lkml.org 
[lkml]   [2012]   [Aug]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH 08/11] perf tool: precise mode requires exclude_guest
    On 26.07.12 10:08:29, Peter Zijlstra wrote:
    > On Wed, 2012-07-25 at 23:16 -0600, David Ahern wrote:
    >
    > > Peter's patch (see https://lkml.org/lkml/2012/7/9/298) changes kernel
    > > side to require the use of exclude_guest if the precise modifier is
    > > used, returning -EOPNOTSUPP if exclude_guest is not set. This patch goes
    > > after the user experience: Today if a user specifies -e <event>:p all
    > > other modifiers are reset - including exclude_guest. Going forward we
    > > need :p to imply :pH if a user has not specified a GH modifer.
    > >
    > > We could do nothing and handle the unsupported error and try setting the
    > > exclude_guest option - like perf handles other new parameters. But
    > > EOPNOTSUPP is not uniquely tied to this error -- e.g., it could be the
    > > BTS is not supported (:pp). Also, we have no easy way to discriminate :p
    > > from :pG or :pGH. It seems to me perf should not silently undo a user
    > > request on the modifier, but inform the user the request is wrong. For
    > > example if a user request -e cycles:pG it should not be silently turned
    > > into :pH.
    > >
    > > And then yesterday, Robert stated that none of the exclude_xxxx
    > > modifiers can be set for the AMD if the precise modifier is used, so we
    > > cannot blindly set exclude_guest if precise_ip is set.
    > >
    > > So, seems to me perf need's one action for Intel processors and another
    > > for AMD.
    >
    > No, we just need to teach the IBS code about SVM enter/exit.

    I aggree that this could be emulated in software by enabling/disabling
    the event with a guest/host switch. And, even better, we add this for
    every pmu in a generic way. E.g. northbridge counter and I guess also
    Intel uncore events do not support G/H counting in hardware. Same to
    other pmus that could be imaginable in the future like counters for
    IOMMUs or other hardware devices.

    But, as some pmus are not related to virtualization or other features
    they simply do not need to support those attributes, or we want other
    defaults, e.g. enable it system wide. Detecting features with syscall
    error checking and then falling back to other defaults does not seem
    the right approach to me, because it may require several syscalls to
    check *combinations* of supported attributes, makes error logging and
    detection more difficult due to noisy log messages and because there
    is no strict attribute flag checking in current and older kernels.

    I better would like to see a pmu feature flag in the same style as
    with /proc/cpuinfo, e.g.:

    $ cat /sys/bus/event_source/devices/cpu/flags
    exclude_host exclude_guest

    We also need stricter attribute flag checking, esp. of reseved flags
    and for unsupported features in some pmus (I already work on some
    patches for this). Userland then checks flags and sets up syscalls
    according to the reported flags. The goal should be to avoid syscall
    errors at all. Thus, we are able to improve dmesg logging in case of
    errors, currently we do not see any message if a syscall fails.

    And finally, if a feature could be emulated, we could provide this
    emulation of an attr flag to all pmus.

    Does this make sense?

    -Robert

    --
    Advanced Micro Devices, Inc.
    Operating System Research Center



    \
     
     \ /
      Last update: 2012-08-03 16:41    [W:2.744 / U:0.432 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site