Re: [PATCH 4/4] tools: add total/local memory bandwith monitoring

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 06/01/15 10:09, Chao Peng wrote:
> On Mon, Jan 05, 2015 at 12:39:42PM +0000, Wei Liu wrote:
>> On Tue, Dec 23, 2014 at 04:54:39PM +0800, Chao Peng wrote:
>> [...]
>>> +static int libxl__psr_cmt_get_mem_bandwidth(libxl__gc *gc, uint32_t domid,
>>> +    xc_psr_cmt_type type, uint32_t socketid, uint32_t *bandwidth)
>>> +{
>>> +    uint64_t sample1, sample2;
>>> +    uint32_t upscaling_factor;
>>> +    int rc;
>>> +
>>> +    rc = libxl__psr_cmt_get_l3_monitoring_data(gc, domid,
>>> +                    type, socketid, &sample1);
>>> +    if (rc < 0)
>>> +        return ERROR_FAIL;
>>> +
>>> +    usleep(10000);
>>> +
>>> +    rc = libxl__psr_cmt_get_l3_monitoring_data(gc, domid,
>>> +                    type, socketid, &sample2);
>>> +    if (rc < 0)
>>> +       return ERROR_FAIL;
>>> +
>>> +    if (sample2 < sample1) {
>>> +         LOGE(ERROR, "event counter overflowed between two samplings");
>>> +         return ERROR_FAIL;
>>> +    }
>>> +
>> What's the likelihood of counter overflows? Can we handle this more
>> gracefully? Say, retry (with maximum retry cap) when counter overflows?
> The likelihood is very small here. Hardware guarantees the counter will
> not overflow in one second even under maximum platform bandwidth conditions.
> And we only sleep 0.01 second here. 
>
> I'd like to adopt your suggestion to retry another time once that happens.
> But only one retry and it should correct the overflow.
>
> Thanks,
> Chao

You have no possible way of guaranteeing that the actual elapsed time
between the two samples is less than 1 second.  On a very heavily loaded
system, even regular task scheduling could cause an actual elapsed time
of more than one second in that snippet of code.

~Andrew