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

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

On Tue, 31 Mar 2020 00:34:10 -0700, Lionel Landwerlin wrote:
>
> On 31/03/2020 08:22, 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)
> > v3: Mistake during patch send (Ashutosh)
> > v4: Remove -EAGAIN from comment (Umesh)
> > v5: Improve condition for clearing pollin and return (Lionel)
> >
> > 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>
>
> I forgot to mention this needs to be Cc: stable.

I will Cc stable or send them the patch after it's finalized, hope that
will be ok?

>
> Still one nit below which should make the remaining function a bit simpler.
>
> Thanks for your time.
>
> -Lionel
>
>
> > ---
> >   drivers/gpu/drm/i915/i915_perf.c | 62 +++++++-------------------------
> >   1 file changed, 13 insertions(+), 49 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
> > index c74ebac50015..9c21f28f89a7 100644
> > --- a/drivers/gpu/drm/i915/i915_perf.c
> > +++ b/drivers/gpu/drm/i915/i915_perf.c
> > @@ -2914,49 +2914,6 @@ 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
> > @@ -2982,7 +2939,8 @@ static ssize_t i915_perf_read(struct file *file,
> >   {
> >	struct i915_perf_stream *stream = file->private_data;
> >	struct i915_perf *perf = stream->perf;
> > -	ssize_t ret;
> > +	size_t offset = 0;
> > +	int __ret;
> >		/* To ensure it's handled consistently we simply treat all reads of
> > a
> >	 * disabled stream as an error. In particular it might otherwise lead
> > @@ -2992,6 +2950,8 @@ static ssize_t i915_perf_read(struct file *file,
> >		return -EIO;
> >		if (!(file->f_flags & O_NONBLOCK)) {
> > +		ssize_t ret;
> > +
> >		/* There's the small chance of false positives from
> >		 * stream->ops->wait_unlocked.
> >		 *
> > @@ -3005,13 +2965,13 @@ 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);
>
>
> I think you can keep using ret and just change the loop to while (ret >= 0)
> (which means no failure).
>
> You will get -ENOSPC when the whole buffer is filled or some other error
> which should trigger stream closure.

Sorry, but I did not follow you here. Are you saying even when we have data
to return (offset > 0) you want to go back and block (in the
wait_unlocked())? I am not sure if that will be acceptable, I'd think the
expectation would be to return data at the rate of the OA timer.

> Finally you can 0 if nothing was written but there was nothing to read and
> that keeps the read going.

Here I really lost you. Are you talking about -ENOSPC returning after
blocking multiple times and then resetting the error to 0 somehow?

Could you please look at v6 and let me know what you think? I have
eliminated the second ret variable by changing the do-while loop to "do { }
while (!offset && !ret);" and thus mostly retain the original logic.

Thanks!
--
Ashutosh

>
>
> > +			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);
> >		mutex_unlock(&perf->lock);
> >	}
> >   @@ -3022,11 +2982,15 @@ 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)
> > +	if (__ret != -ENOSPC)
> >		stream->pollin = false;
> >   -	return ret;
> > +	/* Possible values for __ret are 0, -EFAULT, -ENOSPC, -EIO, ... */
> > +	return offset ?: (__ret ?: -EAGAIN);
> >   }
> >     static enum hrtimer_restart oa_poll_check_timer_cb(struct hrtimer
> > *hrtimer)
>
>
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