Re: [Intel-gfx] [PATCH 7/7] drm/i915/perf: add flushing ioctl

["Dixit, Ashutosh" <ashutosh.dixit@intel.com>]

On Wed, 04 Mar 2020 00:52:34 -0800, Lionel Landwerlin wrote:
>
> On 04/03/2020 07:48, Dixit, Ashutosh wrote:
> > On Tue, 03 Mar 2020 14:19:05 -0800, Umesh Nerlige Ramappa wrote:
> >> From: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
> >>
> >> With the currently available parameters for the i915-perf stream,
> >> there are still situations that are not well covered :
> >>
> >> If an application opens the stream with polling disable or at very low
> >> frequency and OA interrupt enabled, no data will be available even
> >> though somewhere between nothing and half of the OA buffer worth of
> >> data might have landed in memory.
> >>
> >> To solve this issue we have a new flush ioctl on the perf stream that
> >> forces the i915-perf driver to look at the state of the buffer when
> >> called and makes any data available through both poll() & read() type
> >> syscalls.
> >>
> >> v2: Version the ioctl (Joonas)
> >> v3: Rebase (Umesh)
> >>
> >> Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
> >> Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
> > [snip]
> >
> >> +/**
> >> + * i915_perf_flush_data - handle `I915_PERF_IOCTL_FLUSH_DATA` ioctl
> >> + * @stream: An enabled i915 perf stream
> >> + *
> >> + * The intention is to flush all the data available for reading from the OA
> >> + * buffer
> >> + */
> >> +static void i915_perf_flush_data(struct i915_perf_stream *stream)
> >> +{
> >> +	stream->pollin = oa_buffer_check(stream, true);
> >> +}
> > Since this function doesn't actually wake up any thread (which anyway can
> > be done by sending a signal to the blocked thread), is the only purpose of
> > this function to update OA buffer head/tail? But in that it is not clear
> > why a separate ioctl should be created for this, can't the read() call
> > itself call oa_buffer_check() to update the OA buffer head/tail?
> >
> > Again just trying to minimize uapi changes if possible.
>
> Most applications will call read() after being notified by poll()/select()
> that some data is available.

Correct this is the standard non blocking read behavior.

> Changing that behavior will break some of the existing perf tests .

I am not suggesting changing that (that standard non blocking read
behavior).

> If any data is available, this new ioctl will wake up existing waiters on
> poll()/select().

The issue is we are not calling wake_up() in the above function to wake up
any blocked waiters. The ioctl will just update the OA buffer head/tail so
that (a) a subsequent blocking read will not block, or (b) a subsequent non
blocking read will return valid data (not -EAGAIN), or (c) a poll/select
will not block but return immediately saying data is available.

That is why it seems to me the ioctl is not required, updating the OA
buffer head/tail can be done as part of the read() (and the poll/select)
calls themselves.

We will investigate if this can be done and update the patches in the next
revision accordingly. Thanks!
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