Re: [PATCH v9 7/7] drm/i915: add a sysfs entry to let users set sseu configs

[Joonas Lahtinen <joonas.lahtinen@linux.intel.com>]

Quoting Bloomfield, Jon (2018-07-18 19:44:14)
> > -----Original Message-----
> > From: Intel-gfx <intel-gfx-bounces@lists.freedesktop.org> On Behalf Of
> > Tvrtko Ursulin
> > Sent: Thursday, June 14, 2018 1:29 AM
> > To: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>; Chris Wilson
> > <chris@chris-wilson.co.uk>; Landwerlin, Lionel G
> > <lionel.g.landwerlin@intel.com>; intel-gfx@lists.freedesktop.org
> > Subject: Re: [Intel-gfx] [PATCH v9 7/7] drm/i915: add a sysfs entry to let users
> > set sseu configs
> > 
> > 
> > On 13/06/2018 13:49, Joonas Lahtinen wrote:
> > > Quoting Tvrtko Ursulin (2018-06-12 15:02:07)
> > >>
> > >> On 12/06/2018 11:52, Lionel Landwerlin wrote:
> > >>> On 12/06/18 11:37, Chris Wilson wrote:
> > >>>> Quoting Lionel Landwerlin (2018-06-12 11:33:34)
> > >>>>> On 12/06/18 10:20, Joonas Lahtinen wrote:
> > >>>>>> Quoting Chris Wilson (2018-06-11 18:02:37)
> > >>>>>>> Quoting Lionel Landwerlin (2018-06-11 14:46:07)
> > >>>>>>>> On 11/06/18 13:10, Tvrtko Ursulin wrote:
> > >>>>>>>>> On 30/05/2018 15:33, Lionel Landwerlin wrote:
> > >>>>>>>>>> There are concerns about denial of service around the per
> > >>>>>>>>>> context sseu
> > >>>>>>>>>> configuration capability. In a previous commit introducing the
> > >>>>>>>>>> capability we allowed it only for capable users. This changes
> > >>>>>>>>>> adds a
> > >>>>>>>>>> new debugfs entry to let any user configure its own context
> > >>>>>>>>>> powergating setup.
> > >>>>>>>>> As far as I understood it, Joonas' concerns here are:
> > >>>>>>>>>
> > >>>>>>>>> 1) That in the containers use case individual containers wouldn't
> > be
> > >>>>>>>>> able to turn on the sysfs toggle for them.
> > >>>>>>>>>
> > >>>>>>>>> 2) That also in the containers use case if box admin turns on the
> > >>>>>>>>> feature, some containers would potentially start negatively
> > >>>>>>>>> affecting
> > >>>>>>>>> the others (via the accumulated cost of slice re-configuration on
> > >>>>>>>>> context switching).
> > >>>>>>>>>
> > >>>>>>>>> I am not familiar with typical container setups to be authoritative
> > >>>>>>>>> here, but intuitively I find it reasonable that a low-level hardware
> > >>>>>>>>> switch like this would be under the control of a master domain
> > >>>>>>>>> administrator. ("If you are installing our product in the container
> > >>>>>>>>> environment, make sure your system administrator enables this
> > >>>>>>>>> hardware
> > >>>>>>>>> feature.", "Note to system administrators: Enabling this features
> > >>>>>>>>> may
> > >>>>>>>>> negatively affect the performance of other containers.")
> > >>>>>>>>>
> > >>>>>>>>> Alternative proposal is for the i915 to apply an "or" filter on all
> > >>>>>>>>> requested masks and in that way ensure dynamic re-
> > configuration
> > >>>>>>>>> doesn't happen on context switches, but driven from userspace
> > via
> > >>>>>>>>> ioctls.
> > >>>>>>>>>
> > >>>>>>>>> In other words, should _all_ userspace agree between
> > themselves that
> > >>>>>>>>> they want to turn off a slice, they would then need to send out a
> > >>>>>>>>> concerted ioctl storm, where number of needed ioctls equals
> > the
> > >>>>>>>>> number
> > >>>>>>>>> of currently active contexts. (This may have its own performance
> > >>>>>>>>> consequences caused by the barriers needed to modify all
> > context
> > >>>>>>>>> images.)
> > >>>>>>>>>
> > >>>>>>>>> This was deemed acceptable the the media use case, but my
> > concern is
> > >>>>>>>>> the approach is not elegant and will tie us with the "or" policy in
> > >>>>>>>>> the ABI. (Performance concerns I haven't evaluated yet, but
> > they
> > >>>>>>>>> also
> > >>>>>>>>> may be significant.)
> > >>>>>>>>>
> > >>>>>>>>> If we go back thinking about the containers use case, then it
> > >>>>>>>>> transpires that even though the "or" policy does prevent one
> > >>>>>>>>> container
> > >>>>>>>>> from affecting the other from one angle, it also prevents one
> > >>>>>>>>> container from exercising the feature unless all containers
> > >>>>>>>>> co-operate.
> > >>>>>>>>>
> > >>>>>>>>> As such, we can view the original problem statement where we
> > have an
> > >>>>>>>>> issue if not everyone co-operates, as conceptually the same just
> > >>>>>>>>> from
> > >>>>>>>>> an opposite angle. (Rather than one container incurring the
> > >>>>>>>>> increased
> > >>>>>>>>> cost of context switches to the rest, we would have one
> > container
> > >>>>>>>>> preventing the optimized slice configuration to the other.)
> > >>>>>>>>>
> > >>>>>>>>>    From this follows that both proposals require complete
> > >>>>>>>>> co-operation
> > >>>>>>>>> from all running userspace to avoid complete control of the
> > feature.
> > >>>>>>>>>
> > >>>>>>>>> Since the balance between the benefit of optimized slice
> > >>>>>>>>> configuration
> > >>>>>>>>> (or penalty of suboptimal one), versus the penalty of increased
> > >>>>>>>>> context switch times, cannot be know by the driver (barring
> > >>>>>>>>> venturing
> > >>>>>>>>> into the heuristics territory), that is another reason why I find
> > >>>>>>>>> the
> > >>>>>>>>> "or" policy in the driver questionable.
> > >>>>>>>>>
> > >>>>>>>>> We can also ask a question of - If we go with the "or" policy, why
> > >>>>>>>>> require N per-context ioctls to modify the global GPU
> > configuration
> > >>>>>>>>> and not instead add a global driver ioctl to modify the state?
> > >>>>>>>>>
> > >>>>>>>>> If a future hardware requires, or enables, the per-context
> > behaviour
> > >>>>>>>>> in a more efficient way, we could then revisit the problem space.
> > >>>>>>>>>
> > >>>>>>>>> In the mean time I see the "or" policy solution as adding some
> > ABI
> > >>>>>>>>> which doesn't do anything for many use cases without any way
> > for the
> > >>>>>>>>> sysadmin to enable it. At the same time master sysfs knob at
> > least
> > >>>>>>>>> enables the sysadmin to make a decision. Here I am thinking
> > about a
> > >>>>>>>>> random client environment where not all userspace co-
> > operates,
> > >>>>>>>>> but for
> > >>>>>>>>> instance user is running the feature aware media stack, and
> > >>>>>>>>> non-feature aware OpenCL/3d stack.
> > >>>>>>>>>
> > >>>>>>>>> I guess the complete story boils down to - is the master sysfs
> > knob
> > >>>>>>>>> really a problem in container use cases.
> > >>>>>>>>>
> > >>>>>>>>> Regards,
> > >>>>>>>>>
> > >>>>>>>>> Tvrtko
> > >>>>>>>> Hey Tvrtko,
> > >>>>>>>>
> > >>>>>>>> Thanks for summarizing a bunch of discussions.
> > >>>>>>>> Essentially I agree with every you wrote above.
> > >>>>>>>>
> > >>>>>>>> If we have a global setting (determined by the OR policy), what's
> > the
> > >>>>>>>> point of per context settings?
> > >>>>>>>>
> > >>>>>>>> In Dmitry's scenario, all userspace applications will work
> > >>>>>>>> together to
> > >>>>>>>> reach the consensus so it sounds like we're reimplementing the
> > policy
> > >>>>>>>> that is already existing in userspace.
> > >>>>>>>>
> > >>>>>>>> Anyway, I'm implementing Joonas' suggestion. Hopefully
> > somebody else
> > >>>>>>>> than me pick one or the other :)
> > >>>>>>> I'll just mention the voting/consensus approach to see if anyone
> > else
> > >>>>>>> likes it.
> > >>>>>>>
> > >>>>>>> Each context has a CONTEXT_PARAM_HINT_SSEU { small,
> > dontcare, large }
> > >>>>>>> (or some other abstract names).
> > >>>>>> Yeah, the param name should have the word _HINT_ in it when it's
> > not a
> > >>>>>> definitive set.
> > >>>>>>
> > >>>>>> There's no global setter across containers, only a scenario when
> > >>>>>> everyone agrees or not. Tallying up the votes and going with a
> > majority
> > >>>>>> vote might be an option, too.
> > >>>>>>
> > >>>>>> Regards, Joonas
> > >>>>> Trying to test the "everyone agrees" approach here.
> > >>>> It's not everyone agrees, but the greater good.
> > >>>
> > >>> I'm looking forward to the definition of the greater good :)
> > >>> Tvrtko wanted to avoid the heuristic territory, it seems like we're just
> > >>> stepping into it.
> > >>
> > >> I am not sure that "small, dontcare, large" models brings much. No one
> > >> would probably set "dontcare" since we have to default new contexts to
> > >> large to be compatible.
> > >>
> > >> Don't know, I still prefer the master knob option. Honestly don't yet
> > >> see the containers use case as a problem. There is always a master
> > >> domain in any system where the knob can be enabled if the customers on
> > >> the system are such to warrant it. On mixed systems enable it or not
> > >> someone always suffers. And with the knob we are free to add heuristics
> > >> later, keep the uapi, and just default the knob to on.
> > >
> > > Master knob effectively means dead code behind a switch, that's not very
> > > upstream friendly.
> > 
> > Hey at least I wasn't proposing a modparam! :)))
> > 
> > Yes it is not the best software practice, upstream or not, however I am
> > trying to be pragmatical here and choose the simplest, smallest, good
> > enough, and least headache inducing in the future solution.
> > 
> > One way of of looking at the master switch could be like tune your
> > system for XYZ - change CPU frequency governor, disable SATA link
> > saving, allow i915 media optimized mode. Some live code out, some dead
> > code in.
> > 
> > But perhaps discussion is moot since we don't have userspace anyway.
> > 
> > Regards,
> > 
> > Tvrtko
> 
> I'm surprised by the "no deadcode behind knobs comment". We do this all
> the time - "display=0" anyone? Or "enable_cmdparser=false"?

Display-less machines are in the progress of being added to CI as we
speak. And what goes for enable_rc6=0 and enable_execlists=0 options, if
you look closely, they have been gotten rid of already. Something
existing in the codebase doesn't mean it's fine to add more of the same.

> Allowing user space to reduce EU performance for the system as a whole
> is not a great idea imho.

When all the clients ask for it, there should be no problem in reducing
the slice count.

> Only sysadmin should have the right to let that
> happen, and an admin switch (I WOULD go with a modparam personally) is
> a good way to ensure that we can get the optimal media configuration for
> dedicated media systems, while for general systems we get the best overall
> performance (not just favouring media).

There definitely won't be a modparam for it.

Regards, Joonas
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