[Xen-devel] Inaccurate CPU utilization of Xentop.

["Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx>]

Hi, all 
   Recently, we used Xentop to collect CPU utilization data for perf analysis, 
and found the CPU utilization reported by Xentop is inaccurate, especially when 
run heavy-stress workloads.  After analyzing the code logic of xentop, we 
thought the issue maybe related to Xentop's implementation, and it is hard to 
get the accurate result with current Xentop. In the Xentop, we list the 
possible reasons which lead to the inaccurate issue here: 
 1.  vCPU info of Xentop is not collected by a single hypercall but each 
hypercall is issued for each vCPU, so vCPU utilization is not collected by a 
same timestamp, especially when system is running with heavy workloads.  
 2.  vCPUs of dom0 don't always have the timeslice to service these hypercalls, 
if dom0 is scheduled out, the issue maybe worse. 
 3. Xentop uses gettimeofday to get the timestamp for the calculation, but its 
call point is not near to with the hypercalls, so the timing maybe not accurate 
? 
 Any thoughts ?  
Xiantao
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