lkml.org 
[lkml]   [2007]   [Aug]   [17]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [kvm-devel] [PATCH/RFC 3/4]Introduce "account modifiers" mechanism
    Laurent Vivier wrote:
    >>
    >>> - remove PATCH 3, and add in task_struct a "ktime vtime" where we accumulate
    >>> guest time (by calling something like guest_enter() and guest_exit() from the
    >>> virtualization engine), and when in account_system_time() we have cputime >
    >>> vtime we substrate vtime from cputime and add vtime to user time and guest time.
    >>> But doing like this we freeze in kernel/sched.c the link between system time,
    >>> user time and guest time (i.e. system time = system time - vtime, user time =
    >>> user time + vtime and guest time = guest time + vtime).
    >>>
    >> Actually, I think we can set a per-cpu "in_guest" flag for the scheduler
    >> code, which then knows to add the tick to the guest time. That seems
    >> the simplest possible solution.
    >>
    >> lguest or kvm would set the flag before running the guest (which is done
    >> with preempt disabled or using preemption hooks), and reset it
    >> afterwards.
    >>
    >> Thoughts?
    >>
    >
    > It was my first attempt (except I didn't have a per-cpu flag, but a per-task
    > flag), it's not visible but I love simplicity... ;-)
    >
    > A KVM VCPU is stopped by preemption, so when we enter in scheduler we have
    > exited from VCPU and thus this flags is off (so we account 0 to the guest). What
    > I did then is "set the flag on when we enter in the VCPU, and
    > "account_system_time()" sets the flag off when it adds this timeslice to cpustat
    > (and compute correctly guest, user, system time). But I didn't like this idea
    > because all code executed after we entered in the VCPU is accounted to the guest
    > until we have an account_system_time() and I suppose we can have real system
    > time in this part. And I guess a VCPU can be less than 1 ms (unit of cputime) in
    > a timeslice.
    >
    > So ? What's best ?
    >

    The normal user/system accounting has the same issue, no? Whereever we
    happen to land (kernel or user) gets the whole tick.

    So I think it is okay to have the same limitation for guest time.

    --
    Do not meddle in the internals of kernels, for they are subtle and quick to panic.

    -
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.kernel.org
    More majordomo info at http://vger.kernel.org/majordomo-info.html
    Please read the FAQ at http://www.tux.org/lkml/

    \
     
     \ /
      Last update: 2007-08-17 15:27    [W:0.022 / U:29.916 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site