Re: [Intel-gfx] [PATCH v5 1/7] drm/i915/pmu: Change bitmask of enabled events to u32

[Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>]

On Wed, May 17, 2023 at 09:25:03AM -0700, Dixit, Ashutosh wrote:
>On Wed, 17 May 2023 01:26:15 -0700, Tvrtko Ursulin wrote:
>>
>>
>> On 17/05/2023 07:55, Umesh Nerlige Ramappa wrote:
>> > On Tue, May 16, 2023 at 05:25:50PM -0700, Dixit, Ashutosh wrote:
>> >> On Tue, 16 May 2023 16:35:28 -0700, Umesh Nerlige Ramappa wrote:
>> >>>
>> >>
>> >> Hi Umesh/Tvrtko,
>> >>
>> >> Mostly repeating comments/questions made on the previous patch below.
>>
>> First of all thanks for improving this, my v1 obviously wasn't good enough.
>>
>> >>
>> >>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>> >>>
>> >>> Having it as u64 was a confusing (but harmless) mistake.
>> >>>
>> >>> Also add some asserts to make sure the internal field does not overflow
>> >>> in the future.
>> >>>
>> >>> v2: Fix WARN_ON firing for INTERRUPT event (Umesh)
>> >>>
>> >>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>> >>> Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
>> >>> Cc: Ashutosh Dixit <ashutosh.dixit@intel.com>
>> >>> ---
>> >>>  drivers/gpu/drm/i915/i915_pmu.c | 26 ++++++++++++++++++--------
>> >>>  1 file changed, 18 insertions(+), 8 deletions(-)
>> >>>
>> >>> diff --git a/drivers/gpu/drm/i915/i915_pmu.c
>> >>> b/drivers/gpu/drm/i915/i915_pmu.c
>> >>> index 7ece883a7d95..96543dce2db1 100644
>> >>> --- a/drivers/gpu/drm/i915/i915_pmu.c
>> >>> +++ b/drivers/gpu/drm/i915/i915_pmu.c
>> >>> @@ -50,7 +50,7 @@ static u8 engine_event_instance(struct perf_event
>> >>> *event)
>> >>>     return (event->attr.config >> I915_PMU_SAMPLE_BITS) & 0xff;
>> >>>  }
>> >>>
>> >>> -static bool is_engine_config(u64 config)
>> >>> +static bool is_engine_config(const u64 config)
>> >>>  {
>> >>>     return config < __I915_PMU_OTHER(0);
>> >>>  }
>> >>> @@ -88,9 +88,20 @@ static unsigned int config_bit(const u64 config)
>> >>>         return other_bit(config);
>> >>>  }
>> >>>
>> >>> -static u64 config_mask(u64 config)
>> >>> +static u32 config_mask(const u64 config)
>> >>>  {
>> >>> -    return BIT_ULL(config_bit(config));
>> >>> +    unsigned int bit = config_bit(config);
>> >>
>> >> Give that config_bit() can return -1 (I understand it is avoided in
>> >> moving
>> >> the code to config_mask from config_bit), maybe the code below should
>> >> also
>> >> have that check?
>> >
>> > config_mask is only called to check frequency related events in the code,
>> > so I don't see it returing -1 here.
>>
>> Yeah that should be fine since -1 would make the below asserts fire
>> anyway. (If it would get called from a different path in the future.)
>>
>> >>
>> >>     int bit = config_bit(config);
>> >>
>> >>     if (bit != -1)
>> >>     {
>> >>         ...
>> >>     }
>> >>
>> >> Though as mentioned below the 'if (__builtin_constant_p())' would have to
>> >> go. Maybe the code could even have stayed in config_bit with the check.
>> >>
>> >>> +
>> >>> +    if (__builtin_constant_p(config))
>> >>> +        BUILD_BUG_ON(bit >
>> >>> +                 BITS_PER_TYPE(typeof_member(struct i915_pmu,
>> >>> +                             enable)) - 1);
>> >>
>> >> Given that config comes from the event (it is event->attr.config), can
>> >> this
>> >> ever be a builtin constant?
>> >
>> > Not sure about earlier code where these checks were inside config_bit(),
>> > but with changes I made, I don't see this being a builtin
>> > constant. However, nothing prevents a caller from just passing a
>> > builtin_constant to this in future.
>>
>> Are you sure? I would have thought it would always be a compile time
>> constant now that the check is in config_mask. Aahhh.. with the multi-tile
>> changes maybe it can't unroll the loops and calculate the masks at compile
>> time. Maybe it is a bit too much and we should drop the
>> __builtin_constant_p branch? Probably..
>
>Ah yes, with the code move to config_mask, they really all are compile time
>constants (provided compiler can unroll the loops) so at least that is the
>justfication for leaving the __builtin_constant_p in. So I'd probably just
>leave it as is (though it is a bit too much).
>
>> But I guess it is safe to use GEM_WARN_ON_ONCE instead of WARN_ON_ONCE
>> since there are no external callers (nothing coming from event) now. That
>> way at least production builds don't have to have the check.
>
>Hmm, there's a GEM_WARN_ON but no GEM_WARN_ON_ONCE. So leave that as is too
>I guess.
>
>So I'm ok with the code staying as is. Enough bike-shed on this already.

Leaving it as is. @Ashutosh, okay to use your R-b without any changes to 
this patch?

Thanks,
Umesh

>
>Thanks.
>--
>Ashutosh
>
>
>>
>> Regards,
>>
>> Tvrtko
>>
>> >
>> > Thanks,
>> > Umesh
>> >
>> >>
>> >>> +    else
>> >>> +        WARN_ON_ONCE(bit >
>> >>> +                 BITS_PER_TYPE(typeof_member(struct i915_pmu,
>> >>> +                             enable)) - 1);
>> >>
>> >> There is really an even stricter limit on what the bit can be, which is
>> >> the
>> >> total number of possible events but anyway this is good enough.
>> >>
>> >> After addressing the above, this patch is:
>> >>
>> >> Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
>> >>
>> >>> +
>> >>> +    return BIT(config_bit(config));
>> >>>  }
>> >>>
>> >>>  static bool is_engine_event(struct perf_event *event)
>> >>> @@ -633,11 +644,10 @@ static void i915_pmu_enable(struct perf_event
>> >>> *event)
>> >>>  {
>> >>>     struct drm_i915_private *i915 =
>> >>>         container_of(event->pmu, typeof(*i915), pmu.base);
>> >>> +    const unsigned int bit = event_bit(event);
>> >>>     struct i915_pmu *pmu = &i915->pmu;
>> >>>     unsigned long flags;
>> >>> -    unsigned int bit;
>> >>>
>> >>> -    bit = event_bit(event);
>> >>>     if (bit == -1)
>> >>>         goto update;
>> >>>
>> >>> @@ -651,7 +661,7 @@ static void i915_pmu_enable(struct perf_event
>> >>> *event)
>> >>>     GEM_BUG_ON(bit >= ARRAY_SIZE(pmu->enable_count));
>> >>>     GEM_BUG_ON(pmu->enable_count[bit] == ~0);
>> >>>
>> >>> -    pmu->enable |= BIT_ULL(bit);
>> >>> +    pmu->enable |= BIT(bit);
>> >>>     pmu->enable_count[bit]++;
>> >>>
>> >>>     /*
>> >>> @@ -698,7 +708,7 @@ static void i915_pmu_disable(struct perf_event
>> >>> *event)
>> >>>  {
>> >>>     struct drm_i915_private *i915 =
>> >>>         container_of(event->pmu, typeof(*i915), pmu.base);
>> >>> -    unsigned int bit = event_bit(event);
>> >>> +    const unsigned int bit = event_bit(event);
>> >>>     struct i915_pmu *pmu = &i915->pmu;
>> >>>     unsigned long flags;
>> >>>
>> >>> @@ -734,7 +744,7 @@ static void i915_pmu_disable(struct perf_event
>> >>> *event)
>> >>>      * bitmask when the last listener on an event goes away.
>> >>>      */
>> >>>     if (--pmu->enable_count[bit] == 0) {
>> >>> -        pmu->enable &= ~BIT_ULL(bit);
>> >>> +        pmu->enable &= ~BIT(bit);
>> >>>         pmu->timer_enabled &= pmu_needs_timer(pmu, true);
>> >>>     }
>> >>>
>> >>> --
>> >>> 2.36.1
>> >>>