On Fri, 24 Apr 2020 at 13:53, Davidlohr Bueso <dave@xxxxxxxxxxxx> 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
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