On 11/03/2020 05:05, Dixit, Ashutosh wrote:
On Tue, 10 Mar 2020 13:44:30 -0700, Lionel Landwerlin wrote:
On 09/03/2020 21:51, Umesh Nerlige Ramappa wrote:
On Wed, Mar 04, 2020 at 09:56:28PM -0800, Dixit, Ashutosh wrote:
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@xxxxxxxxx>
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@xxxxxxxxx>
Signed-off-by: Umesh Nerlige Ramappa
<umesh.nerlige.ramappa@xxxxxxxxx>
[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!
In this case, where we are trying to determine if there is any data in
the oa buffer before the next interrupt has fired, user could call poll
with a reasonable timeout to determine if data is available or not. That
would eliminate the need for the flush ioctl. Thoughts?
Thanks,
Umesh
I almost forgot why this would cause problem.
Checking the state of the buffer every time you call poll() will pretty
much guarantee you have at least one report to read every time.
So that would lead to lot more wakeups :(
The whole system has to stay "unidirectional" with either interrupts or
timeout driving the wakeups.
This additional ioctl is the only solution I could find to add one more
input to the wakeup mechanism.
Well, aren't we asking the app to sleep for time T and then call flush
(followed by read)? Then we might as well ask them to sleep for time T and
call poll? Or we can ask them set the hrtimer to T, skip the sleep and call
poll (followed by read)? Aren't these 3 mechanisms equivalent? To me the
last option seems to be the cleanest. Thanks!
I guess one thing that could work is to call oa_buffer_check() at the
top of i915_oa_read().
Maybe you proposed this in a previous email.
That way poll() doesn't trigger too many wake ups and right before
closing the FD will pull the remaining data.
One side effect of this is that read() might almost always return
something because the HW is faster to write the OA buffer than the app
can call read().
Still I think it still works out.
What do you think?
-Lionel
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