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Date: Mon, 19 Feb 2024 22:30:07 -0800
From: Saurabh Singh Sengar <ssengar@linux.microsoft.com>
To: mhklinux@outlook.com
Cc: haiyangz@microsoft.com, wei.liu@kernel.org, decui@microsoft.com,
	boqun.feng@gmail.com, linux-kernel@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
Message-ID: <20240220063007.GA17584@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>
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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))
>  
>  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
>