RE: [PATCH v4 1/2] dma-direct: provide the ability to reserve per-numa CMA

Song Bao Hua (Barry Song)
2020-07-28T12:19:27Z

> -----Original Message-----
> From: Christoph Hellwig [mailto:hch@lst.de]
> Sent: Tuesday, July 28, 2020 11:53 PM
> To: Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com>
> Cc: hch@lst.de; m.szyprowski@samsung.com; robin.murphy@arm.com;
> will@kernel.org; ganapatrao.kulkarni@cavium.com;
> catalin.marinas@arm.com; iommu@lists.linux-foundation.org; Linuxarm
> <linuxarm@huawei.com>; linux-arm-kernel@lists.infradead.org;
> linux-kernel@vger.kernel.org; Zengtao (B) <prime.zeng@hisilicon.com>;
> huangdaode <huangdaode@huawei.com>; Jonathan Cameron
> <jonathan.cameron@huawei.com>; Nicolas Saenz Julienne
> <nsaenzjulienne@suse.de>; Steve Capper <steve.capper@arm.com>; Andrew
> Morton <akpm@linux-foundation.org>; Mike Rapoport <rppt@linux.ibm.com>
> Subject: Re: [PATCH v4 1/2] dma-direct: provide the ability to reserve
> per-numa CMA
>
> On Fri, Jul 24, 2020 at 01:13:43AM +1200, Barry Song wrote:
> > +config CMA_PERNUMA_SIZE_MBYTES
> > + int "Size in Mega Bytes for per-numa CMA areas"
> > + depends on NUMA
> > + default 16 if ARM64
> > + default 0
> > + help
> > + Defines the size (in MiB) of the per-numa memory area for Contiguous
> > + Memory Allocator. Every numa node will get a separate CMA with this
> > + size. If the size of 0 is selected, per-numa CMA is disabled.
>
> I'm still not a fan of the config option. You can just hardcode the
> value in CONFIG_CMDLINE based on the kernel parameter. Also I wonder

I am sorry I haven't got your point yet. Do you mean something like the below?

arch/arm64/Kconfig:
config CMDLINE
string "Default kernel command string"
- default ""
+ default "pernuma_cma=16M"
help
Provide a set of default command-line options at build time by
entering them here. As a minimum, you should specify the the
root device (e.g. root=/dev/nfs).

A background of the current code is that Linux distributions can usually use arch/arm64/configs/defconfig
directly to build kernel. cmdline can be easily ignored during the generation of Linux distributions.

> if a way to expose this in the device tree might be useful, but people
> more familiar with the device tree and the arm code will have to chime
> in on that.

Not sure if it is an useful user case as we are using ACPI but not device tree since it is an ARM64
server with NUMA.

>
> > struct cma *dma_contiguous_default_area;
> > +static struct cma *dma_contiguous_pernuma_area[MAX_NUMNODES];
> >
> > /*
> > * Default global CMA area size can be defined in kernel's .config.
> > @@ -44,6 +51,8 @@ struct cma *dma_contiguous_default_area;
> > */
> > static const phys_addr_t size_bytes __initconst =
> > (phys_addr_t)CMA_SIZE_MBYTES * SZ_1M;
> > +static phys_addr_t pernuma_size_bytes __initdata =
> > + (phys_addr_t)CMA_SIZE_PERNUMA_MBYTES * SZ_1M;
> > static phys_addr_t size_cmdline __initdata = -1;
> > static phys_addr_t base_cmdline __initdata;
> > static phys_addr_t limit_cmdline __initdata;
> > @@ -69,6 +78,13 @@ static int __init early_cma(char *p)
> > }
> > early_param("cma", early_cma);
> >
> > +static int __init early_pernuma_cma(char *p)
> > +{
> > + pernuma_size_bytes = memparse(p, &p);
> > + return 0;
> > +}
> > +early_param("pernuma_cma", early_pernuma_cma);
> > +
> > #ifdef CONFIG_CMA_SIZE_PERCENTAGE
> >
> > static phys_addr_t __init __maybe_unused
> cma_early_percent_memory(void)
> > @@ -96,6 +112,33 @@ static inline __maybe_unused phys_addr_t
> cma_early_percent_memory(void)
> >
> > #endif
> >
> > +void __init dma_pernuma_cma_reserve(void)
> > +{
> > + int nid;
> > +
> > + if (!pernuma_size_bytes)
> > + return;
> > +
> > + for_each_node_state(nid, N_MEMORY) {
> > + int ret;
> > + char name[20];
> > +
> > + snprintf(name, sizeof(name), "pernuma%d", nid);
> > + ret = cma_declare_contiguous_nid(0, pernuma_size_bytes, 0, 0,
> > + 0, false, name,
> > + &dma_contiguous_pernuma_area[nid],
> > + nid);
>
> This adds a > 80 char line.

Will refine.

>
> > struct page *dma_alloc_contiguous(struct device *dev, size_t size, gfp_t
> gfp)
> > {
> > + int nid = dev_to_node(dev);
> > +
> > /* CMA can be used only in the context which permits sleeping */
> > if (!gfpflags_allow_blocking(gfp))
> > return NULL;
> > if (dev->cma_area)
> > return cma_alloc_aligned(dev->cma_area, size, gfp);
> > - if (size <= PAGE_SIZE || !dma_contiguous_default_area)
> > + if (size <= PAGE_SIZE)
> > return NULL;
> > +
> > + if ((nid != NUMA_NO_NODE) && !(gfp & (GFP_DMA | GFP_DMA32))) {
>
> No need for the braces around the nid check.

Will refine.

>
> > + struct cma *cma = dma_contiguous_pernuma_area[nid];
> > + struct page *page;
> > +
> > + if (cma) {
> > + page = cma_alloc_aligned(cma, size, gfp);
> > + if (page)
> > + return page;
> > + }
> > + }
> > +
> > return cma_alloc_aligned(dma_contiguous_default_area, size, gfp);
>
> This seems to have lost the dma_contiguous_default_area NULL check.

cma_alloc() is doing the check by returning NULL if cma is NULL.

struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align,
bool no_warn)
{
...
if (!cma || !cma->count)
return NULL;
}

But I agree here the code can check before calling cma_alloc_aligned.

>
> > + /* if dev has its own cma, free page from there */
> > + if (dev->cma_area) {
> > + if (cma_release(dev->cma_area, page, PAGE_ALIGN(size) >>
> PAGE_SHIFT))
> > + return;
>
> Another overly long line.

Will refine.

>
> > + } else {
> > + /*
> > + * otherwise, page is from either per-numa cma or default cma
> > + */
> > + if (cma_release(dma_contiguous_pernuma_area[page_to_nid(page)],
> > + page, PAGE_ALIGN(size) >> PAGE_SHIFT))
> > + return;
> > +
> > + if (cma_release(dma_contiguous_default_area, page,
> > + PAGE_ALIGN(size) >> PAGE_SHIFT))
> > + return;
> > + }
>
> I'd introduce a count variable for the value of "PAGE_ALIGN(size) >>
> PAGE_SHIFT" to clean al lthis up a bit.

Good idea.

>
> Also please add a CONFIG_PERCPU_DMA_CMA config variable so that we
> don't build this code for the vast majority of users that don't need it.

agreed.

Thanks
Barry