Re: [PATCH v4 10/13] x86: perf/core: use PERF_PMU_CAP_NO_EXCLUDE for exclude incapable PMUs

[Boris Ostrovsky]

On 1/8/19 5:48 AM, Peter Zijlstra wrote:
> On Mon, Jan 07, 2019 at 04:27:27PM +0000, Andrew Murray wrote:
>> For drivers that do not support context exclusion let's advertise the
>> PERF_PMU_CAP_NOEXCLUDE capability. This ensures that perf will
>> prevent us from handling events where any exclusion flags are set.
>> Let's also remove the now unnecessary check for exclusion flags.
>>
>> Signed-off-by: Andrew Murray <andrew.murray@xxxxxxx>
>> ---
>>  arch/x86/events/amd/ibs.c          | 13 +------------
>>  arch/x86/events/amd/power.c        | 10 ++--------
>>  arch/x86/events/intel/cstate.c     | 12 +++---------
>>  arch/x86/events/intel/rapl.c       |  9 ++-------
>>  arch/x86/events/intel/uncore_snb.c |  9 ++-------
>>  arch/x86/events/msr.c              | 10 ++--------
>>  6 files changed, 12 insertions(+), 51 deletions(-)
> You (correctly) don't add CAP_NO_EXCLUDE to the main x86 pmu code, but
> then you also don't check if it handles all the various exclude options
> correctly/consistently.
>
> Now; I must admit that that is a bit of a maze, but I think we can at
> least add exclude_idle and exclude_hv fails in there, nothing uses those
> afaict.
>
> On the various exclude options; they are as follows (IIUC):
>
>   - exclude_guest: we're a HV/host-kernel and we don't want the counter
>                    to run when we run a guest context.
>
>   - exclude_host: we're a HV/host-kernel and we don't want the counter
>                   to run when we run in host context.
>
>   - exclude_hv: we're a guest and don't want the counter to run in HV
>                 context.
>
> Now, KVM always implies exclude_hv afaict (for guests), I'm not sure
> what, if anything Xen does on x86 (IIRC Brendan Gregg once said perf
> works on Xen) -- nor quite sure who to ask, Boris, Jeurgen?

perf does work inside guests.

VPMU is managed by the Xen and it presents to the guest only samples
that are associated with the guest. So from that perspective exclude_hv
doesn't seem to be needed.

There is a VPMU mode that allows profiling whole system (host and
guests) from dom0, and this where exclude_hv might be useful. But this
mode, ahem, needs some work.


-boris