Re: [PATCH 3/5] KVM: Conditionally reschedule when resetting the dirty ring

[Yan Zhao <yan.y.zhao@intel.com>]

On Fri, Jan 10, 2025 at 05:04:07PM -0800, Sean Christopherson wrote:
> When resetting a dirty ring, conditionally reschedule on each iteration
> after the first.  The recently introduced hard limit mitigates the issue
> of an endless reset, but isn't sufficient to completely prevent RCU
> stalls, soft lockups, etc., nor is the hard limit intended to guard
> against such badness.
> 
> Note!  Take care to check for reschedule even in the "continue" paths,
> as a pathological scenario (or malicious userspace) could dirty the same
> gfn over and over, i.e. always hit the continue path.
> 
>   rcu: INFO: rcu_sched self-detected stall on CPU
>   rcu: 	4-....: (5249 ticks this GP) idle=51e4/1/0x4000000000000000 softirq=309/309 fqs=2563
>   rcu: 	(t=5250 jiffies g=-319 q=608 ncpus=24)
>   CPU: 4 UID: 1000 PID: 1067 Comm: dirty_log_test Tainted: G             L     6.13.0-rc3-17fa7a24ea1e-HEAD-vm #814
>   Tainted: [L]=SOFTLOCKUP
>   Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
>   RIP: 0010:kvm_arch_mmu_enable_log_dirty_pt_masked+0x26/0x200 [kvm]
>   Call Trace:
>    <TASK>
>    kvm_reset_dirty_gfn.part.0+0xb4/0xe0 [kvm]
>    kvm_dirty_ring_reset+0x58/0x220 [kvm]
>    kvm_vm_ioctl+0x10eb/0x15d0 [kvm]
>    __x64_sys_ioctl+0x8b/0xb0
>    do_syscall_64+0x5b/0x160
>    entry_SYSCALL_64_after_hwframe+0x4b/0x53
>    </TASK>
>   Tainted: [L]=SOFTLOCKUP
>   Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
>   RIP: 0010:kvm_arch_mmu_enable_log_dirty_pt_masked+0x17/0x200 [kvm]
>   Call Trace:
>    <TASK>
>    kvm_reset_dirty_gfn.part.0+0xb4/0xe0 [kvm]
>    kvm_dirty_ring_reset+0x58/0x220 [kvm]
>    kvm_vm_ioctl+0x10eb/0x15d0 [kvm]
>    __x64_sys_ioctl+0x8b/0xb0
>    do_syscall_64+0x5b/0x160
>    entry_SYSCALL_64_after_hwframe+0x4b/0x53
>    </TASK>
> 
> Fixes: fb04a1eddb1a ("KVM: X86: Implement ring-based dirty memory tracking")
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  virt/kvm/dirty_ring.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/virt/kvm/dirty_ring.c b/virt/kvm/dirty_ring.c
> index a81ad17d5eef..37eb2b7142bd 100644
> --- a/virt/kvm/dirty_ring.c
> +++ b/virt/kvm/dirty_ring.c
> @@ -133,6 +133,16 @@ int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring,
>  
>  		ring->reset_index++;
>  		(*nr_entries_reset)++;
> +
> +		/*
> +		 * While the size of each ring is fixed, it's possible for the
> +		 * ring to be constantly re-dirtied/harvested while the reset
> +		 * is in-progress (the hard limit exists only to guard against
> +		 * wrapping the count into negative space).
> +		 */
> +		if (!first_round)
> +			cond_resched();
> +
Will cond_resched() per entry be too frequent?
Could we combine the cond_resched() per ring? e.g.

if (count >= ring->soft_limit)
	cond_resched();

or simply
while (count < ring->size) {
	...
}


>  		/*
>  		 * Try to coalesce the reset operations when the guest is
>  		 * scanning pages in the same slot.
> -- 
> 2.47.1.613.gc27f4b7a9f-goog
>