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Date: Sun, 25 Feb 2024 20:25:20 -0800
From: Boqun Feng <boqun.feng@gmail.com>
To: Michael Kelley <mhklinux@outlook.com>
Cc: Saurabh Singh Sengar <ssengar@linux.microsoft.com>,
	"haiyangz@microsoft.com" <haiyangz@microsoft.com>,
	"wei.liu@kernel.org" <wei.liu@kernel.org>,
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	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>
Subject: Re: [PATCH 1/1] Drivers: hv: vmbus: Calculate ring buffer size for
 more efficient use of memory
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On Tue, Feb 20, 2024 at 06:15:52PM +0000, Michael Kelley wrote:
> From: Boqun Feng <boqun.feng@gmail.com> Sent: Tuesday, February 20, 2024 9:29 AM
> > 
> > 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.
> 
> Storvsc and netvsc were previously not using VMBUS_RING_SIZE(),
> so space for the ring buffer header was already being "stolen" from
> the specified ring size.  But if this new version of VMBUS_RING_SIZE()
> still allows the ring buffer header space to be "stolen" from the
> large-ish ring buffers, I don't see that as a problem.  The ring buffer
> sizes have always been a swag value for the low-speed devices with
> small ring buffers.  For the high-speed devices (netvsc and storvsc)
> the ring buffer sizes have been somewhat determined by perf
> measurements, but the ring buffers are much bigger, so stealing
> a little bit of space doesn't have any noticeable effect.  Even with
> the perf measurements, the sizes aren't very exact. Performance
> just isn't sensitive to the size of the ring buffer except at a gross level.
> 

Fair enough.

> > 
> > 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?
> 
> Yes, that seems like a desirable long-term solution.  While the GPADL
> code handles vmalloc memory correctly (because the netvsc send and
> receive buffers are vmalloc memory), hv_ringbuffer_init() assumes
> physically contiguous memory.  It would need to use vmalloc_to_page()
> in building the pages_wraparound array instead of just indexing into
> the struct page array.  But that's an easy fix.  Another problem is the
> uio_hv_generic.c driver, where hv_uio_ring_mmap() assumes a physically
> contiguous ring. Maybe there's a straightforward way to fix it as well,
> but it isn't immediately obvious to me.
> 
> Using vmalloc memory for ring buffers has another benefit as well.
> Today, adding a new NIC to a VM that has been running for a while
> could fail because of not being able to allocate 1 Mbyte contiguous
> memory for the ring buffers used by each VMBus channel.  There
> could be plenty of memory available, but fragmentation could prevent
> getting 1 Mbyte contiguous.  Using vmalloc'ed ring buffers would
> solve this problem.
> 

Yep, that will be ideal.

Regards,
Boqun

> Michael
> 
> > 
> > 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
> > > >