[Intel-gfx] [PATCH 1/3] drm/i915/perf: Reduce cpu overhead for blocking perf OA reads

[Lionel Landwerlin]

On 13/04/2020 18:48, Umesh Nerlige Ramappa wrote:
> A condition in wait_event_interruptible seems to be checked twice before
> waiting on the event to occur. These checks are redundant when hrtimer
> events will call oa_buffer_check_unlocked to update the oa_buffer tail
> pointers. The redundant checks add cpu overhead. Simplify the check
> to reduce cpu overhead when using blocking io to read oa buffer reports.
>
> Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa at intel.com>


I cherry picked this patch alone and it seems to break the 
disabled-read-error test.


> ---
>   drivers/gpu/drm/i915/i915_perf.c | 28 +++++++++++++++++++++++++++-
>   1 file changed, 27 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
> index 5cde3e4e7be6..e28a3ab83fde 100644
> --- a/drivers/gpu/drm/i915/i915_perf.c
> +++ b/drivers/gpu/drm/i915/i915_perf.c
> @@ -541,6 +541,32 @@ static bool oa_buffer_check_unlocked(struct i915_perf_stream *stream)
>   	return pollin;
>   }
>   
> +/**
> + * oa_buffer_check_reports - quick check if reports are available
> + * @stream: i915 stream instance
> + *
> + * The return from this function is used as a condition for
> + * wait_event_interruptible in blocking read. This is used to detect
> + * available reports.
> + *
> + * A condition in wait_event_interruptible seems to be checked twice before
> + * waiting on an event to occur. These checks are redundant when hrtimer events
> + * will call oa_buffer_check_unlocked to update the oa_buffer tail pointers. The
> + * redundant checks add cpu overhead. We simplify the check to reduce cpu
> + * overhead.
> + */
> +static bool oa_buffer_check_reports(struct i915_perf_stream *stream)
> +{
> +	unsigned long flags;
> +	bool available;
> +
> +	spin_lock_irqsave(&stream->oa_buffer.ptr_lock, flags);
> +	available = stream->oa_buffer.tail != stream->oa_buffer.head;
> +	spin_unlock_irqrestore(&stream->oa_buffer.ptr_lock, flags);
> +
> +	return available;
> +}
> +
>   /**
>    * append_oa_status - Appends a status record to a userspace read() buffer.
>    * @stream: An i915-perf stream opened for OA metrics
> @@ -1150,7 +1176,7 @@ static int i915_oa_wait_unlocked(struct i915_perf_stream *stream)
>   		return -EIO;
>   
>   	return wait_event_interruptible(stream->poll_wq,
> -					oa_buffer_check_unlocked(stream));
> +					oa_buffer_check_reports(stream));
>   }
>   
>   /**