Re: [RFC 1/4] drm/i915: Implement a framework for batch buffer pools

["Volkin, Bradley D" <bradley.d.volkin@intel.com>]

On Thu, Jun 19, 2014 at 02:48:29AM -0700, Tvrtko Ursulin wrote:
> 
> Hi Brad,
> 
> On 06/18/2014 05:36 PM, bradley.d.volkin@intel.com wrote:
> > From: Brad Volkin <bradley.d.volkin@intel.com>
> >
> > This adds a small module for managing a pool of batch buffers.
> > The only current use case is for the command parser, as described
> > in the kerneldoc in the patch. The code is simple, but separating
> > it out makes it easier to change the underlying algorithms and to
> > extend to future use cases should they arise.
> >
> > The interface is simple: alloc to create an empty pool, free to
> > clean it up; get to obtain a new buffer, put to return it to the
> > pool. Note that all buffers must be returned to the pool before
> > freeing it.
> >
> > The pool has a maximum number of buffers allowed due to some tests
> > (e.g. gem_exec_nop) creating a very large number of buffers (e.g.
> > ___). Buffers are purgeable while in the pool, but not explicitly
> > truncated in order to avoid overhead during execbuf.
> >
> > Locking is currently based on the caller holding the struct_mutex.
> > We already do that in the places where we will use the batch pool
> > for the command parser.
> >
> > Signed-off-by: Brad Volkin <bradley.d.volkin@intel.com>
> > ---
> >
> > r.e. pool capacity
> > My original testing showed something like thousands of buffers in
> > the pool after a gem_exec_nop run. But when I reran with the max
> > check disabled just now to get an actual number for the commit
> > message, the number was more like 130.  I developed and tested the
> > changes incrementally, and suspect that the original run was before
> > I implemented the actual copy operation. So I'm inclined to remove
> > or at least increase the cap in the final version. Thoughts?
> 
> Some random thoughts:
> 
> Is it strictly necessary to cap the pool size? I ask because it seems to 
> be introducing a limit where so far there wasn't an explicit one.

No, I only added it because there were a huge number of buffers in the
pool at one point. But that seems to have been an artifact of my
development process, so unless someone says they really want to keep
the cap, I'm going to drop it in the next rev.

> 
> Are object sizes generally page aligned or all you've seen all sizes in 
> the distribution? Either way, I am thinking whether some sort of 
> rounding up would be more efficient? Would it cause a problem if 
> slightly larger object was returned?

I believe objects must have page-aligned sizes.

> 
> Given that objects are only ever added to the pool, once max number is 
> allocated and there are no free ones of exact size it nags userspace 
> with EAGAIN and retires objects. But I wonder if the above points could 
> reduce that behaviour?
> 
> Could we get away without tracking the given out objects in a list and 
> just keep a list of available ones? In which case if object can only 
> ever be either in the free pool or on one of the existing GEM 
> active/inactive lists the same list head could be used?
> 
> Could it use its own locking just as easily? Just thinking if the future 
> goal is to fine grain locking, this seems self contained enough to be 
> doable straight away unless I am missing something.

I thought about putting a spinlock in the batch pool struct for
exactly that reason. But we have to hold the struct_mutex for at least
drm_gem_object_unreference(). I guess we don't actually need it for
i915_gem_alloc_object(). I opted to just put the mutex_is_locked()
checks so the dependency on the lock being held was hopefully easy to
find by the poor soul who eventually tries to tackle the locking
improvements :). But if people would rather I add a lock to the batch
pool struct, I can do that.

Thanks,
Brad

> 
> The above would make the pool more generic, but then I read Chris's 
> reply which perhaps suggests to make it more specialised so I don't know.
> 
> Regards,
> 
> Tvrtko
> 
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