From: Jamie Iles <ji...@qti.qualcomm.com> The round-robin scheduler will iterate over the CPU list with an assigned budget until the next timer expiry and may exit early because of a TB exit. This is fine under normal operation but with icount enabled and SMP it is possible for a CPU to be starved of run time and the system live-locks.
For example, booting a riscv64 platform with '-icount shift=0,align=off,sleep=on -smp 2' we observe a livelock once the kernel has timers enabled and starts performing TLB shootdowns. In this case we have CPU 0 in M-mode with interrupts disabled sending an IPI to CPU 1. As we enter the TCG loop, we assign the icount budget to next timer interrupt to CPU 0 and begin executing where the guest is sat in a busy loop exhausting all of the budget before we try to execute CPU 1 which is the target of the IPI but CPU 1 is left with no budget with which to execute and the process repeats. We try here to add some fairness by splitting the budget across all of the CPUs on the thread fairly before entering each one. The CPU count is cached on CPU list generation ID to avoid iterating the list on each loop iteration. With this change it is possible to boot an SMP rv64 guest with icount enabled and no hangs. New in v3 (address feedback from Richard Henderson): - Additional patch to use QEMU_LOCK_GUARD with qemu_cpu_list_lock where appropriate - Move rr_cpu_count() call to be conditional on icount_enabled() - Initialize cpu_budget to 0 Jamie Iles (2): cpu: expose qemu_cpu_list_lock for lock-guard use accel/tcg/tcg-accel-ops-rr: ensure fairness with icount accel/tcg/tcg-accel-ops-icount.c | 17 +++++++++++++-- accel/tcg/tcg-accel-ops-icount.h | 3 ++- accel/tcg/tcg-accel-ops-rr.c | 37 +++++++++++++++++++++++++++++++- cpus-common.c | 2 +- include/exec/cpu-common.h | 1 + linux-user/elfload.c | 12 +++++------ migration/dirtyrate.c | 26 +++++++++++----------- trace/control-target.c | 9 ++++---- 8 files changed, 78 insertions(+), 29 deletions(-) -- 2.25.1
