I am trying to create a system which has a deterministic realtime response.
I create a number of cpusets, move all non-critical tasks and unpinned kernel threads to one set, and then pin each of my realtime threads to its own cpuset, each of which consists of a single cpu.
$ non-critical tasks and unpinned kernel threads
cset proc --move --fromset=root --toset=system
cset proc --kthread --fromset=root --toset=system
$ realtime threads
cset proc --move --toset=shield/RealtimeTest1/thread1 --pid=17651
cset proc --move --toset=shield/RealtimeTest1/thread2 --pid=17654
My scenario is this:
- Thread 1:
SCHED_OTHER, pinned toset1, waiting onstd::future<void> - Thread 2:
SCHED_FIFO, pinned toset2, callsstd::promise<void>::set_value()
Thread 1 blocks forever.
However, if I change Thread 2 so be SCHED_OTHER, Thread 1 is able to continue.
I have run an strace -f to get more insight; it seems Thread 1 is waiting on a futex (I assume the internals of std::future) but is never woken up.
I’m absolutely stymied – is there any way to have a thread pin itself to a core and set its scheduler to FIFO , and then use a std::promise to wake up another thread which is waiting for it to complete this so-called realtime setup?
The code for thread1 creating thread2 is as follows:
// Thread1:
std::promise<void> p;
std::future <void> f = p.get_future();
_thread = std::move(std::thread(std::bind(&Dispatcher::Run, this, std::ref(p))));
LOG_INFO << "waiting for thread2 to start" << std::endl;
if (f.valid())
f.wait();
and the Run function for thread2 is as follows:
// Thread2:
LOG_INFO << "started: threadId=" << Thread::GetId() << std::endl;
Realtime::Service* rs = Service::Registry::Lookup<Realtime::Service>();
if (rs)
rs->ConfigureThread(this->Name()); // this does the pinning and FIFO etc
LOG_INFO << "thread2 has started" << std::endl;
p.set_value(); // indicate fact that the thread has started
The strace output follows:
- Thread 1 is
[pid 17651] - Thread 2 is
[pid 17654]
In the interests of brevity I have removed some of the output.
//////// Thread 1 creates thread 2 and waits on a future ////////
[pid 17654] gettid() = 17654
[pid 17651] write(2, "09:29:52 INFO waiting for thread"..., 4309:29:52 INFO waiting for thread2 to start
<unfinished ...>
[pid 17654] gettid( <unfinished ...>
[pid 17651] <... write resumed> ) = 43
[pid 17654] <... gettid resumed> ) = 17654
[pid 17651] futex(0xd52294, FUTEX_WAIT_PRIVATE, 1, NULL <unfinished ...>
[pid 17654] gettid() = 17654
[pid 17654] write(2, "09:29:52 INFO thread2 started: t"..., 6109:29:52 INFO thread2 started: threadId=17654
) = 61
//////// <snip> thread2 performs pinning, FIFO, etc </snip> ////////
[pid 17654] write(2, "09:29:52 INFO thread2 has starte"..., 3409:29:52 INFO thread2 has started
) = 34
[pid 17654] futex(0xd52294, FUTEX_CMP_REQUEUE_PRIVATE, 1, 2147483647, 0xd52268, 2) = 1
[pid 17651] <... futex resumed> ) = 0
[pid 17654] futex(0xd522c4, FUTEX_WAKE_PRIVATE, 2147483647 <unfinished ...>
[pid 17651] futex(0xd52268, FUTEX_WAKE_PRIVATE, 1 <unfinished ...>
[pid 17654] <... futex resumed> ) = 0
[pid 17651] <... futex resumed> ) = 0
//////// blocks here forever ////////
You can see that pid 17651 (thread1) reports futex resumed, but is it maybe running on the wrong cpu and getting blocked behind thread2 which is running as FIFO?
Update: It seems this is an issue with threads not running on the cpus they have been pinned to.
top -p 17649 -H with f,j to bring up the last used cpu shows that thread 1 is indeed running on thread 2’s cpu.
top - 10:00:59 up 18:17, 3 users, load average: 7.16, 7.61, 4.18
Tasks: 3 total, 2 running, 1 sleeping, 0 stopped, 0 zombie
Cpu(s): 7.1%us, 0.1%sy, 0.0%ni, 89.5%id, 0.0%wa, 0.0%hi, 3.3%si, 0.0%st
Mem: 8180892k total, 722800k used, 7458092k free, 43364k buffers
Swap: 8393952k total, 0k used, 8393952k free, 193324k cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ P COMMAND
17654 root -2 0 54080 35m 7064 R 100 0.4 5:00.77 3 RealtimeTest
17649 root 20 0 54080 35m 7064 S 0 0.4 0:00.05 2 RealtimeTest
17651 root 20 0 54080 35m 7064 R 0 0.4 0:00.00 3 RealtimeTest
However, if I look at the cpuset filesystem, I can see that my tasks are supposedly pinned to the cpus I requested:
/cpusets/shield/RealtimeTest1 $ for i in `find -name tasks`; do echo $i; cat $i; echo "------------"; done
./thread1/tasks
17651
------------
./main/tasks
17649
------------
./thread2/tasks
17654
------------
Displaying the cpuset config:
$ cset set --list -r
cset:
Name CPUs-X MEMs-X Tasks Subs Path
------------ ---------- - ------- - ----- ---- ----------
root 0-23 y 0-1 y 279 2 /
system 0,2,4,6,8,10 n 0 n 202 0 /system
shield 1,3,5,7,9,11 n 1 n 0 2 /shield
RealtimeTest1 1,3,5,7 n 1 n 0 4 /shield/RealtimeTest1
thread1 3 n 1 n 1 0 /shield/RealtimeTest1/thread1
thread2 5 n 1 n 1 0 /shield/RealtimeTest1/thread2
main 1 n 1 n 1 0 /shield/RealtimeTest1/main
From this I would say that thread2 is supposed to be on cpu 5, but top says it’s running on cpu 3.
Interestingly, sched_getaffinity reports what cpuset does – that thread1 is on cpu 3 and thread2 is on cpu 5.
However, looking at /proc/17649/task to find the last_cpu each of its tasks ran on:
/proc/17649/task $ for i in `ls -1`; do cat $i/stat | awk '{print $1 " is on " $(NF - 5)}'; done
17649 is on 2
17651 is on 3
17654 is on 3
sched_getaffinity reports one thing, but reality is another
Interestingly, main thread [pid 17649] is supposed to be on cpu 1 (according to the cset output), but in fact it is running on cpu 2 (which is on another socket)
So I would say that cpuset is not working?
My machine configuration is:
$ cat /etc/SuSE-release
SUSE Linux Enterprise Server 11 (x86_64)
VERSION = 11
PATCHLEVEL = 1
$ uname -a
Linux foobar 2.6.32.12-0.7-default #1 SMP 2010-05-20 11:14:20 +0200 x86_64 x86_64 x86_64 GNU/Linux
I have reran the tests on a SLES 11 / SP 2 box, and the pinning works.
As such, I’m going to mark this as an answer, which is: This is an issue related to SP 1