Re: [PATCH v4 1/1] xen/time: do not decrease steal time after live migration on xen

[Juergen Gross]

On 30/10/17 06:26, 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
> 
> ---
>  drivers/xen/manage.c         |  2 ++
>  drivers/xen/time.c           | 83 ++++++++++++++++++++++++++++++++++++++++++--
>  include/xen/interface/vcpu.h |  2 ++
>  include/xen/xen-ops.h        |  1 +
>  4 files changed, 86 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);
>  	gnttab_resume();
>  
>  	if (!si->cancelled) {
> diff --git a/drivers/xen/time.c b/drivers/xen/time.c
> index ac5f23f..18e2b76 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 u64 **runstate_time_delta;
> +
>  /* 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 +69,82 @@ 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_time_delta));
> +
> +		runstate_time_delta = kcalloc(num_possible_cpus(),
> +						  sizeof(*runstate_time_delta),
> +						  GFP_KERNEL);

You know the number of cpus, so you can just allocate an array of
struct vcpu_runstate_info:

struct vcpu_runstate_info *runstate_time_delta;

kcalloc(num_possible_cpus(), sizeof(*runstate_time_delta), GFP_KERNEL);

then ...

> +		if (unlikely(!runstate_time_delta)) {
> +			pr_alert("%s: failed to allocate runstate_time_delta\n",
> +					__func__);
> +			return;
> +		}
> +
> +		for_each_possible_cpu(cpu) {
> +			runstate_time_delta[cpu] = kmalloc_array(RUNSTATE_max,
> +						  sizeof(**runstate_time_delta),
> +						  GFP_KERNEL);

... this allocation isn't needed any more.


Juergen