Subject: Re: [PATCH v8 20/22] counter: Implement events_queue_size sysfs attribute

From: Jonathan Cameron <[email-masked]>
Date: Sun, 21 Feb 2021 15:51:40 +0000

On Thu, 18 Feb 2021 19:32:16 +0900
William Breathitt Gray <[email-masked]> wrote:

> On Sun, Feb 14, 2021 at 06:11:46PM +0000, Jonathan Cameron wrote:
> > On Fri, 12 Feb 2021 21:13:44 +0900
> > William Breathitt Gray <[email-masked]> wrote:
> >   
> > > The events_queue_size sysfs attribute provides a way for users to
> > > dynamically configure the Counter events queue size for the Counter
> > > character device interface. The size is in number of struct
> > > counter_event data structures. The number of elements will be rounded-up
> > > to a power of 2 due to a requirement of the kfifo_alloc function called
> > > during reallocation of the queue.
> > > 
> > > Cc: Oleksij Rempel <[email-masked]>
> > > Signed-off-by: William Breathitt Gray <[email-masked]>
> > > ---
> > >  Documentation/ABI/testing/sysfs-bus-counter |  8 +++++++
> > >  drivers/counter/counter-chrdev.c            | 23 +++++++++++++++++++
> > >  drivers/counter/counter-chrdev.h            |  2 ++
> > >  drivers/counter/counter-sysfs.c             | 25 +++++++++++++++++++++
> > >  4 files changed, 58 insertions(+)
> > > 
> > > diff --git a/Documentation/ABI/testing/sysfs-bus-counter b/Documentation/ABI/testing/sysfs-bus-counter
> > > index 847e96f19d19..f6cb2a8b08a7 100644
> > > --- a/Documentation/ABI/testing/sysfs-bus-counter
> > > +++ b/Documentation/ABI/testing/sysfs-bus-counter
> > > @@ -212,6 +212,14 @@ Description:
> > >  		both edges:
> > >  			Any state transition.
> > >  
> > > +What:		/sys/bus/counter/devices/counterX/events_queue_size
> > > +KernelVersion:	5.13
> > > +Contact:	[email-masked]
> > > +Description:
> > > +		Size of the Counter events queue in number of struct
> > > +		counter_event data structures. The number of elements will be
> > > +		rounded-up to a power of 2.
> > > +
> > >  What:		/sys/bus/counter/devices/counterX/name
> > >  KernelVersion:	5.2
> > >  Contact:	[email-masked]
> > > diff --git a/drivers/counter/counter-chrdev.c b/drivers/counter/counter-chrdev.c
> > > index 16f02df7f73d..53eea894e13f 100644
> > > --- a/drivers/counter/counter-chrdev.c
> > > +++ b/drivers/counter/counter-chrdev.c
> > > @@ -375,6 +375,29 @@ void counter_chrdev_remove(struct counter_device *const counter)
> > >  	cdev_del(&counter->chrdev);
> > >  }
> > >  
> > > +int counter_chrdev_realloc_queue(struct counter_device *const counter,
> > > +				 size_t queue_size)
> > > +{
> > > +	int err;
> > > +	DECLARE_KFIFO_PTR(events, struct counter_event);
> > > +	unsigned long flags;
> > > +
> > > +	/* Allocate new events queue */
> > > +	err = kfifo_alloc(&events, queue_size, GFP_ATOMIC);  
> > 
> > Is there any potential for losing events?  
> 
> We take the events_list_lock down below so we're safe against missing an
> event, but past events currently unread in the queue will be lost.
> 
> Shortening the size of the queue is inherently a destructive process if
> we have more events in the current queue than can fit in the new queue.
> Because we a liable to lose some events in such a case, I think it's
> best to keep the behavior of this reallocation consistent and have it
> provide a fresh empty queue every time, as opposed to sometimes dropping
> events and sometimes not.
> 
> I also suspect an actual user would be setting the size of their queue
> to the required amount before they begin watching events, rather than
> adjusting it sporadically during a live operation.
>

Absolutely agree.   As such I wonder if you are better off enforcing this
behaviour?  If the cdev is open for reading, don't allow the fifo to be
resized. 

Jonathan

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