On 26.05.24 г. 4:58 ч., Chen Yu wrote:
The kernel can change spinlock behavior when running as a guest. But
this guest-friendly behavior causes performance problems on bare metal.
So there's a 'virt_spin_lock_key' static key to switch between the two
modes.
The static key is always enabled by default (run in guest mode) and
should be disabled for bare metal (and in some guests that want native
behavior).
Performance drop is reported when running encode/decode workload and
BenchSEE cache sub-workload.
Bisect points to commit ce0a1b608bfc ("x86/paravirt: Silence unused
native_pv_lock_init() function warning"). When CONFIG_PARAVIRT_SPINLOCKS
is disabled the virt_spin_lock_key is incorrectly set to true on bare
metal. The qspinlock degenerates to test-and-set spinlock, which
decrease the performance on bare metal.
Fix this by disabling virt_spin_lock_key if it is on bare metal,
regardless of CONFIG_PARAVIRT_SPINLOCKS.
nit:
This bug wouldn't have happened if the key was defined FALSE by default
and only enabled in the appropriate case. I think it makes more sense to
invert the logic and have the key FALSE by default and only enable it
iff the kernel is running under a hypervisor... At worst only the
virtualization case would suffer if the lock is falsely not enabled.
