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

Tvrtko Ursulin
2022-11-24T12:42:13Z

On 23/11/2022 06:03, Dixit, Ashutosh wrote:
> On Mon, 21 Nov 2022 01:09:55 -0800, Tvrtko Ursulin wrote:
>>
>
> 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?

> (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..

> 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?

>
> 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?

Regards,

Tvrtko

>
> Thanks.
> --
> Ashutosh
>
>
>>> igt_spin_free(gem_fd, spin);
>>> gem_quiescent_gpu(gem_fd); /* Don't leak busy bo into the next phase */
>>> @@ -1597,11 +1602,11 @@ test_frequency(int gem_fd)
>>> * Set GPU to max frequency and read PMU counters.
>>> */
>>> igt_require(igt_sysfs_set_u32(sysfs, "gt_max_freq_mhz", max_freq));
>>> - igt_require(igt_sysfs_get_u32(sysfs, "gt_max_freq_mhz") == max_freq);
>>> igt_require(igt_sysfs_set_u32(sysfs, "gt_boost_freq_mhz", boost_freq));
>>> - igt_require(igt_sysfs_get_u32(sysfs, "gt_boost_freq_mhz") == boost_freq);
>>> -
>>> igt_require(igt_sysfs_set_u32(sysfs, "gt_min_freq_mhz", max_freq));
>>> + wait_freq_set();
>>> + igt_require(igt_sysfs_get_u32(sysfs, "gt_max_freq_mhz") == max_freq);
>>> + igt_require(igt_sysfs_get_u32(sysfs, "gt_boost_freq_mhz") == boost_freq);
>>> igt_require(igt_sysfs_get_u32(sysfs, "gt_min_freq_mhz") == max_freq);
>>> gem_quiescent_gpu(gem_fd);
>>> @@ -1613,6 +1618,7 @@ test_frequency(int gem_fd)
>>> max[0] = 1e9*(val[0] - start[0]) / slept;
>>> max[1] = 1e9*(val[1] - start[1]) / slept;
>>> + max_req = igt_sysfs_get_u32(sysfs, "gt_cur_freq_mhz");
>>
>> For the max freq use case we had 15% downward tolerance to account for
>> thermally challenged.
>>
>> a) Is that not enough with SLPC? Like there is a new "failure" mode?
>>
>> b) Is the extra 10% needed with the sysfs readback added? Aka do we need
>> both?
>>
>>> igt_spin_free(gem_fd, spin);
>>> gem_quiescent_gpu(gem_fd);
>>> @@ -1621,6 +1627,7 @@ test_frequency(int gem_fd)
>>> * Restore min/max.
>>> */
>>> igt_sysfs_set_u32(sysfs, "gt_min_freq_mhz", min_freq);
>>> + wait_freq_set();
>>> if (igt_sysfs_get_u32(sysfs, "gt_min_freq_mhz") != min_freq)
>>> igt_warn("Unable to restore min frequency to saved value [%u MHz], now %u MHz\n",
>>> min_freq, igt_sysfs_get_u32(sysfs, "gt_min_freq_mhz"));
>>> @@ -1633,12 +1640,12 @@ test_frequency(int gem_fd)
>>> igt_info("Max frequency: requested %.1f, actual %.1f\n",
>>> max[0], max[1]);
>>> - assert_within_epsilon(min[0], min_freq, tolerance);
>>> + assert_within_epsilon(min[0], min_req, tolerance);
>>> /*
>>> * On thermally throttled devices we cannot be sure maximum frequency
>>> * can be reached so use larger tolerance downards.
>>> */
>>> - __assert_within_epsilon(max[0], max_freq, tolerance, 0.15f);
>>> + __assert_within_epsilon(max[0], max_req, tolerance, 0.15f);
>>
>>
>> This 0.15f is what I was talking about above.
>>
>> Regards,
>>
>> Tvrtko
>>
>>> }
>>> static void