umtx: allow to configure minimal timeout (in nanoseconds)

PR:	270785
Reviewed by:	markj, mav
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D39584

lib/libc/sys/_umtx_op.2

@@ -1270,6 +1270,17 @@ in which case list processing is continued.
 See
 .Sx ROBUST UMUTEXES
 subsection for details.
+.It Dv UMTX_OP_GET_MIN_TIMEOUT
+Writes out the current value of minimal umtx operations timeout,
+in nanoseconds, into the long integer variable pointed to by
+.Fa uaddr1 .
+.It Dv UMTX_OP_SET_MIN_TIMEOUT
+Set the minimal amount of time, in nanoseconds, the thread is required
+to sleep for umtx operations specifying a timeout using absolute clocks.
+The value is taken from the
+.Fa val
+argument of the call.
+Zero means no minimum.
 .El
 .Pp
 The

lib/libthr/libthr.3

@@ -196,6 +196,12 @@ The integer value of the variable specifies how often blocked
 threads are inserted at the head of the sleep queue, instead of its tail.
 Bigger values reduce the frequency of the FIFO discipline.
 The value must be between 0 and 255.
+.It Dv LIBPTHREAD_UMTX_MIN_TIMEOUT
+The minimal amount of time, in nanoseconds, the thread is required to sleep
+for pthread operations specifying a timeout.
+If the operation requests a timeout less than the value provided,
+it is silently increased to the value.
+The value of zero means no minimum (default).
 .Pp
 .El
 The following

lib/libthr/thread/thr_init.c

@@ -522,6 +522,17 @@ init_private(void)
 		if (env)
 			_thr_queuefifo = atoi(env);
 		TAILQ_INIT(&_thr_atfork_list);
+		env = getenv("LIBPTHREAD_UMTX_MIN_TIMEOUT");
+		if (env) {
+			char *endptr;
+			long mint;
+
+			mint = strtol(env, &endptr, 0);
+			if (*endptr == '\0' && mint >= 0) {
+				_umtx_op(NULL, UMTX_OP_SET_MIN_TIMEOUT,
+				    mint, NULL, NULL);
+			}
+		}
 	}
 	init_once = 1;
 }

sys/kern/kern_thread.c

@@ -101,7 +101,7 @@ _Static_assert(offsetof(struct proc, p_filemon) == 0x3c8,
     "struct proc KBI p_filemon");
 _Static_assert(offsetof(struct proc, p_comm) == 0x3e0,
     "struct proc KBI p_comm");
-_Static_assert(offsetof(struct proc, p_emuldata) == 0x4c8,
+_Static_assert(offsetof(struct proc, p_emuldata) == 0x4d0,
     "struct proc KBI p_emuldata");
 #endif
 #ifdef __i386__
@@ -121,7 +121,7 @@ _Static_assert(offsetof(struct proc, p_filemon) == 0x270,
     "struct proc KBI p_filemon");
 _Static_assert(offsetof(struct proc, p_comm) == 0x284,
     "struct proc KBI p_comm");
-_Static_assert(offsetof(struct proc, p_emuldata) == 0x310,
+_Static_assert(offsetof(struct proc, p_emuldata) == 0x318,
     "struct proc KBI p_emuldata");
 #endif
 

sys/kern/kern_umtx.c

@@ -700,6 +700,19 @@ umtx_abs_timeout_init2(struct umtx_abs_timeout *timo,
 	    (umtxtime->_flags & UMTX_ABSTIME) != 0, &umtxtime->_timeout);
 }
 
+static void
+umtx_abs_timeout_enforce_min(sbintime_t *sbt)
+{
+	sbintime_t when, mint;
+
+	mint = curproc->p_umtx_min_timeout;
+	if (__predict_false(mint != 0)) {
+		when = sbinuptime() + mint;
+		if (*sbt < when)
+			*sbt = when;
+	}
+}
+
 static int
 umtx_abs_timeout_getsbt(struct umtx_abs_timeout *timo, sbintime_t *sbt,
     int *flags)
@@ -739,6 +752,7 @@ umtx_abs_timeout_getsbt(struct umtx_abs_timeout *timo, sbintime_t *sbt,
 			return (0);
 		}
 		*sbt = bttosbt(bt);
+		umtx_abs_timeout_enforce_min(sbt);
 
 		/*
 		 * Check if the absolute time should be aligned to
@@ -4594,6 +4608,33 @@ __umtx_op_robust_lists(struct thread *td, struct _umtx_op_args *uap,
 	return (0);
 }
 
+static int
+__umtx_op_get_min_timeout(struct thread *td, struct _umtx_op_args *uap,
+    const struct umtx_copyops *ops)
+{
+	long val;
+	int error, val1;
+
+	val = sbttons(td->td_proc->p_umtx_min_timeout);
+	if (ops->compat32) {
+		val1 = (int)val;
+		error = copyout(&val1, uap->uaddr1, sizeof(val1));
+	} else {
+		error = copyout(&val, uap->uaddr1, sizeof(val));
+	}
+	return (error);
+}
+
+static int
+__umtx_op_set_min_timeout(struct thread *td, struct _umtx_op_args *uap,
+    const struct umtx_copyops *ops)
+{
+	if (uap->val < 0)
+		return (EINVAL);
+	td->td_proc->p_umtx_min_timeout = nstosbt(uap->val);
+	return (0);
+}
+
 #if defined(__i386__) || defined(__amd64__)
 /*
  * Provide the standard 32-bit definitions for x86, since native/compat32 use a
@@ -4816,6 +4857,8 @@ static const _umtx_op_func op_table[] = {
 	[UMTX_OP_SEM2_WAKE]	= __umtx_op_sem2_wake,
 	[UMTX_OP_SHM]		= __umtx_op_shm,
 	[UMTX_OP_ROBUST_LISTS]	= __umtx_op_robust_lists,
+	[UMTX_OP_GET_MIN_TIMEOUT] = __umtx_op_get_min_timeout,
+	[UMTX_OP_SET_MIN_TIMEOUT] = __umtx_op_set_min_timeout,
 };
 
 static const struct umtx_copyops umtx_native_ops = {
@@ -4990,6 +5033,8 @@ umtx_exec(struct proc *p)
 		umtx_thread_cleanup(td);
 		td->td_rb_list = td->td_rbp_list = td->td_rb_inact = 0;
 	}
+
+	p->p_umtx_min_timeout = 0;
 }
 
 /*

sys/sys/proc.h

@@ -741,6 +741,7 @@ struct proc {
 	uint64_t	p_elf_flags;	/* (x) ELF flags */
 	void		*p_elf_brandinfo; /* (x) Elf_Brandinfo, NULL for
 						 non ELF binaries. */
+	sbintime_t	p_umtx_min_timeout;
 /* End area that is copied on creation. */
 #define	p_endcopy	p_xexit
 

sys/sys/umtx.h

@@ -103,6 +103,8 @@
 #define	UMTX_OP_SEM2_WAKE	24
 #define	UMTX_OP_SHM		25
 #define	UMTX_OP_ROBUST_LISTS	26
+#define	UMTX_OP_GET_MIN_TIMEOUT	27
+#define	UMTX_OP_SET_MIN_TIMEOUT	28
 
 /*
  * Flags for ops; the double-underbar convention must be maintained for future