wq_has_sleeper(9) [centos man page]

WQ_HAS_SLEEPER(9)						 Linux Networking						 WQ_HAS_SLEEPER(9)

NAME

       wq_has_sleeper - check if there are any waiting processes

SYNOPSIS

       bool wq_has_sleeper(struct socket_wq * wq);

ARGUMENTS

       wq
	   struct socket_wq

DESCRIPTION

       Returns true if socket_wq has waiting processes

       The purpose of the wq_has_sleeper and sock_poll_wait is to wrap the memory barrier call. They were added due to the race found within the
       tcp code.

CONSIDER FOLLOWING TCP CODE PATHS

       CPU1 CPU2

       sys_select receive packet ... ... __add_wait_queue update tp->rcv_nxt ... ... tp->rcv_nxt check sock_def_readable ... { schedule
       rcu_read_lock; wq = rcu_dereference(sk->sk_wq); if (wq && waitqueue_active(wq->wait)) wake_up_interruptible(wq->wait) ... }

       The race for tcp fires when the __add_wait_queue changes done by CPU1 stay in its cache, and so does the tp->rcv_nxt update on CPU2 side.
       The CPU1 could then endup calling schedule and sleep forever if there are no more data on the socket.

COPYRIGHT

Kernel Hackers Manual 3.10					     June 2014							 WQ_HAS_SLEEPER(9)

sigwaittest(8)															    sigwaittest(8)

NAME

       sigwaittest - Start two threads or fork two processes and measure the latency between sending and receiving a signal

SYNTAX

       sigwaittest [-a|-a PROC] [-b USEC] [-d DIST] [-f] [-i INTV] [-l loops] [-p PRIO] [-t|-t NUM]

DESCRIPTION

       The  program  sigwaittest  starts two threads or, optionally, forks two processes that are synchonized via signals and measures the latency
       between sending a signal and returning from sigwait().

OPTIONS

       -a, --affinity[=PROC]
	      Run on procesor number PROC. If PROC is not specified, run on current processor.

       -b, --breaktrace=USEC
	      Send break trace command when latency > USEC. This is a debugging option to control the latency tracer in  the  realtime	preemption
	      patch.  It is useful to track down unexpected large latencies of a system.

       -d, --distance=DIST
	      Set the distance of thread intervals in microseconds (default is 500 us). When  cylictest is called with the -t option and more than
	      one thread is created, then this distance value is added to the interval of the threads: Interval(thread N) = Interval(thread N-1) +
	      DIST

       -f, --fork
	      Instead of creating threads (which is the default), fork new processes

       -i, --interval=INTV
	      Set  the	base  interval of the thread(s) in microseconds (default is 1000 us). This sets the interval of the first thread. See also
	      -d.

       -l, --loops=LOOPS
	      Set the number of loops. The default is 0 (endless). This option is useful for automated tests with a given number of  test  cycles.
	      sigwaittest is stopped once the number of timer intervals has been reached.

       -p, --prio=PRIO
	      Set the priority of the process.

       -t, --threads[=NUM]
	      Set  the number of test threads (default is 1, if this option is not given). If NUM is specified, create NUM test threads. If NUM is
	      not specifed, NUM is set to the number of available CPUs.

EXAMPLES

       The following example was running on a 4-way CPU:

       # sigwaittest -a -t -p99 -i100 -d25 -l1000000
       #0: ID11510, P99, CPU0, I100; #1: ID11511, P99, CPU0, Cycles 1000000
       #2: ID11512, P98, CPU1, I125; #3: ID11513, P98, CPU1, Cycles 817484
       #4: ID11514, P97, CPU2, I150; #5: ID11515, P97, CPU2, Cycles 668213
       #6: ID11516, P96, CPU3, I175; #7: ID11517, P96, CPU3, Cycles 597344
       #1 -> #0, Min	1, Cur	  2, Avg    3, Max   30
       #3 -> #2, Min	1, Cur	 26, Avg    3, Max   42
       #5 -> #4, Min	1, Cur	 46, Avg    4, Max   67
       #7 -> #6, Min	1, Cur	  2, Avg    3, Max   74

AUTHORS

       Carsten Emde <C.Emde@osadl.org>

SEE ALSO

       kill(2), sigwait(3)

									0.1							    sigwaittest(8)