Re: [igt-dev] [PATCH i-g-t] tests/perf_pmu: Compare against requested freq in frequency subtest

Tvrtko Ursulin
2022-12-19T08:47:33Z

On 17/12/2022 02:49, Dixit, Ashutosh wrote:
> On Fri, 16 Dec 2022 07:39:52 -0800, Rodrigo Vivi wrote:
>> On Fri, Dec 16, 2022 at 09:37:31AM +0000, Tvrtko Ursulin wrote:
>>>
>>> On 16/12/2022 06:21, Dixit, Ashutosh wrote:
>>>> On Thu, 24 Nov 2022 04:42:08 -0800, Tvrtko Ursulin wrote:
>>>>>
>>>>> On 23/11/2022 06:03, Dixit, Ashutosh wrote:
>>>>>> On Mon, 21 Nov 2022 01:09:55 -0800, Tvrtko Ursulin wrote:
>>>>>>>
>>>>>>
>>>>
>>>> Hi Tvrtko,
>>>>
>>>> Sorry for the delay in replying on this. We are discussing this but still
>>>> no conclusion. I have replied what I can below, will update again after I
>>>> know more. Wanted to send a reply before disappearing for the
>>>> holidays. Please see below.
>>>>
>>>>>> Hi Tvrtko,
>>>>>>
>>>>>> I am only offering an overall clarification below first, not answering all
>>>>>> of your points for now.
>>>>>>
>>>>>>>
>>>>>>> On 19/11/2022 02:00, Ashutosh Dixit wrote:
>>>>>>>> With SLPC, even when we set the same min and max freq's, the requested and
>>>>>>>> actual freq's can differ from the min/max freq set. For example "efficient
>>>>>>>> freq" (when in effect) can override set min/max freq. In general FW is the
>>>>>>>> final arbiter in determining freq and can override set values.
>>>>>>>>
>>>>>>>> Therefore in igt@perf_pmu@frequency subtest, compare the requested freq
>>>>>>>> reported by PMU not against the set freq's but against the requested freq
>>>>>>>> reported in sysfs. Also add a delay after setting freq's to account for
>>>>>>>> messaging delays in setting freq's in GuC.
>>>>>>>>
>>>>>>>> v2: Introduce a 100 ms delay after setting freq
>>>>>>>> v3: Update commit message, code identical to v2
>>>>>>>>
>>>>>>>> Bug: https://gitlab.freedesktop.org/drm/intel/-/issues/6806
>>>>>>>> Cc: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
>>>>>>>> Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
>>>>>>>> Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
>>>>>>>> ---
>>>>>>>> tests/i915/perf_pmu.c | 23 +++++++++++++++--------
>>>>>>>> 1 file changed, 15 insertions(+), 8 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/tests/i915/perf_pmu.c b/tests/i915/perf_pmu.c
>>>>>>>> index f363db2ba13..02f6ae989b1 100644
>>>>>>>> --- a/tests/i915/perf_pmu.c
>>>>>>>> +++ b/tests/i915/perf_pmu.c
>>>>>>>> @@ -1543,10 +1543,13 @@ test_interrupts_sync(int gem_fd)
>>>>>>>> igt_assert_lte(target, busy);
>>>>>>>> }
>>>>>>>> +/* Wait for GuC SLPC freq changes to take effect */
>>>>>>>> +#define wait_freq_set() usleep(100000)
>>>>>>>
>>>>>>> A future task of maybe adding a drop_caches flag to wait for this?
>>>>>>>
>>>>>>> Alternatively, if we have a sysfs or debugfs file which reflects the value
>>>>>>> once GuC has processed it, keep reading it until it is updated? Even if to
>>>>>>> something other than it was before the write, to handle the inability to
>>>>>>> set the exact requested frequency sometimes?
>>>>>>>>
>>>>>>>> +
>>>>>>>> static void
>>>>>>>> test_frequency(int gem_fd)
>>>>>>>> {
>>>>>>>> - uint32_t min_freq, max_freq, boost_freq;
>>>>>>>> + uint32_t min_freq, max_freq, boost_freq, min_req, max_req;
>>>>>>>> uint64_t val[2], start[2], slept;
>>>>>>>> double min[2], max[2];
>>>>>>>> igt_spin_t *spin;
>>>>>>>> @@ -1572,10 +1575,11 @@ test_frequency(int gem_fd)
>>>>>>>> * Set GPU to min frequency and read PMU counters.
>>>>>>>> */
>>>>>>>> igt_require(igt_sysfs_set_u32(sysfs, "gt_min_freq_mhz", min_freq));
>>>>>>>> - igt_require(igt_sysfs_get_u32(sysfs, "gt_min_freq_mhz") == min_freq);
>>>>>>>> igt_require(igt_sysfs_set_u32(sysfs, "gt_max_freq_mhz", min_freq));
>>>>>>>> - igt_require(igt_sysfs_get_u32(sysfs, "gt_max_freq_mhz") == min_freq);
>>>>>>>> igt_require(igt_sysfs_set_u32(sysfs, "gt_boost_freq_mhz", min_freq));
>>>>>>>> + wait_freq_set();
>>>>>>>> + igt_require(igt_sysfs_get_u32(sysfs, "gt_min_freq_mhz") == min_freq);
>>>>>>>> + igt_require(igt_sysfs_get_u32(sysfs, "gt_max_freq_mhz") == min_freq);
>>>>>>>> igt_require(igt_sysfs_get_u32(sysfs, "gt_boost_freq_mhz") == min_freq);
>>>>>>>> gem_quiescent_gpu(gem_fd); /* Idle to be sure the change takes
>>>>>>>> effect */
>>>>>>>> @@ -1587,6 +1591,7 @@ test_frequency(int gem_fd)
>>>>>>>> min[0] = 1e9*(val[0] - start[0]) / slept;
>>>>>>>> min[1] = 1e9*(val[1] - start[1]) / slept;
>>>>>>>> + min_req = igt_sysfs_get_u32(sysfs, "gt_cur_freq_mhz");
>>>>>>>
>>>>>>> This really leaves a bad taste for me still, sorry. First of all, it may
>>>>>>> make it more palatable if readback was immediately after wait_freq_set(),
>>>>>>> or as part of it. Otherwise it seems like even more confused test and sysfs
>>>>>>> ABI.
>>>>>>>
>>>>>>> Did we close on my question of whether we can make readback of
>>>>>>> gt_max_freq_mhz actually represent the real max? Correct me please if I got
>>>>>>> confused - with min freq checking here we write max=300 and get 350? If so
>>>>>>> can we make gt_max_freq_mhz return 350 after the write of 300? Or return an
>>>>>>> error?
>>>>>>
>>>>>> Did you read Vinay's email on v1 here:
>>>>>>
>>>>>> https://patchwork.freedesktop.org/patch/510304/?series=110574&rev=1#comment_921571
>>>>>>
>>>>>> As Vinay says, SLPC is requesting an entirely different freq (the effecient
>>>>>> freq rather than the set 'min == max freq') and no amount of waiting will
>>>>>> result in the requested freq being the same as the set 'min == max
>>>>>> freq'. Also the efficient freq cannot be controlled from user space.
>>>>>
>>>>> Why is that not a break in our ABI? What is the point in allowing users to
>>>>> write into this field if the desired value can be silently ignored?
>>>>
>>>> Yes, this is in discussion. FW is doing this and we need to see if FW can
>>>> be made to obey the i915 ABI. IMHO what matters is performance per watt and
>>>> if the ABI has to be tossed so be it. I will keep this list posted about
>>>> what happens here. Merging any patches for these issues (GL #6806 and
>>>> #6786) is blocked till the ABI breakage issue is resolved one way or
>>>> another.
>>>
>>> Two thoughts here, one has been mentioned before.
>>>
>>> New one is that performance per watt perhaps does not matter (is not a
>>> primary concern at least) for use cases where "users" (for some definition)
>>> tweak the min/max in sysfs. Presumably in those cases user wants control,
>>> for some reason, so as long as it is safe and within the limits why not give
>>> it to them.
>>>
>>> Same argument from a different angle - what is the use case for allowing
>>> min/max control which does not really do what it says on the tin?
>>>
>>> Old thought is this - if SLPC refuses to be controlled using our existing
>>> controls then we could just say -ENODEV on attempts to modify them. Whether
>>> for any input value, or some, depending on what SLPC can support (or wants
>>> to support).
>>
>> The problem is not just SLPC. The freq is ultimately decided by PCODE and it
>> has sudo and all the rights to disrespect what drivers (including SLPC as a
>> a driver from PCODE perspective) are requesting.
>>
>>>
>>> Important to note I am primarily focusing on the min freq here, which AFAIR
>>> we concluded can behave that user writes in 300, but SLPC decides it will
>>> only go as low as 350.
>>
>> The other caveat here with the efficient freq is that it is dynamic and PCODE
>> decides it with runtime conditions. It may start with 300, then move to 350,
>> then back to 300. i915 is only stashing the initial value.
>>
>> We could potentially update our rpe everytime any freq control is
>> touched.
>
> We thought of doing this but two points:
>
> 1. i915 now reads the same register as GuC for rpe but I never see the
> value in this register change.
> 2. Even if we could know the real rpe, the freq's might be set before a
> workload start running and rpe might change after the workload starts
> running. So knowing rpe before the workload might not be very useful.
>
> I am of the opposite opinion that anyway the previous kernel ABI is
> probably now irrecoverably bust, so we should give FW free reign and stop
> disabling efficient freq the way we do now when min_freq < rpe_freq.

We have to answer the question of why we need to keep exposing the sysfs
controls if they don't/can't/won't do what they say on the tin. Could we
make a break with DG2 for instance? Some use cases might suffer but it's
probably better than having them think it does what they think it does,
or file bugs when it does not.

Regards,

Tvrtko

>
> Cheers.
> --
> Ashutosh
>
>> Just to highlight how bad this is, I've seen cases where rpe in runtime
>> goes below the minimal fused freq (our rpn).
>>
>>> Because that is different from the existing, should
>>> be well known, situation where we can write a max which cannot be achieved
>>> due thermal throttling.
>>
>> yeap, usually when things got disrespected by PCODE there will be some thermal
>> throttling information. But I have seen cases where this indicators simply do
>> not exist. What complicate the api expectation to a very bad level.
>>
>>>
>>> Perhaps we need a table, or a summary of some kind, which explains the
>>> possible behaviours for both min and max settings.
>>
>> This is probably a good idea. At least document everything and what and when
>> things can be disrespected, etc...
>>
>> (and update our rpe info dynamically)
>>
>>>
>>>>>> (So in the example above min and max are set to 300 MHz and SLPC is
>>>>>> requesting efficient freq which is 350 or 400 MHz. So min and max values
>>>>>> are correct at 300 MHz and requested/actual freq will show 350/400 MHz).
>>>>>>
>>>>>> Let me also explain what happens in GuC FW when we change min/max freq:
>>>>>> * i915 receives min/max freq from sysfs and sends a message to GuC to
>>>>>> change min/max freq.
>>>>>> * The message itself might be queued behind other messages in the host to
>>>>>> GuC queue. i915 is still blocked in the sysfs handler.
>>>>>> * Now when GuC receives the message interrupt, in its top-half handler it
>>>>>> validates the freq parameters and sends a message back to host saying the
>>>>>> freq change has been accepted. But min/max freq is actually changed later
>>>>>> in the bottom-half handler in GuC FW. So the top half only validates the
>>>>>> freq value.
>>>>>> * When i915 receives the message from GuC top half, the sysfs handler
>>>>>> unblocks and starts reflecting the new min/max freq value.
>>>>>>
>>>>>> So the delay for the new min/max freq values to take effect is between
>>>>>> returning from the sysfs handler and when the min/max freq are really set
>>>>>> in GuC bottom half (an indeterminate possibly large amount of time if GuC
>>>>>> is busy).
>>>>>
>>>>> Do we have a drop_caches flag which would force waiting of idle GuC, which
>>>>> would cover the bottom half as well? Could we or should we add it if not?
>>>>> Would beat the usleep approach..
>>>>
>>>> Agree, waiting for GuC to idle would do it, but not sure "how" to do it. As
>>>> I said there is no notification after the bottom half is done applying the
>>>> freq settings. We may need to schedule something after the freq setting
>>>> host to guc command and wait for that to be done. A little bit out of my
>>>> depth on this, let me know if you have any ideas here. I'll ask the GuC
>>>> guys too. Currently we don't have a drop_caches flag for this.
>>>
>>> Okay, I was wondering if DROP_IDLE would do it, but perhaps this bottom half
>>> can run independently from those flows so it was just a thought, not a
>>> claim.
>>>
>>>>>> So as you see this introduces an indeterminacy due to to GuC interface not
>>>>>> being synchronous. If this needs to be fixed, it needs to be fixed in GuC
>>>>>> FW rather than waiting etc. in i915. GuC needs to implement another
>>>>>> notification to host /after/ the min/max freq change has been applied in
>>>>>> its bottom half. Doing this is not a priority for the GuC team at this
>>>>>> point.
>>>>>
>>>>> Is there a method for querying the real min from GuC, which would maybe
>>>>> work after waiting for GuC to idle?
>>>>
>>>> Will find out about this too. Afaik, at present querying the real min from
>>>> GuC is not possible. As soon as the top half has completed the freq
>>>> settings are updated so querying will return the new settings even though
>>>> they might not have been applied by the bottom half yet.
>>>
>>> And it will return the written value, or the real one? Going back to the 300
>>> written in, 350 actual min freq example. Presumably 300 and 350 SLPC picks
>>> is completely invisible to i915 via the FW interface?
>>>
>>>>>> However, this bug is not due to these delays but due to GuC requesting the
>>>>>> efficient freq instead of the set 'min == max freq' as mentioned above.
>>>>>>
>>>>>> If all these delays are confusing we can revert to v1 of the patch:
>>>>>>
>>>>>> https://patchwork.freedesktop.org/patch/510304/?series=110574&rev=1
>>>>>>
>>>>>> v1 is simple and does not introduce these delays but just reads the
>>>>>> requested freq from sysfs after the 500 ms PMU measurement but while the
>>>>>> spinner is still running (which we do in v3 too). Without load SLPC will
>>>>>> only request RPn so to compare with PMU measurement we must read the
>>>>>> requested freq from sysfs with the spinner load. v1 is sufficient to fix
>>>>>> the bug but I thought to be completely safe it is better to add
>>>>>> wait_freq_set() after changing min/max freq (and hence v3).
>>>>>>
>>>>>> I hope this clarifies things somewhat. If not please let me know.
>>>>>
>>>>> It clarifies a large chunk thank you.
>>>>>
>>>>> Another question which you missed is why is it needed for the max mode?
>>>>> There we allow a 15% error (more than 5% default). Is that not enough with
>>>>> SLPC?
>>>>
>>>> Yes I think you are right, should not be needed for the max mode. The real
>>>> issue is that efficient freq can exceed the max freq set but I think that
>>>> should not be an issue if the max is RP0.
>>>
>>> Ah okay, so SLPC not respecting max same as with min. I think the question
>>> of what is the use case for min/max controls in the SLPC world is the one to
>>> answer then.
>>>
>>> Regards,
>>>
>>> Tvrtko