Re: [PATCH 5/5] kvm: Replace vcpu->swait with rcuwait

[Wanpeng Li <kernellwp@gmail.com>]

On Fri, 24 Apr 2020 at 13:53, Davidlohr Bueso <dave@stgolabs.net> wrote:
>
> The use of any sort of waitqueue (simple or regular) for
> wait/waking vcpus has always been an overkill and semantically
> wrong. Because this is per-vcpu (which is blocked) there is
> only ever a single waiting vcpu, thus no need for any sort of
> queue.
>
> As such, make use of the rcuwait primitive, with the following
> considerations:
>
>   - rcuwait already provides the proper barriers that serialize
>   concurrent waiter and waker.
>
>   - Task wakeup is done in rcu read critical region, with a
>   stable task pointer.
>
>   - Because there is no concurrency among waiters, we need
>   not worry about rcuwait_wait_event() calls corrupting
>   the wait->task. As a consequence, this saves the locking
>   done in swait when modifying the queue. This also applies
>   to per-vcore wait for powerpc kvm-hv.
>
> The x86 tscdeadline_latency test mentioned in 8577370fb0cb
> ("KVM: Use simple waitqueue for vcpu->wq") shows that, on avg,
> latency is reduced by around 15-20% with this change.
>

This is splatting when I run linux guest on latest kvm/queue.

[24726.009187] =============================
[24726.009193] WARNING: suspicious RCU usage
[24726.009201] 5.7.0-rc2+ #3 Not tainted
[24726.009207] -----------------------------
[24726.009215] ./include/linux/rcuwait.h:34 suspicious
rcu_dereference_check() usage!
[24726.009222]
other info that might help us debug this:

[24726.009229]
rcu_scheduler_active = 2, debug_locks = 1
[24726.009237] 2 locks held by qemu-system-x86/6094:
[24726.009244]  #0: ffff88837b6cb990 (&vcpu->mutex){+.+.}-{3:3}, at:
kvm_vcpu_ioctl+0x191/0xbb0 [kvm]
[24726.009347]  #1: ffffc900036c2c68 (&kvm->srcu){....}-{0:0}, at:
kvm_arch_vcpu_ioctl_run+0x17f1/0x5680 [kvm]
[24726.009386]
stack backtrace:
[24726.009394] CPU: 5 PID: 6094 Comm: qemu-system-x86 Not tainted 5.7.0-rc2+ #3
[24726.009400] Hardware name: LENOVO ThinkCentre M8500t-N000/SHARKBAY,
BIOS FBKTC1AUS 02/16/2016
[24726.009405] Call Trace:
[24726.009418]  dump_stack+0x98/0xd5
[24726.009432]  lockdep_rcu_suspicious+0x123/0x170
[24726.009465]  kvm_vcpu_on_spin+0x46f/0x5d0 [kvm]
[24726.009497]  handle_pause+0x7e/0x3e0 [kvm_intel]
[24726.009517]  vmx_handle_exit+0x1fe/0x1000 [kvm_intel]
[24726.009547]  ? kvm_arch_vcpu_ioctl_run+0x17c5/0x5680 [kvm]
[24726.009586]  kvm_arch_vcpu_ioctl_run+0x18f5/0x5680 [kvm]
[24726.009595]  ? check_chain_key+0x26e/0x670
[24726.009651]  ? kvm_arch_vcpu_runnable+0x540/0x540 [kvm]
[24726.009667]  ? tomoyo_execute_permission+0x4b0/0x4b0
[24726.009677]  ? sched_clock+0x31/0x40
[24726.009726]  kvm_vcpu_ioctl+0x5d2/0xbb0 [kvm]
[24726.009754]  ? kvm_vcpu_ioctl+0x5d2/0xbb0 [kvm]
[24726.009786]  ? kvm_set_memory_region+0x90/0x90 [kvm]
[24726.009819]  ? __fget_files+0x2d4/0x420
[24726.009837]  ? do_dup2+0x660/0x660
[24726.009847]  ? lock_acquire+0x1e5/0x660
[24726.009864]  ? tomoyo_file_ioctl+0x19/0x20
[24726.009880]  ksys_ioctl+0x3d2/0x1390
[24726.009900]  ? generic_block_fiemap+0x70/0x70
[24726.009911]  ? rcu_read_lock_sched_held+0xb4/0xe0
[24726.009920]  ? rcu_read_lock_any_held.part.9+0x20/0x20
[24726.009935]  ? __x64_sys_futex+0x1a1/0x400
[24726.009943]  ? __kasan_check_write+0x14/0x20
[24726.009951]  ? switch_fpu_return+0x181/0x3e0
[24726.009963]  ? do_futex+0x14e0/0x14e0
[24726.009970]  ? lockdep_hardirqs_off+0x1df/0x2d0
[24726.009977]  ? syscall_return_slowpath+0x66/0x9d0
[24726.009987]  ? do_syscall_64+0x8e/0xae0
[24726.009995]  ? entry_SYSCALL_64_after_hwframe+0x49/0xb3
[24726.010012]  __x64_sys_ioctl+0x73/0xb0
[24726.010023]  do_syscall_64+0x108/0xae0
[24726.010032]  ? trace_hardirqs_on_thunk+0x1a/0x1c
[24726.010042]  ? syscall_return_slowpath+0x9d0/0x9d0
[24726.010048]  ? trace_hardirqs_off_thunk+0x1a/0x1c
[24726.010059]  ? trace_hardirqs_off_caller+0x28/0x1b0
[24726.010074]  ? trace_hardirqs_off_thunk+0x1a/0x1c
[24726.010092]  entry_SYSCALL_64_after_hwframe+0x49/0xb3


[25372.530417] =============================
[25372.530420] WARNING: suspicious RCU usage
[25372.530425] 5.7.0-rc2+ #3 Not tainted
[25372.530428] -----------------------------
[25372.530432] ./include/linux/rcuwait.h:34 suspicious
rcu_dereference_check() usage!
[25372.530436]
other info that might help us debug this:

[25372.530440]
rcu_scheduler_active = 2, debug_locks = 1
[25372.530443] 1 lock held by qemu-system-x86/6433:
[25372.530447]  #0: ffff88837b6cb990 (&vcpu->mutex){+.+.}-{3:3}, at:
kvm_vcpu_ioctl+0x191/0xbb0 [kvm]
[25372.530483]
stack backtrace:
[25372.530487] CPU: 1 PID: 6433 Comm: qemu-system-x86 Not tainted 5.7.0-rc2+ #3
[25372.530491] Hardware name: LENOVO ThinkCentre M8500t-N000/SHARKBAY,
BIOS FBKTC1AUS 02/16/2016
[25372.530495] Call Trace:
[25372.530504]  dump_stack+0x98/0xd5
[25372.530513]  lockdep_rcu_suspicious+0x123/0x170
[25372.530547]  kvm_lapic_expired_hv_timer+0x1ad/0x1f0 [kvm]
[25372.530559]  vmx_vcpu_run+0x1892/0x2c60 [kvm_intel]
[25372.530568]  ? rcu_preempt_deferred_qs_handler+0x20/0x40
[25372.530579]  ? clear_atomic_switch_msr+0x970/0x970 [kvm_intel]
[25372.530586]  ? rcu_read_lock_any_held.part.9+0x20/0x20
[25372.530613]  kvm_arch_vcpu_ioctl_run+0x1579/0x5680 [kvm]
[25372.530619]  ? check_chain_key+0x26e/0x670
[25372.530654]  ? kvm_arch_vcpu_runnable+0x540/0x540 [kvm]
[25372.530665]  ? tomoyo_execute_permission+0x4b0/0x4b0
[25372.530671]  ? sched_clock+0x31/0x40
[25372.530678]  ? sched_clock_cpu+0x1b/0x1b0
[25372.530706]  kvm_vcpu_ioctl+0x5d2/0xbb0 [kvm]
[25372.530724]  ? kvm_vcpu_ioctl+0x5d2/0xbb0 [kvm]
[25372.530744]  ? kvm_set_memory_region+0x90/0x90 [kvm]
[25372.530764]  ? __fget_files+0x2d4/0x420
[25372.530775]  ? do_dup2+0x660/0x660
[25372.530779]  ? vfs_iter_write+0xb0/0xb0
[25372.530785]  ? rcu_read_lock_held+0xb4/0xc0
[25372.530795]  ? tomoyo_file_ioctl+0x19/0x20
[25372.530806]  ksys_ioctl+0x3d2/0x1390
[25372.530814]  ? do_dup2+0x660/0x660
[25372.530821]  ? generic_block_fiemap+0x70/0x70
[25372.530833]  ? __kasan_check_write+0x14/0x20
[25372.530838]  ? fput_many+0x20/0x140
[25372.530844]  ? fput+0x13/0x20
[25372.530848]  ? do_writev+0x175/0x320
[25372.530856]  ? lockdep_hardirqs_off+0x1df/0x2d0
[25372.530861]  ? syscall_return_slowpath+0x66/0x9d0
[25372.530867]  ? do_syscall_64+0x8e/0xae0
[25372.530872]  ? entry_SYSCALL_64_after_hwframe+0x49/0xb3
[25372.530883]  __x64_sys_ioctl+0x73/0xb0
[25372.530890]  do_syscall_64+0x108/0xae0
[25372.530894]  ? switch_fpu_return+0x181/0x3e0
[25372.530899]  ? trace_hardirqs_on_thunk+0x1a/0x1c
[25372.530905]  ? syscall_return_slowpath+0x9d0/0x9d0
[25372.530908]  ? trace_hardirqs_off_thunk+0x1a/0x1c
[25372.530917]  ? trace_hardirqs_off_caller+0x28/0x1b0
[25372.530926]  ? trace_hardirqs_off_thunk+0x1a/0x1c
[25372.530937]  entry_SYSCALL_64_after_hwframe+0x49/0xb3