On Wed, Mar 8, 2023 at 2:14 PM Junyan Lin <junyan.lin@xxxxxxxxxxxx> wrote: > > When Sstc is enabled, kvm_riscv_vcpu_timer_blocking() starts a hrtimer > to kick the sleeping VCPU when its next_cycles arrives. The hrtimer > would be started only if delta_ns is not 0. But sometimes when code runs > here, current_cycles has already become greater than the VCPU > next_cycles, and delta_ns is set to 0. At this time, the hrtimer won't > be started and the sleeping VCPU will never be woken up (unless kicked > elsewhere). > > To fix this issue, we can start the hrtimer as usual when delta_ns is 0. > It seems no harm is done if a hrtimer is started with relative expiry 0, > and it can trigger kvm_riscv_vcpu_vstimer_expired() ASAP to kick the > VCPU. > > Signed-off-by: Junyan Lin Similar patch is already queued for Linux-6.3 RC fixes. Refer, https://patchwork.ozlabs.org/project/kvm-riscv/patch/20230210142711.1177212-1-rkanwal@xxxxxxxxxxxx/ Regards, Anup > --- > > I used QEMU (v7.2.0, with Sstc support, running on a x86_64 Ubuntu PC) > to run RISC-V KVM (with the application kvmtool). Sometimes when I > operate the VM console, the following VM kernel messages occured: > > [ 193.615374] rcu: INFO: rcu_sched self-detected stall on CPU > [ 193.817218] rcu: 0-...!: (1 ticks this GP) idle=1f4c/0/0x1 softirq=391/391 fqs=0 > [ 194.081286] (t=26093 jiffies g=-367 q=1 ncpus=1) > [ 194.259913] rcu: rcu_sched kthread timer wakeup didn't happen for 26092 jiffies! g-367 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x402 > [ 194.469957] rcu: Possible timer handling issue on cpu=0 timer-softirq=531 > [ 194.604240] rcu: rcu_sched kthread starved for 26093 jiffies! g-367 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x402 ->cpu=0 > [ 194.804629] rcu: Unless rcu_sched kthread gets sufficient CPU time, OOM is now expected behavior. > [ 194.990219] rcu: RCU grace-period kthread stack dump: > [ 195.090119] task:rcu_sched state:I stack:0 pid:14 ppid:2 flags:0x00000000 > [ 195.224749] Call Trace: > [ 195.262612] [] schedule+0x5a/0xe6 > [ 195.369865] [] schedule_timeout+0x96/0x184 > [ 195.467307] [] rcu_gp_fqs_loop+0x2f8/0x3c6 > [ 195.571945] [] rcu_gp_kthread+0x132/0x15a > [ 195.713804] [] kthread+0xea/0x100 > [ 195.797756] [] ret_from_exception+0x0/0xc > [ 195.889635] rcu: Stack dump where RCU GP kthread last ran: > [ 196.010148] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 6.1.0 #27 > [ 196.109710] Hardware name: linux,dummy-virt (DT) > [ 196.220722] epc : arch_cpu_idle+0x22/0x28 > [ 196.301151] ra : default_idle_call+0x3c/0xfc > [ 196.370436] epc : ffffffff80003ffa ra : ffffffff808e85c8 sp : ffffffff80c4beb0 > [ 196.470957] gp : ffffffff80e252e0 tp : ffffffff80c57e40 t0 : ff6000001f5ea600 > [ 196.591430] t1 : 0000000000000000 t2 : ffffffff809aab10 s0 : ffffffff80c4bec0 > [ 196.754133] s1 : 0000000000000000 a0 : 0000000000000000 a1 : 0000000000000000 > [ 196.893724] a2 : ffffffff809a1368 a3 : ff6000001f5e7368 a4 : 0000000000001f44 > [ 196.999888] a5 : ff6000001f5e7368 a6 : 4000000000000000 a7 : 0000000000000000 > [ 197.096473] s2 : ffffffff80e28248 s3 : ffffffff80e283f8 s4 : ffffffff80e2521c > [ 197.200699] s5 : 0000000000000000 s6 : ffffffff809a23d8 s7 : ffffffff80e724f0 > [ 197.316841] s8 : 0000000000000000 s9 : 0000000000000000 s10: 0000000000000000 > [ 197.426412] s11: 0000000000000000 t3 : 000000000000005d t4 : 0000000000000000 > [ 197.591666] t5 : 00000000a9a15ab4 t6 : 0000000000000000 > [ 197.689520] status: 0000000200000120 badaddr: 0000000000000000 cause: 8000000000000005 > [ 197.830312] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 6.1.0 #27 > [ 197.940338] Hardware name: linux,dummy-virt (DT) > [ 198.028434] epc : arch_cpu_idle+0x22/0x28 > [ 198.122230] ra : default_idle_call+0x3c/0xfc > [ 198.206204] epc : ffffffff80003ffa ra : ffffffff808e85c8 sp : ffffffff80c4beb0 > [ 198.358393] gp : ffffffff80e252e0 tp : ffffffff80c57e40 t0 : ff6000001f5ea600 > [ 198.485282] t1 : 0000000000000000 t2 : ffffffff809aab10 s0 : ffffffff80c4bec0 > [ 198.573230] s1 : 0000000000000000 a0 : 0000000000000000 a1 : 0000000000000000 > [ 198.712281] a2 : ffffffff809a1368 a3 : ff6000001f5e7368 a4 : 0000000000001f44 > [ 198.845871] a5 : ff6000001f5e7368 a6 : 4000000000000000 a7 : 0000000000000000 > [ 198.991388] s2 : ffffffff80e28248 s3 : ffffffff80e283f8 s4 : ffffffff80e2521c > [ 199.139266] s5 : 0000000000000000 s6 : ffffffff809a23d8 s7 : ffffffff80e724f0 > [ 199.277888] s8 : 0000000000000000 s9 : 0000000000000000 s10: 0000000000000000 > [ 199.414241] s11: 0000000000000000 t3 : 000000000000005d t4 : 0000000000000000 > [ 199.553449] t5 : 00000000a9a15ab4 t6 : 0000000000000000 > [ 199.613065] status: 0000000200000120 badaddr: 0000000000000000 cause: 8000000000000005 > > It seems that VCPU stalled for a long time. I found it is because > sometimes the delta_ns in kvm_riscv_vcpu_timer_blocking() is 0, and the > hrtimer won't be started to wake the VCPU up again later. The VCPU keeps > sleeping until I operate the VM console, and kvmtool calls ioctl() to > inject a interrupt and finally calls kvm_vcpu_kick() to wake it up. > > If we configure QEMU to have only one physical CPU, this issue is easier > to reproduce. It looks as expected. The less CPU resources the VCPU > thread has, the more likely the circumstance 'delta_ns == 0' is to > happen. > > I tried starting the hrtimer as usual when delta_ns is 0, it did solve > this problem. I looked into the implementation of hrtimer, there seems > no harm if we start it with relative expiry 0 (but I'm not entirely > sure). > > Some details of my environment: > > - QEMU: v7.2.0 > - Linux kernel: v6.2 > - OpenSBI: commit-id 506928a1be9952abc52f05c0c661338aa1d66f6d (higher > than v1.1) > - kvmtool: commit-id 717a3ab0a195626360f40e9d3b4d6d8e072e83ba > > Running QEMU: > ``` > qemu-system-riscv64 \ > -cpu rv64 \ > -machine virt \ > -smp 1 \ > -m 1024M \ > -nographic \ > -bios MY_OPENSBI_FIRMWARE \ > -kernel MY_KERNEL.Image \ > -initrd MY_INITRD \ > -append "root=/dev/ram rw console=ttyS0,115200n8 earlycon=sbi" > ``` > > Running VM with kvmtool on QEMU: > ``` > lkvm run \ > --mem 512 \ > --console serial \ > --kernel MY_KERNEL.Image \ > --params "console=ttyS0,115200n8 earlycon=uart8250,mmio,0x3f8" > ``` > > arch/riscv/kvm/vcpu_timer.c | 6 ++---- > 1 file changed, 2 insertions(+), 4 deletions(-) > > diff --git a/arch/riscv/kvm/vcpu_timer.c b/arch/riscv/kvm/vcpu_timer.c > index ad34519c8a13..3ac2ff6a65da 100644 > --- a/arch/riscv/kvm/vcpu_timer.c > +++ b/arch/riscv/kvm/vcpu_timer.c > @@ -147,10 +147,8 @@ static void kvm_riscv_vcpu_timer_blocking(struct kvm_vcpu *vcpu) > return; > > delta_ns = kvm_riscv_delta_cycles2ns(t->next_cycles, gt, t); > - if (delta_ns) { > - hrtimer_start(&t->hrt, ktime_set(0, delta_ns), HRTIMER_MODE_REL); > - t->next_set = true; > - } > + hrtimer_start(&t->hrt, ktime_set(0, delta_ns), HRTIMER_MODE_REL); > + t->next_set = true; > } > > static void kvm_riscv_vcpu_timer_unblocking(struct kvm_vcpu *vcpu) > -- > 2.34.1 > > > This message and any attachment are confidential and may be privileged or otherwise protected from disclosure. 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