Date:   Mon, 30 Oct 2017 20:58:38 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Dongli Zhang <dongli.zhang@...cle.com>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Cc:     jgross@...e.com, joao.m.martins@...cle.com
Subject: Re: [Xen-devel] [PATCH v5 1/1] xen/time: do not decrease steal time
 after live migration on xen



On 10/30/2017 08:14 PM, Dongli Zhang wrote:
> Hi Boris,
> 
> On 10/30/2017 09:34 PM, Boris Ostrovsky wrote:
>> 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@...cle.com>
>>>
>>> ---
>>> 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.
> 
> I would split xen_accumulate_runstate_time() into two functions to avoid the
> -EPERM issue, as one is for saving and another is for accumulation, respectively.
> 
> Otherwise, can you use xen_accumulate_runstate_time(2) for saving before suspend
> and xen_accumulate_runstate_time(si->cancelled) after resume?


I'd probably just say something like

si->cancelled = HYPERVISOR_suspend() ? 1 : 0;

and keep xen_accumulate_runstate_time() as is (maybe rename it to 
xen_manage_runstate_time()). And also remove the comment above the 
hypercall as it is incorrect (but please mention the reason in the 
commit message)

> 
>>
>>
>>>   	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
> 
> If we split xen_accumulate_runstate_time() into two functions, we would leave
> runstate_delta as global static.
> 
>> only function that uses it. And why does it need to be
>> vcpu_runstate_info and not u64[4]?
> 
> This was suggested by Juergen to avoid the allocation and reclaim of the second
> dimensional array as in v4 of this patch?
> 
> Or would you like to allocate sizeof(u64[4]) * num_possible_cpus() and emulate
> the 2d array with this 1d array and move the pointer forward sizeof(u64[4]) in
> each iteration?


I was thinking of

u64 **runstate_delta = (u64 **)kmalloc(sizeof(xen_runstate.time) * 
num_possible_cpus())

and then you should be able to access runstate_delta[cpu][RUNSTATE_*].

> 
>>
>>> +
>>>   /* 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?
> 
> I would prefer to not return if it is true but just warn the administrator that
> there is memory leakage issue while leaving runstate accumulation works normally.
> 
>>
>>> +
>>> +		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?
> 
> Yes. kmalloc_array() is better than kcalloc, unless we have 2 dimensional array
> and we need to guarantee the value of first dimension is always 0.


That's not what was thinking about. GFP_KERNEL may sleep and I don't 
know how sleep is handled at this point. Everything is pretty much dead 
now. Perhaps GFP_ATOMIC might be a better choice.

-boris

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