Re: [Qemu-devel] [RFC v4 71/71] cpus-common: wait on the CPU lock for exclusive work completion

[Alex Bennée]

Emilio G. Cota <address@hidden> writes:

> The current implementation of exclusive work can suffer from high
> contention when the number of guest CPUs is large (> 16 or so). The
> reason is that all CPUs end up waiting on the same condvar/mutex pair,
> which unnecessarily slows them down to wake up.
<snip>

FWIW this is the commit that breaks the linux-test code. I get the
following:

(gdb) thread apply all bt

Thread 2 (Thread 0x7ffff64d3700 (LWP 33607)):
#0  syscall () at ../sysdeps/unix/sysv/linux/x86_64/syscall.S:38
#1  0x000055555574d6db in qemu_futex_wait (val=<optimized out>, f=<optimized 
out>) at /home/alex.bennee/lsrc/qemu.git/include/qemu/futex.h:29
#2  qemu_event_wait (address@hidden <rcu_call_ready_event>) at 
util/qemu-thread-posix.c:442
#3  0x0000555555754158 in call_rcu_thread (opaque=<optimized out>) at 
util/rcu.c:261
#4  0x00007ffff6b3e6db in start_thread (arg=0x7ffff64d3700) at 
pthread_create.c:463
#5  0x00007ffff686788f in clone () at 
../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Thread 1 (Thread 0x7ffff7fda500 (LWP 33603)):
#0  safe_syscall_base () at 
/home/alex.bennee/lsrc/qemu.git/linux-user/host/x86_64/safe-syscall.inc.S:75
#1  0x000055555563793a in safe_wait4 (rusage=0x0, options=<optimized out>, 
status=0x7fffffffc350, pid=1472725552) at 
/home/alex.bennee/lsrc/qemu.git/linux-user/syscall.c:730
#2  do_syscall1 (address@hidden, address@hidden, address@hidden, 
address@hidden, arg3=<optimized out>, arg4=0, arg5=4743376, arg6=4743168, 
arg8=0, arg7=0) at /home/alex.bennee/lsrc/qemu.git/linux-user/syscall.c:8784
#3  0x000055555564225a in do_syscall (address@hidden, num=260, arg1=33608, 
arg2=274886295360, arg3=<optimized out>, arg4=<optimized out>, arg5=4743376, 
arg6=4743168, arg7=0, arg8=0) at 
/home/alex.bennee/lsrc/qemu.git/linux-user/syscall.c:11432
#4  0x000055555564bc8a in cpu_loop (env=0x555557c88550) at 
/home/alex.bennee/lsrc/qemu.git/linux-user/aarch64/cpu_loop.c:88
#5  0x00005555555e581c in main (argc=<optimized out>, argv=0x7fffffffe038, 
envp=<optimized out>) at /home/alex.bennee/lsrc/qemu.git/linux-user/main.c:819

I don't seem to be able to step through it but it looks like the newly
spawned thread is blocked on the futex and the safe_wait4 never
progresses.

--
Alex Bennée