[PATCH RFC v2] kernel: decouple TASK_WORK TWA_SIGNAL handling from signals
From: Jens Axboe
Date: Thu Oct 01 2020 - 11:03:55 EST
Users of TWA_SIGNAL need to have system calls interrupted and go through
a kernel/user transition to ensure they are run. This currently works
well from a functional standpoint, but it is heavy handed on a
multithreaded application where sighand is shared between the 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 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 <romger@xxxxxxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
---
V2: Ensure we don't restart syscall if we did deliver a signal
This likely needs some splitting into two parts, but figured I'd toss
this one out there for comments. It's fixing a performance regression
for me, while still fixing the original issue that introduced the
regression.
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/fs/io_uring.c b/fs/io_uring.c
index eed0d068904c..e0fec19b5faa 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1767,7 +1767,7 @@ static int io_req_task_work_add(struct io_kiocb *req, struct callback_head *cb,
notify = TWA_SIGNAL;
ret = task_work_add(tsk, cb, notify);
- if (!ret)
+ if (!ret && !notify)
wake_up_process(tsk);
return ret;
@@ -6744,27 +6744,17 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
iowq.nr_timeouts = atomic_read(&ctx->cq_timeouts);
trace_io_uring_cqring_wait(ctx, min_events);
- do {
+ while (!io_should_wake(&iowq, false)) {
prepare_to_wait_exclusive(&ctx->wait, &iowq.wq,
TASK_INTERRUPTIBLE);
- /* make sure we run task_work before checking for signals */
if (io_run_task_work())
continue;
if (signal_pending(current)) {
- if (current->jobctl & JOBCTL_TASK_WORK) {
- spin_lock_irq(¤t->sighand->siglock);
- current->jobctl &= ~JOBCTL_TASK_WORK;
- recalc_sigpending();
- spin_unlock_irq(¤t->sighand->siglock);
- continue;
- }
ret = -EINTR;
break;
}
- if (io_should_wake(&iowq, false))
- break;
schedule();
- } while (1);
+ }
finish_wait(&ctx->wait, &iowq.wq);
restore_saved_sigmask_unless(ret == -EINTR);
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 1bad18a1d8ba..d1a02749df74 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -355,7 +355,17 @@ static inline int restart_syscall(void)
static inline int signal_pending(struct task_struct *p)
{
- return unlikely(test_tsk_thread_flag(p,TIF_SIGPENDING));
+#ifdef TIF_TASKWORK
+ /*
+ * TIF_TASKWORK isn't really a signal, but it requires the same
+ * behavior of restarting the system call to force a kernel/user
+ * transition.
+ */
+ return unlikely(test_tsk_thread_flag(p, TIF_SIGPENDING) ||
+ test_tsk_thread_flag(p, TIF_TASKWORK));
+#else
+ return unlikely(test_tsk_thread_flag(p, TIF_SIGPENDING));
+#endif
}
static inline int __fatal_signal_pending(struct task_struct *p)
@@ -501,10 +511,16 @@ 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;
}
diff --git a/kernel/task_work.c b/kernel/task_work.c
index 613b2d634af8..3345a12df643 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -28,7 +28,6 @@ int
task_work_add(struct task_struct *task, struct callback_head *work, int notify)
{
struct callback_head *head;
- unsigned long flags;
do {
head = READ_ONCE(task->task_works);
@@ -41,7 +40,10 @@ task_work_add(struct task_struct *task, struct callback_head *work, int notify)
case TWA_RESUME:
set_notify_resume(task);
break;
- case TWA_SIGNAL:
+ case TWA_SIGNAL: {
+#ifndef TIF_TASKWORK
+ unsigned long flags;
+
/*
* Only grab the sighand lock if we don't already have some
* task_work pending. This pairs with the smp_store_mb()
@@ -53,7 +55,12 @@ task_work_add(struct task_struct *task, struct callback_head *work, int notify)
signal_wake_up(task, 0);
unlock_task_sighand(task, &flags);
}
+#else
+ set_tsk_thread_flag(task, TIF_TASKWORK);
+ wake_up_process(task);
+#endif
break;
+ }
}
return 0;
@@ -110,6 +117,11 @@ void task_work_run(void)
struct task_struct *task = current;
struct callback_head *work, *head, *next;
+#ifdef TIF_TASKWORK
+ if (test_tsk_thread_flag(task, TIF_TASKWORK))
+ clear_tsk_thread_flag(task, TIF_TASKWORK);
+#endif
+
for (;;) {
/*
* work->func() can do task_work_add(), do not set
--
Jens Axboe