Re: [PATCH v2] arch/sparc: support NR_CPUS = 4096

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On 06/05/2017 05:57 PM, David Miller wrote:

From: Jane Chu <jane.chu@xxxxxxxxxx>
Date: Mon,  5 Jun 2017 16:48:31 -0600

Linux SPARC64 limits NR_CPUS to 4064 because init_cpu_send_mondo_info()
only allocates a single page for NR_CPUS mondo entries. Thus we cannot
use all 4096 CPUs on some SPARC platforms.

To fix, allocate (2^order) pages where order is set according to the size
of cpu_list for possible cpus. Since cpu_list_pa and cpu_mondo_block_pa
are not used in asm code, there are no imm13 offsets from the base PA
that will break because they can only reach one page.

Orabug: 25505750

Signed-off-by: Jane Chu <jane.chu@xxxxxxxxxx>

Reviewed-by: Bob Picco <bob.picco@xxxxxxxxxx>
Reviewed-by: Atish Patra <atish.patra@xxxxxxxxxx>
---
  arch/sparc/Kconfig         |    4 ++--
  arch/sparc/kernel/irq_64.c |   16 ++++++++++++----
  2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/arch/sparc/Kconfig b/arch/sparc/Kconfig
index 58243b0..4399be7 100644
--- a/arch/sparc/Kconfig
+++ b/arch/sparc/Kconfig
@@ -192,9 +192,9 @@ config NR_CPUS
  	int "Maximum number of CPUs"
  	depends on SMP
  	range 2 32 if SPARC32
-	range 2 1024 if SPARC64
+	range 2 4096 if SPARC64
  	default 32 if SPARC32
-	default 64 if SPARC64
+	default 4096 if SPARC64
source kernel/Kconfig.hz diff --git a/arch/sparc/kernel/irq_64.c b/arch/sparc/kernel/irq_64.c
index 4d0248a..cc8f6c3 100644
--- a/arch/sparc/kernel/irq_64.c
+++ b/arch/sparc/kernel/irq_64.c
@@ -1034,17 +1034,25 @@ static void __init init_cpu_send_mondo_info(struct trap_per_cpu *tb)
  {
  #ifdef CONFIG_SMP
  	unsigned long page;
+	void *mondo;
- BUILD_BUG_ON((NR_CPUS * sizeof(u16)) > (PAGE_SIZE - 64));
+	BUILD_BUG_ON((NR_CPUS * sizeof(u16)) > PAGE_SIZE);
+
+	/* Make sure mondo block is 64byte aligned */
+	mondo = kzalloc(64, GFP_KERNEL);
+	if (!mondo) {
+		prom_printf("SUN4V: Error, cannot allocate mondo block.\n");
+		prom_halt();
+	}
+	tb->cpu_mondo_block_pa = __pa(mondo);
Hmmm, you said that this has to be 64 byte aligned right?  We might have
to do something in order to insure that, as kmalloc() only guarantees
ARCH_KMALLOC_MINALIGN which I think is 8 on sparc.
Yes.

I suppose this would work:

	mondo = kzalloc(64 + 63, GFP_KERNEL);

and then 64-byte align that pointer.

On sun4v sparc,  it looks like kzalloc(64, GFP_KERNEL)  ends up allocating
from kmalloc_caches[6] - a 64-byte kmem-cache allocated by
kmem_cache_init() with SLAB_HWCACHE_ALIGN flag set, so it's in l3-cache-line-size
alignment, i.e. 64byte.  I printed out the 'mondo' pa and verified that.

thanks!
-jane


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