Re: Calculating real cpu usage of Xen domains correctly!

[Anthony Liguori <aliguori@us.ibm.com>]

Hi John,

John L Griffin wrote:

>My quick perusal of the current xeno-unstable code suggests that this 
>SCHED_OP call is a null call.  The SCHED_OP macro attempts to call the 
>"do_block" function pointed to in the "struct scheduler sched_bvt_def" 
>array, but this function pointer is never initialized, so it just does a 
>NOP.
>  
>
Yes.  I agree.

>It appears that what your patch does is limit when cpu_time gets updated, 
>such that the time only gets updated when the exec_domain is in the 
>BLOCKED state:
>  
>
No.  Currently, cpu_time will always get updated for the prev domain 
when __enter_scheduler() is called.  My patch modifies that behavior the 
behavior so that it only gets updated if prev->blocked is false or if 
prev->blocked is true but there are events pending for the domain.  The 
last part may not be right.

>    if ( test_bit(EDF_BLOCKED, &prev->ed_flags) )
>        prev->cpu_time += now - prev->lastschd;
>    }
>  
>
Sorry, this is my fault, my mail client badly munged the patch when I 
copy-pasted it.  Let me show you the code:

    if ( test_bit(EDF_BLOCKED, &prev->ed_flags) )
    {
        /* This check is needed to avoid a race condition. */
        if ( event_pending(prev) )
            clear_bit(EDF_BLOCKED, &prev->ed_flags);
        else
            SCHED_OP(do_block, prev);
    }

    prev->cpu_time += now - prev->lastschd;

Was changed to:

    if ( test_bit(EDF_BLOCKED, &prev->ed_flags) )
    {
        /* This check is needed to avoid a race condition. */
        if ( event_pending(prev) )
        {
            clear_bit(EDF_BLOCKED, &prev->ed_flags);
            prev->cpu_time += now - prev->lastschd;
        } else
            SCHED_OP(do_block, prev);
    } else {
        prev->cpu_time += now - prev->lastschd;
    }

It's not pretty at all.  I'm not sure if the update after clear_bit() is 
necessary either.

>What this seems to be saying (in regard to your patch working) is that the 
>cpu_time is updated when the domain relinquishes the CPU by block()ing, 
>but cpu_time doesn't get updated if it relinquishes the CPU by yield()ing.
>  
>
No, it's the opposite.  Sorry, I think the whitespace munging made the 
patch confusing.

>I wonder why this works.  Is anyone familiar with block() vs yield(), that 
>could lend some insight into what's going on? 
>  
>
I'm quite confident this is the cause of the problem.  I'm just not sure 
about that one cpu_time update.

Regards,
Anthony Liguori

>JLG
>
>
>  
>



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