Re: [PATCH v5 1/1] xen/time: do not decrease steal time after live migration on xen

[Boris Ostrovsky]

On 10/30/2017 04:03 AM, Dongli Zhang wrote:
> After guest live migration on xen, steal time in /proc/stat
> (cpustat[CPUTIME_STEAL]) might decrease because steal returned by
> xen_steal_lock() might be less than this_rq()->prev_steal_time which is
> derived from previous return value of xen_steal_clock().
>
> For instance, steal time of each vcpu is 335 before live migration.
>
> cpu  198 0 368 200064 1962 0 0 1340 0 0
> cpu0 38 0 81 50063 492 0 0 335 0 0
> cpu1 65 0 97 49763 634 0 0 335 0 0
> cpu2 38 0 81 50098 462 0 0 335 0 0
> cpu3 56 0 107 50138 374 0 0 335 0 0
>
> After live migration, steal time is reduced to 312.
>
> cpu  200 0 370 200330 1971 0 0 1248 0 0
> cpu0 38 0 82 50123 500 0 0 312 0 0
> cpu1 65 0 97 49832 634 0 0 312 0 0
> cpu2 39 0 82 50167 462 0 0 312 0 0
> cpu3 56 0 107 50207 374 0 0 312 0 0
>
> Since runstate times are cumulative and cleared during xen live migration
> by xen hypervisor, the idea of this patch is to accumulate runstate times
> to global percpu variables before live migration suspend. Once guest VM is
> resumed, xen_get_runstate_snapshot_cpu() would always return the sum of new
> runstate times and previously accumulated times stored in global percpu
> variables.
>
> Similar and more severe issue would impact prior linux 4.8-4.10 as
> discussed by Michael Las at
> https://0xstubs.org/debugging-a-flaky-cpu-steal-time-counter-on-a-paravirtualized-xen-guest,
> which would overflow steal time and lead to 100% st usage in top command
> for linux 4.8-4.10. A backport of this patch would fix that issue.
>
> References: https://0xstubs.org/debugging-a-flaky-cpu-steal-time-counter-on-a-paravirtualized-xen-guest
> Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
>
> ---
> Changed since v1:
>   * relocate modification to xen_get_runstate_snapshot_cpu
>
> Changed since v2:
>   * accumulate runstate times before live migration
>
> Changed since v3:
>   * do not accumulate times in the case of guest checkpointing
>
> Changed since v4:
>   * allocate array of vcpu_runstate_info to reduce number of memory allocation
>
> ---
>  drivers/xen/manage.c         |  2 ++
>  drivers/xen/time.c           | 68 ++++++++++++++++++++++++++++++++++++++++++--
>  include/xen/interface/vcpu.h |  2 ++
>  include/xen/xen-ops.h        |  1 +
>  4 files changed, 71 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/xen/manage.c b/drivers/xen/manage.c
> index c425d03..3dc085d 100644
> --- a/drivers/xen/manage.c
> +++ b/drivers/xen/manage.c
> @@ -72,6 +72,7 @@ static int xen_suspend(void *data)
>  	}
>  
>  	gnttab_suspend();
> +	xen_accumulate_runstate_time(-1);
>  	xen_arch_pre_suspend();
>  
>  	/*
> @@ -84,6 +85,7 @@ static int xen_suspend(void *data)
>                                             : 0);
>  
>  	xen_arch_post_suspend(si->cancelled);
> +	xen_accumulate_runstate_time(si->cancelled);

I am not convinced that the comment above HYPERVISOR_suspend() is
correct. The call can return an error code and so if it returns -EPERM
(which AFAICS it can't now but might in the future) then
xen_accumulate_runstate_time() will do wrong thing.


>  	gnttab_resume();
>  
>  	if (!si->cancelled) {
> diff --git a/drivers/xen/time.c b/drivers/xen/time.c
> index ac5f23f..cf3afb9 100644
> --- a/drivers/xen/time.c
> +++ b/drivers/xen/time.c
> @@ -19,6 +19,9 @@
>  /* runstate info updated by Xen */
>  static DEFINE_PER_CPU(struct vcpu_runstate_info, xen_runstate);
>  
> +static DEFINE_PER_CPU(u64[RUNSTATE_max], old_runstate_time);
> +static struct vcpu_runstate_info *runstate_delta;

I'd move this inside xen_accumulate_runstate_time() since that's the
only function that uses it. And why does it need to be
vcpu_runstate_info and not u64[4]?

> +
>  /* return an consistent snapshot of 64-bit time/counter value */
>  static u64 get64(const u64 *p)
>  {
> @@ -47,8 +50,8 @@ static u64 get64(const u64 *p)
>  	return ret;
>  }
>  
> -static void xen_get_runstate_snapshot_cpu(struct vcpu_runstate_info *res,
> -					  unsigned int cpu)
> +static void xen_get_runstate_snapshot_cpu_delta(
> +			struct vcpu_runstate_info *res, unsigned int cpu)
>  {
>  	u64 state_time;
>  	struct vcpu_runstate_info *state;
> @@ -66,6 +69,67 @@ static void xen_get_runstate_snapshot_cpu(struct vcpu_runstate_info *res,
>  		 (state_time & XEN_RUNSTATE_UPDATE));
>  }
>  
> +static void xen_get_runstate_snapshot_cpu(struct vcpu_runstate_info *res,
> +					  unsigned int cpu)
> +{
> +	int i;
> +
> +	xen_get_runstate_snapshot_cpu_delta(res, cpu);
> +
> +	for (i = 0; i < RUNSTATE_max; i++)
> +		res->time[i] += per_cpu(old_runstate_time, cpu)[i];
> +}
> +
> +void xen_accumulate_runstate_time(int action)
> +{
> +	struct vcpu_runstate_info state;
> +	int cpu, i;
> +
> +	switch (action) {
> +	case -1: /* backup runstate time before suspend */
> +		WARN_ON_ONCE(unlikely(runstate_delta));

pr_warn_once(), to be consistent with the rest of the file. And then
should you return if this is true?

> +
> +		runstate_delta = kcalloc(num_possible_cpus(),
> +					 sizeof(*runstate_delta),
> +					 GFP_KERNEL);
> +		if (unlikely(!runstate_delta)) {
> +			pr_alert("%s: failed to allocate runstate_delta\n",
> +				    __func__);

pr_warn() should be sufficient. Below too.

Also, as a side question --- can we do kmalloc() at this point?

> +			return;
> +		}
> +
> +		for_each_possible_cpu(cpu) {
> +			xen_get_runstate_snapshot_cpu_delta(&state, cpu);
> +			memcpy(runstate_delta[cpu].time, state.time,
> +			      RUNSTATE_max * sizeof(*runstate_delta[cpu].time));

sizeof(runstate_delta[cpu].time).

> +		}
> +
> +		break;
> +
> +	case 0: /* backup runstate time after resume */
> +		if (unlikely(!runstate_delta)) {
> +			pr_alert("%s: cannot accumulate runstate time as runstate_delta is NULL\n",
> +				    __func__);
> +			return;
> +		}
> +
> +		for_each_possible_cpu(cpu) {
> +			for (i = 0; i < RUNSTATE_max; i++)
> +				per_cpu(old_runstate_time, cpu)[i] +=
> +					runstate_delta[cpu].time[i];
> +		}
> +		break;
> +
> +	default: /* do not accumulate runstate time for checkpointing */
> +		break;
> +	}
> +
> +	if (action != -1 && runstate_delta) {
> +		kfree(runstate_delta);
> +		runstate_delta = NULL;
> +	}
> +}
> +
>  /*
>   * Runstate accounting
>   */
> diff --git a/include/xen/interface/vcpu.h b/include/xen/interface/vcpu.h
> index 98188c8..85e81ce 100644
> --- a/include/xen/interface/vcpu.h
> +++ b/include/xen/interface/vcpu.h
> @@ -110,6 +110,8 @@ DEFINE_GUEST_HANDLE_STRUCT(vcpu_runstate_info);
>   */
>  #define RUNSTATE_offline  3
>  
> +#define RUNSTATE_max      4

This file is part of Xen ABI. While this macro technically doesn't
change anything I'd rather have those updates first appear in Xen code.

Besides, this change leaves vcpu_runstate_info.time[4] definition
intact. I think all RUNSTATE_* macros would need to be moved above
vcpu_runstate_info definition.

TBH, for purposes of this patch I'd use 4.


-boris