On Wed, Apr 26, 2017 at 11:10:21AM +0100, Will Deacon wrote: > On Wed, Apr 26, 2017 at 07:22:46AM +0000, Pinski, Andrew wrote: > > On 4/25/2017 11:53 PM, Jayachandran C. wrote: > > > On Tue, Apr 25, 2017 at 10:23 PM, Will Deacon <will.dea...@arm.com> wrote: > > >> On Tue, Apr 25, 2017 at 09:13:40AM +0530, Ganapatrao Kulkarni wrote: > > >>> On Mon, Apr 24, 2017 at 9:15 PM, Will Deacon <will.dea...@arm.com> > > >>> wrote: > > >>>> On Thu, Apr 20, 2017 at 02:56:50PM +0530, Ganapatrao Kulkarni wrote: > > >>>>> OK, if you are ok with sysfs part, i can send next version with that > > >>>>> change only?. > > >>>> I think the sysfs part is still a little dodgy, since you still expose > > >>>> the > > >>>> "exclude_hv" file with a value of 0 when not running at EL2, which > > >>>> would > > >>>> imply that exclude_hv is forced to zero. I don't think that's correct. > > >>> okay, i can make exclude_hv visible only when kernel booted in EL2. > > >>> is it ok to have empty directory "attr" when kernel booted to EL1? > > >>> attr can be place holder for any other miscellaneous attributes, that > > >>> can be added in future. > > >> Sounds good to me, although I'll seek comment from the other perf folks > > >> before merging anything with ABI implications. > > > Do you really think this is the solution given: > > > - this is an arm64 specific sysfs interface that is tied to the perf API > > That's why I want feedback from others. The intention would be that this can > be used by other PMUs as well, since it's not uncommon that parts of the > sizeable perf_event_attr structure are not used by a given PMU. > > > > - the perf API documentation has to be updated for this > > So? If having to update documentation means we shouldn't change the kernel, > then we may as well all find new jobs. > > > > - All the applications that use the perf API have to be modified to > > > check this sysfs interface > > > - If the application fails to do so, a very narrow corner case > > > (exclude_hv != exclude_kernel and VHE enabled) fails. > > See below, but apparently people care about it. > > > > Any application that really cares can already do see if exclude_hv != > > > exclude_kernel case works by calling perf_open_event() with those > > > options and checking the return value. > > That's a good point: there is *something* userspace can do, although that > would be arm64-specific and doesn't really help with the state-space > explosion you get with combinations of invalid/unused perf_event_attr > fields. > > > An example of an application which needs to changed is HHVM. Currently > > it sets exclude_hv to true but exclude_kernel to false as it does not > > care about the hypervisor associated perf events associated with the > > code, only the kernel and userspace associated evnts. > > Yes we could submit a patch to use the sysfs interface to check but it > > would look funny and the facebook folks might reject the patch as it is > > ARM64 specific in generic code. Note this is how all of this discussion > > started was HHVM's call to perf_open_event was failing. > > Hmm, if you're saying that HHVM won't be changed to use the sysfs stuff, > then why are we bothering? > > Not sure where this leaves us.
If my understanding is correct, the sysfs suggestion above is going to add API complexity without solving the issue. Ignoring the exclude_hv if it cannot be honored would be a better solution. If that is not acceptable (which seems to be the case - but I do not see a reason for that), I think the better option for the application is to check if the platform supports the mode exclusion it wants by using the perf_event_open API itself. Thanks, JC.
