Re: [PATCH 1/1] Drivers: hv: vmbus: Calculate ring buffer size for more efficient use of memory

[Boqun Feng <boqun.feng@gmail.com>]

On Mon, Feb 19, 2024 at 10:30:07PM -0800, Saurabh Singh Sengar wrote:
> On Mon, Feb 12, 2024 at 10:19:59PM -0800, mhkelley58@gmail.com wrote:
> > From: Michael Kelley <mhklinux@outlook.com>
> > 
> > The VMBUS_RING_SIZE macro adds space for a ring buffer header to the
> > requested ring buffer size.  The header size is always 1 page, and so
> > its size varies based on the PAGE_SIZE for which the kernel is built.
> > If the requested ring buffer size is a large power-of-2 size and the header
> > size is small, the resulting size is inefficient in its use of memory.
> > For example, a 512 Kbyte ring buffer with a 4 Kbyte page size results in
> > a 516 Kbyte allocation, which is rounded to up 1 Mbyte by the memory
> > allocator, and wastes 508 Kbytes of memory.
> > 
> > In such situations, the exact size of the ring buffer isn't that important,
> > and it's OK to allocate the 4 Kbyte header at the beginning of the 512
> > Kbytes, leaving the ring buffer itself with just 508 Kbytes. The memory
> > allocation can be 512 Kbytes instead of 1 Mbyte and nothing is wasted.
> > 
> > Update VMBUS_RING_SIZE to implement this approach for "large" ring buffer
> > sizes.  "Large" is somewhat arbitrarily defined as 8 times the size of
> > the ring buffer header (which is of size PAGE_SIZE).  For example, for
> > 4 Kbyte PAGE_SIZE, ring buffers of 32 Kbytes and larger use the first
> > 4 Kbytes as the ring buffer header.  For 64 Kbyte PAGE_SIZE, ring buffers
> > of 512 Kbytes and larger use the first 64 Kbytes as the ring buffer
> > header.  In both cases, smaller sizes add space for the header so
> > the ring size isn't reduced too much by using part of the space for
> > the header.  For example, with a 64 Kbyte page size, we don't want
> > a 128 Kbyte ring buffer to be reduced to 64 Kbytes by allocating half
> > of the space for the header.  In such a case, the memory allocation
> > is less efficient, but it's the best that can be done.
> > 
> > Fixes: c1135c7fd0e9 ("Drivers: hv: vmbus: Introduce types of GPADL")
> > Signed-off-by: Michael Kelley <mhklinux@outlook.com>
> > ---
> >  include/linux/hyperv.h | 22 +++++++++++++++++++++-
> >  1 file changed, 21 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
> > index 2b00faf98017..6ef0557b4bff 100644
> > --- a/include/linux/hyperv.h
> > +++ b/include/linux/hyperv.h
> > @@ -164,8 +164,28 @@ struct hv_ring_buffer {
> >  	u8 buffer[];
> >  } __packed;
> >  
> > +
> > +/*
> > + * If the requested ring buffer size is at least 8 times the size of the
> > + * header, steal space from the ring buffer for the header. Otherwise, add
> > + * space for the header so that is doesn't take too much of the ring buffer
> > + * space.
> > + *
> > + * The factor of 8 is somewhat arbitrary. The goal is to prevent adding a
> > + * relatively small header (4 Kbytes on x86) to a large-ish power-of-2 ring
> > + * buffer size (such as 128 Kbytes) and so end up making a nearly twice as
> > + * large allocation that will be almost half wasted. As a contrasting example,
> > + * on ARM64 with 64 Kbyte page size, we don't want to take 64 Kbytes for the
> > + * header from a 128 Kbyte allocation, leaving only 64 Kbytes for the ring.
> > + * In this latter case, we must add 64 Kbytes for the header and not worry
> > + * about what's wasted.
> > + */
> > +#define VMBUS_HEADER_ADJ(payload_sz) \
> > +	((payload_sz) >=  8 * sizeof(struct hv_ring_buffer) ? \
> > +	0 : sizeof(struct hv_ring_buffer))
> > +
> >  /* Calculate the proper size of a ringbuffer, it must be page-aligned */
> > -#define VMBUS_RING_SIZE(payload_sz) PAGE_ALIGN(sizeof(struct hv_ring_buffer) + \
> > +#define VMBUS_RING_SIZE(payload_sz) PAGE_ALIGN(VMBUS_HEADER_ADJ(payload_sz) + \
> >  					       (payload_sz))

I generally see the point of this patch, however, it changes the
semantics of VMBUS_RING_SIZE() (similiar as Saurabh mentioned below),
before VMBUS_RING_SIZE() will give you a ring buffer size which has at
least "payload_sz" bytes, but after the change, you may not get "enough"
bytes for the vmbus ring buffer.

One cause of the waste memory is using alloc_pages() to get physical
continuous, however, after a quick look into GPADL, looks like it also
supports uncontinuous pages. Maybe that's the longer-term solution?

Regards,
Boqun

> >  
> >  struct hv_ring_buffer_info {
> 
> Thanks for the patch.
> It's worth noting that this will affect the size of ringbuffer calculation for
> some of the drivers: netvsc, storvsc_drv, hid-hyperv, and hyperv-keyboard.c.
> It will be nice to have this comment added in commit for future reference.
> 
> Looks a good improvement to me,
> Reviewed-by: Saurabh Sengar <ssengar@linux.microsoft.com>
> 
> > -- 
> > 2.25.1
> >