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authorPaolo Bonzini <pbonzini@redhat.com>2013-11-20 12:54:02 +0100
committerPaolo Bonzini <pbonzini@redhat.com>2013-11-21 17:39:20 +0100
commitb5fc314bcbb80f76b8deaf23a4c45767b87f750b (patch)
tree11abb9fb6a5fef0fdeddec46b4661d01beef076d /hw
parentfbdcec5c487685b46e78f1e40a236ebf83f862fa (diff)
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target-i386: yield to another VCPU on PAUSE
After commit b1bbfe7 (aio / timers: On timer modification, qemu_notify or aio_notify, 2013-08-21) FreeBSD guests report a huge slowdown. The problem shows up as soon as FreeBSD turns out its periodic (~1 ms) tick, but the timers are only the trigger for a pre-existing problem. Before the offending patch, setting a timer did a timer_settime system call. After, setting the timer exits the event loop (which uses poll) and reenters it with a new deadline. This does not cause any slowdown; the difference is between one system call (timer_settime and a signal delivery (SIGALRM) before the patch, and two system calls afterwards (write to a pipe or eventfd + calling poll again when re-entering the event loop). Unfortunately, the exit/enter causes the main loop to grab the iothread lock, which in turns kicks the VCPU thread out of execution. This causes TCG to execute the next VCPU in its round-robin scheduling of VCPUS. When the second VCPU is mostly unused, FreeBSD runs a "pause" instruction in its idle loop which only burns cycles without any progress. As soon as the timer tick expires, the first VCPU runs the interrupt handler but very soon it sets it again---and QEMU then goes back doing nothing in the second VCPU. The fix is to make the pause instruction do "cpu_loop_exit". Reported-by: Luigi Rizzo <rizzo@iet.unipi.it> Reviewed-by: Richard Henderson <rth@twiddle.net> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'hw')
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