Re: [BUG linux-4.9.x] xen hotplug cpu leads to 100% steal usage

[Juergen Gross <jgross@suse.com>]

On 01/03/2019 03:35, Dongli Zhang wrote:
> This issue is only for stable 4.9.x (e.g., 4.9.160), while the root cause is
> still in the lasted mainline kernel.
> 
> This is obviated by new feature patch set ended with b672592f0221
> ("sched/cputime: Remove generic asm headers").
> 
> After xen guest is up for long time, once we hotplug new vcpu, the corresponding
> steal usage might become 100% and the steal time from /proc/stat would increase
> abnormally.
> 
> As we cannot wait for long time to reproduce the issue, here is how I reproduce
> it on purpose by accounting a large initial steal clock for new vcpu 2 and 3.
> 
> 1. Apply the below patch to guest 4.9.160 to account large initial steal clock
> for new vcpu 2 and 3:
> 
> diff --git a/drivers/xen/time.c b/drivers/xen/time.c
> index ac5f23f..3cf629e 100644
> --- a/drivers/xen/time.c
> +++ b/drivers/xen/time.c
> @@ -85,7 +85,14 @@ u64 xen_steal_clock(int cpu)
>         struct vcpu_runstate_info state;
>  
>         xen_get_runstate_snapshot_cpu(&state, cpu);
> -       return state.time[RUNSTATE_runnable] + state.time[RUNSTATE_offline];
> +
> +       if (cpu == 2 || cpu == 3)
> +               return state.time[RUNSTATE_runnable]
> +                      + state.time[RUNSTATE_offline]
> +                      + 0x00071e87e677aa12;
> +       else
> +               return state.time[RUNSTATE_runnable]
> +                      + state.time[RUNSTATE_offline];
>  }
>  
>  void xen_setup_runstate_info(int cpu)
> 
> 
> 2. Boot hvm guest with "vcpus=2" and "maxvcpus=4". By default, VM boot with
> vcpu 0 and 1.
> 
> 3. Hotplug vcpu 2 and 3 via "xl vcpu-set <domid> 4" on dom0.
> 
> In my env, the steal becomes 100% within 10s after the "xl vcpu-set" command on
> dom0.
> 
> I can reproduce on kvm with similar method. However, as the initial steal clock
> on kvm guest is always 0, I do not think it is easy to hit this issue on kvm.
> 
> --------------------------------------------------------
> 
> The root cause is that the return type of jiffies_to_usecs() is 'unsigned int',
> but not 'unsigned long'. As a result, the leading 32 bits are discarded.
> 
> jiffies_to_usecs() is indirectly triggered by cputime_to_nsecs() at line 264.
> If guest is already up for long time, the initial steal time for new vcpu might
> be large and the leading 32 bits of jiffies_to_usecs() would be discarded.
> 
> As a result, the steal at line 259 is always large and the
> this_rq()->prev_steal_time at line 264 is always small. The difference at line
> 260 is always large during each time steal_account_process_time() is involved.
> Finally, the steal time in /proc/stat would increase abnormally.
> 
> 252 static __always_inline cputime_t steal_account_process_time(cputime_t maxtime)
> 253 {
> 254 #ifdef CONFIG_PARAVIRT
> 255         if (static_key_false(&paravirt_steal_enabled)) {
> 256                 cputime_t steal_cputime;
> 257                 u64 steal;
> 258 
> 259                 steal = paravirt_steal_clock(smp_processor_id());
> 260                 steal -= this_rq()->prev_steal_time;
> 261 
> 262                 steal_cputime = min(nsecs_to_cputime(steal), maxtime);
> 263                 account_steal_time(steal_cputime);
> 264                 this_rq()->prev_steal_time += cputime_to_nsecs(steal_cputime);
> 265 
> 266                 return steal_cputime;
> 267         }
> 268 #endif
> 269         return 0;
> 270 }
> 
> --------------------------------------------------------
> 
> I have emailed the kernel mailing list about the return type of
> jiffies_to_usecs() and jiffies_to_msecs():
> 
> https://lkml.org/lkml/2019/2/26/899
> 
> 
> So far, I have two solutions:
> 
> 1. Change the return type from 'unsigned int' to 'unsigned long' as in above
> link and I am afraid it would bring side effect. The return type in latest
> mainline kernel is still 'unsigned int'.
> 
> 2. Something like below based on stable 4.9.160:

3. use jiffies64_to_nsecs() instead of trying to open code it.


Juergen