Re: [Intel-gfx] [PATCH 2/3] drm/i915/guc/slpc: Add waitboost functionality for SLPC

Belgaumkar, Vinay
2021-11-02T00:19:57Z

On 11/1/2021 1:28 PM, Dixit, Ashutosh wrote:
> On Sun, 31 Oct 2021 21:39:36 -0700, Belgaumkar, Vinay wrote:
>>
>> @@ -945,6 +960,17 @@ void intel_rps_boost(struct i915_request *rq)
>> if (!test_and_set_bit(I915_FENCE_FLAG_BOOST, &rq->fence.flags)) {
>> struct intel_rps *rps = &READ_ONCE(rq->engine)->gt->rps;
>>
>> + if (rps_uses_slpc(rps)) {
>> + slpc = rps_to_slpc(rps);
>> +
>> + /* Return if old value is non zero */
>> + if (atomic_fetch_inc(&slpc->num_waiters))
>> + return;
>> +
>> + if (intel_rps_get_requested_frequency(rps) < slpc->boost_freq)
>
> I think this check is not needed because:
>
> a. The waitboost code only changes min_freq. i915 code should not depend on
> how GuC changes requested_freq in response to change in min_freq.
>
> b. What is more worrisome is that when we "de-boost" we set min_freq to
> min_freq_softlimit. If GuC e.g. has a delay in bringing requested_freq
> down and intel_rps_boost() gets called meanwhile we will miss the one
> opportunity we have to boost the freq (when num_waiters goes from 0 to
> 1. Asking GuC to boost when actual_freq is already boost_freq is
> harmless in comparison). So to avoid this risk of missing the chance to
> boost I think we should delete this check and replace the code above
> with something like:
>
> if (rps_uses_slpc(rps)) {
> struct intel_guc_slpc *slpc = rps_to_slpc(rps);
>
> if (slpc->boost_freq <= slpc->min_freq_softlimit)
> return;
>
> if (!atomic_fetch_inc(&slpc->num_waiters))
> schedule_work(&slpc->boost_work);
>
> return;
> }
>
> Note that this check:
>
> if (slpc->boost_freq <= slpc->min_freq_softlimit)
> return;
>
> (which is basically a degenerate case in which we don't have to do
> anything), can be probably be implemented when boost_freq is set in sysfs,
> or may already be encompassed in "val < slpc->min_freq" in
> intel_guc_slpc_set_boost_freq() in which case this check can also be
> skipped from this function.

We already have that check in set_boost_freq function. So, just adding
the atomic_fetch_inc check.

>
>> +void intel_guc_slpc_dec_waiters(struct intel_guc_slpc *slpc)
>> +{
>> + /* Return min back to the softlimit.
>> + * This is called during request retire,
>> + * so we don't need to fail that if the
>> + * set_param fails.
>> + */
>
> nit: maybe follow kernel multi-line comment format.
>
Ok.

Thanks,
Vinay.