aarch64a53-zynqmp-qemu: severe emulation lag and extended sleep times with CPU-intensive threads

[etiaro]

When running on aarch64a53-zynqmp-qemu target with thread=single qemu option, the entire emulation experiences significant lag if any thread in the system is computationally intensive. This is causing sleep() calls and other timers to take orders of magnitude longer than their specified duration.

The behavior can be observed in waitpid test, where due to starting process with dummy loop (for (;;) ;) sleeping takes longer than expected

After moving signals to kernel in #1340, and with that not interrupting sleep by ignored signals as specified in POSIX, another uninturrupted, longer sleep timeouts the waitpid test.

The problem disappears when thread=single option is removed

[nalajcie]

IMHO for (;;) ; is not needed in this test and should be changed to for(;;) sleep(10000) or something similar.

Unnecessary busy loops should be avoided.

[nalajcie]

@damianloew please assign somebody to removing infinite busy loop from this test and re-enabling it on zynq-qemu

[damianloew]

Sure, I thought that testing such case without sleep is more demanding for the system and it would good to leave it in this form. However, after some thinking, I came to the conclusion that the solution with sleep is closer to the realistic scenario and there is no significant benefits from leaving this busy waiting.