLING_NRFLAG) #define _TIF_IO_BITMAP (1 << TIF_IO_BITMAP) #define _TIF_FORCED_TF (1 << TIF_FORCED_TF) diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c index be0d7d4152ec..5dc1eeaf0866 100644 --- a/arch/x86/kernel/signal.c +++ b/arch/x86/kernel/signal.c @@ -799,21 +799,8 @@ static inline unsigned long get_nr_restart_syscall(const struct pt_regs *regs) #endif } -/* - * Note that 'init' is a special process: it doesn't get signals it doesn't - * want to handle. Thus you cannot kill init even with a SIGKILL even by - * mistake. - */ -void arch_do_signal(struct pt_regs *regs) +void arch_restart_syscall(struct pt_regs *regs) { - struct ksignal ksig; - - if (get_signal(&ksig)) { - /* Whee! Actually deliver the signal. */ - handle_signal(&ksig, regs); - return; - } - /* Did we come from a system call? */ if (syscall_get_nr(current, regs) >= 0) { /* Restart the system call - no handlers present */ @@ -831,12 +818,29 @@ void arch_do_signal(struct pt_regs *regs) break; } } +} + +/* + * Note that 'init' is a special process: it doesn't get signals it doesn't + * want to handle. Thus you cannot kill init even with a SIGKILL even by + * mistake. + */ +bool arch_do_signal(struct pt_regs *regs) +{ + struct ksignal ksig; + + if (get_signal(&ksig)) { + /* Whee! Actually deliver the signal. */ + handle_signal(&ksig, regs); + return true; + } /* * If there's no signal to deliver, we just put the saved sigmask * back. */ restore_saved_sigmask(); + return false; } void signal_fault(struct pt_regs *regs, void __user *frame, char *where) diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h index 159c7476b11b..03cab8b9ddab 100644 --- a/include/linux/entry-common.h +++ b/include/linux/entry-common.h @@ -37,6 +37,10 @@ # define _TIF_UPROBE (0) #endif +#ifndef _TIF_TASKWORK +# define _TIF_TASKWORK (0) +#endif + /* * TIF flags handled in syscall_enter_from_usermode() */ @@ -69,7 +73,7 @@ #define EXIT_TO_USER_MODE_WORK \ (_TIF_SIGPENDING | _TIF_NOTIFY_RESUME | _TIF_UPROBE | \ - _TIF_NEED_RESCHED | _TIF_PATCH_PENDING | \ + _TIF_NEED_RESCHED | _TIF_PATCH_PENDING | _TIF_TASKWORK| \ ARCH_EXIT_TO_USER_MODE_WORK) /** @@ -262,9 +266,19 @@ static __always_inline void arch_exit_to_user_mode(void) { } * arch_do_signal - Architecture specific signal delivery function * @regs: Pointer to currents pt_regs * - * Invoked from exit_to_user_mode_loop(). + * Invoked from exit_to_user_mode_loop(). Returns true if a signal was + * handled. + */ +bool arch_do_signal(struct pt_regs *regs); + +/** + * arch_restart_syscall - Architecture specific syscall restarting + * @regs: Pointer to currents pt_regs + * + * Invoked from exit_to_user_mode_loop(), if we need to restart the current + * system call. */ -void arch_do_signal(struct pt_regs *regs); +void arch_restart_syscall(struct pt_regs *regs); /** * arch_syscall_exit_tracehook - Wrapper around tracehook_report_syscall_exit() diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h index e6f34d8fbf4d..3093a7d30a24 100644 --- a/include/linux/sched/signal.h +++ b/include/linux/sched/signal.h @@ -360,6 +360,15 @@ static inline int task_sigpending(struct task_struct *p) static inline int signal_pending(struct task_struct *p) { +#ifdef TIF_TASKWORK + /* + * IF_TASKWORK isn't really a signal, but it requires the same + * behavior in terms of ensuring that we break out of wait loops + * so that task_work can be processed. + */ + if (unlikely(test_tsk_thread_flag(p, TIF_TASKWORK))) + return 1; +#endif return task_sigpending(p); } @@ -506,10 +515,16 @@ extern int set_user_sigmask(const sigset_t __user *umask, size_t sigsetsize); static inline void restore_saved_sigmask_unless(bool interrupted) { - if (interrupted) + if (interrupted) { +#ifdef TIF_TASKWORK + WARN_ON(!test_thread_flag(TIF_SIGPENDING) && + !test_thread_flag(TIF_TASKWORK)); +#else WARN_ON(!test_thread_flag(TIF_SIGPENDING)); - else +#endif + } else { restore_saved_sigmask(); + } } static inline sigset_t *sigmask_to_save(void) diff --git a/kernel/entry/common.c b/kernel/entry/common.c index 6fdb6105e6d6..d25ee8f7f071 100644 --- a/kernel/entry/common.c +++ b/kernel/entry/common.c @@ -135,11 +135,13 @@ static __always_inline void exit_to_user_mode(void) } /* Workaround to allow gradual conversion of architecture code */ -void __weak arch_do_signal(struct pt_regs *regs) { } +bool __weak arch_do_signal(struct pt_regs *regs) { return true; } static unsigned long exit_to_user_mode_loop(struct pt_regs *regs, unsigned long ti_work) { + bool restart_sys = (ti_work & (_TIF_SIGPENDING|_TIF_TASKWORK)) != 0; + /* * Before returning to user space ensure that all pending work * items have been completed. @@ -157,8 +159,11 @@ static unsigned long exit_to_user_mode_loop(struct pt_regs *regs, if (ti_work & _TIF_PATCH_PENDING) klp_update_patch_state(current); - if (ti_work & _TIF_SIGPENDING) - arch_do_signal(regs); + if (ti_work & _TIF_TASKWORK) + task_work_run(); + + if ((ti_work & _TIF_SIGPENDING) && arch_do_signal(regs)) + restart_sys = false; if (ti_work & _TIF_NOTIFY_RESUME) { clear_thread_flag(TIF_NOTIFY_RESUME); @@ -178,6 +183,9 @@ static unsigned long exit_to_user_mode_loop(struct pt_regs *regs, ti_work = READ_ONCE(current_thread_info()->flags); } + if (restart_sys) + arch_restart_syscall(regs); + /* Return the latest work state for arch_exit_to_user_mode() */ return ti_work; } -- 2.28.0
