Re: [Intel-gfx] [PATCH] drm/i915/perf: Do not clear pollin for small user read buffers

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

On Thu, 26 Mar 2020 02:09:34 -0700, Lionel Landwerlin wrote:
>
> On 26/03/2020 06:43, Ashutosh Dixit wrote:
> > It is wrong to block the user thread in the next poll when OA data is
> > already available which could not fit in the user buffer provided in
> > the previous read. In several cases the exact user buffer size is not
> > known. Blocking user space in poll can lead to data loss when the
> > buffer size used is smaller than the available data.
> >
> > This change fixes this issue and allows user space to read all OA data
> > even when using a buffer size smaller than the available data using
> > multiple non-blocking reads rather than staying blocked in poll till
> > the next timer interrupt.
> >
> > v2: Fix ret value for blocking reads (Umesh)
> >
> > Cc: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
> > Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
> > Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
> > ---
> >   drivers/gpu/drm/i915/i915_perf.c | 63 ++++++--------------------------
> >   1 file changed, 12 insertions(+), 51 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
> > index 3222f6cd8255..e2d083efba6d 100644
> > --- a/drivers/gpu/drm/i915/i915_perf.c
> > +++ b/drivers/gpu/drm/i915/i915_perf.c
> > @@ -2957,49 +2957,6 @@ void i915_oa_init_reg_state(const struct intel_context *ce,
> >		gen8_update_reg_state_unlocked(ce, stream);
> >   }
> >   -/**
> > - * i915_perf_read_locked - &i915_perf_stream_ops->read with error normalisation
> > - * @stream: An i915 perf stream
> > - * @file: An i915 perf stream file
> > - * @buf: destination buffer given by userspace
> > - * @count: the number of bytes userspace wants to read
> > - * @ppos: (inout) file seek position (unused)
> > - *
> > - * Besides wrapping &i915_perf_stream_ops->read this provides a common place to
> > - * ensure that if we've successfully copied any data then reporting that takes
> > - * precedence over any internal error status, so the data isn't lost.
> > - *
> > - * For example ret will be -ENOSPC whenever there is more buffered data than
> > - * can be copied to userspace, but that's only interesting if we weren't able
> > - * to copy some data because it implies the userspace buffer is too small to
> > - * receive a single record (and we never split records).
> > - *
> > - * Another case with ret == -EFAULT is more of a grey area since it would seem
> > - * like bad form for userspace to ask us to overrun its buffer, but the user
> > - * knows best:
> > - *
> > - *   http://yarchive.net/comp/linux/partial_reads_writes.html
> > - *
> > - * Returns: The number of bytes copied or a negative error code on failure.
> > - */
> > -static ssize_t i915_perf_read_locked(struct i915_perf_stream *stream,
> > -				     struct file *file,
> > -				     char __user *buf,
> > -				     size_t count,
> > -				     loff_t *ppos)
> > -{
> > -	/* Note we keep the offset (aka bytes read) separate from any
> > -	 * error status so that the final check for whether we return
> > -	 * the bytes read with a higher precedence than any error (see
> > -	 * comment below) doesn't need to be handled/duplicated in
> > -	 * stream->ops->read() implementations.
> > -	 */
> > -	size_t offset = 0;
> > -	int ret = stream->ops->read(stream, buf, count, &offset);
> > -
> > -	return offset ?: (ret ?: -EAGAIN);
> > -}
> > -
> >   /**
> >    * i915_perf_read - handles read() FOP for i915 perf stream FDs
> >    * @file: An i915 perf stream file
> > @@ -3025,6 +2982,8 @@ static ssize_t i915_perf_read(struct file *file,
> >   {
> >	struct i915_perf_stream *stream = file->private_data;
> >	struct i915_perf *perf = stream->perf;
> > +	size_t offset = 0;
> > +	int __ret;
> >	ssize_t ret;
> >		/* To ensure it's handled consistently we simply treat all reads of
> > a
> > @@ -3048,16 +3007,19 @@ static ssize_t i915_perf_read(struct file *file,
> >				return ret;
> >				mutex_lock(&perf->lock);
> > -			ret = i915_perf_read_locked(stream, file,
> > -						    buf, count, ppos);
> > +			__ret = stream->ops->read(stream, buf, count, &offset);
> > +			ret = offset ?: (__ret ?: -EAGAIN);
> >			mutex_unlock(&perf->lock);
> >		} while (ret == -EAGAIN);
> >	} else {
> >		mutex_lock(&perf->lock);
> > -		ret = i915_perf_read_locked(stream, file, buf, count, ppos);
> > +		__ret = stream->ops->read(stream, buf, count, &offset);
> > +		ret = offset ?: (__ret ?: -EAGAIN);
> >		mutex_unlock(&perf->lock);
> >	}
> >   +	/* Possible values for __ret are 0, -EFAULT, -ENOSPC, -EAGAIN,
> > ... */
> > +
> >	/* We allow the poll checking to sometimes report false positive EPOLLIN
> >	 * events where we might actually report EAGAIN on read() if there's
> >	 * not really any data available. In this situation though we don't
> > @@ -3065,13 +3027,12 @@ static ssize_t i915_perf_read(struct file *file,
> >	 * and read() returning -EAGAIN. Clearing the oa.pollin state here
> >	 * effectively ensures we back off until the next hrtimer callback
> >	 * before reporting another EPOLLIN event.
> > +	 * The exception to this is if ops->read() returned -ENOSPC which means
> > +	 * that more OA data is available than could fit in the user provided
> > +	 * buffer. In this case we want the next poll() call to not block.
> >	 */
> > -	if (ret >= 0 || ret == -EAGAIN) {
> > -		/* Maybe make ->pollin per-stream state if we support multiple
> > -		 * concurrent streams in the future.
> > -		 */
> > +	if ((ret > 0 || ret == -EAGAIN) && __ret != -ENOSPC)
> >		stream->pollin = false;
> > -	}
> >		return ret;
> >   }
>
> I think this reset of the pollin field is in the wrong place in the driver.
>
> The decision of whether pollin is true/false should be based off the
> difference between head/tail pointers.
>
> In my opinion the best place to do this in at the end of
> gen7/8_append_oa_reports functions, under the stream->oa_buffer.ptr_lock.
>
> If everything has been read up to the tail pointer, then there is nothing
> to wake up userspace for, otherwise leave pollin untouched.

Hi Lionel,

Are you seeing any problems of correctness in the code? My intention was to
use previously existing mechanisms (viz. -ENOSPC). Afais when
stream->ops->read() returns -ENOSPC it has already looked at head/tail
pointers and determined that there is data to be returned which it is
unable to because the provided buffer was too small.

Also, -ENOSPC can also be returned from append_oa_status(), though that can
probably be ignored.

Following your reasoning we should probably also say that pollin should be
set in oa_buffer_check_unlocked()?

About, stream->oa_buffer.ptr_lock, as I said previously, imo it is a lock
between a ring buffer producer (oa_buffer_check_unlocked()) and consumer
(i915_perf_read) which should not be needed, that ring buffer operation
should be lockless. Though we will need to check before removing it, maybe
I am wrong.

So unless you say there are real correctness problems in the patch or
previously existing code I am leaning towards leaving as it as is.

Thanks!
--
Ashutosh
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