mirror of https://gitlab.com/qemu-project/qemu
6b8f0187a4
icount has become much slower after tcg_cpu_exec has stopped using the BQL. There is also a latent bug that is masked by the slowness. The slowness happens because every occurrence of a QEMU_CLOCK_VIRTUAL timer now has to wake up the I/O thread and wait for it. The rendez-vous is mediated by the BQL QemuMutex: - handle_icount_deadline wakes up the I/O thread with BQL taken - the I/O thread wakes up and waits on the BQL - the VCPU thread releases the BQL a little later - the I/O thread raises an interrupt, which calls qemu_cpu_kick - the VCPU thread notices the interrupt, takes the BQL to process it and waits on it All this back and forth is extremely expensive, causing a 6 to 8-fold slowdown when icount is turned on. One may think that the issue is that the VCPU thread is too dependent on the BQL, but then the latent bug comes in. I first tried removing the BQL completely from the x86 cpu_exec, only to see everything break. The only way to fix it (and make everything slow again) was to add a dummy BQL lock/unlock pair. This is because in -icount mode you really have to process the events before the CPU restarts executing the next instruction. Therefore, this series moves the processing of QEMU_CLOCK_VIRTUAL timers straight in the vCPU thread when running in icount mode. The required changes include: - make the timer notification callback wake up TCG's single vCPU thread when run from another thread. By using async_run_on_cpu, the callback can override all_cpu_threads_idle() when the CPU is halted. - move handle_icount_deadline after qemu_tcg_wait_io_event, so that the timer notification callback is invoked after the dummy work item wakes up the vCPU thread - make handle_icount_deadline run the timers instead of just waking the I/O thread. - stop processing the timers in the main loop Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> |
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| .. | ||
| block | ||
| crypto | ||
| disas | ||
| exec | ||
| fpu | ||
| hw | ||
| io | ||
| libdecnumber | ||
| migration | ||
| monitor | ||
| net | ||
| qapi | ||
| qemu | ||
| qom | ||
| standard-headers | ||
| sysemu | ||
| ui | ||
| elf.h | ||
| glib-compat.h | ||
| qemu-common.h | ||
| qemu-io.h | ||
| trace-tcg.h | ||