[PATCH] [net-next] cxgb4: reduce kernel stack usage in cudbg_collect_mem_region()

[PATCH] [net-next] cxgb4: reduce kernel stack usage in cudbg_collect_mem_region()

[Arnd Bergmann]

The cudbg_collect_mem_region() and cudbg_read_fw_mem() both use several
hundred kilobytes of kernel stack space. One gets inlined into the other,
which causes the stack usage to be combined beyond the warning limit
when building with clang:

drivers/net/ethernet/chelsio/cxgb4/cudbg_lib.c:1057:12: error: stack frame size of 1244 bytes in function 'cudbg_collect_mem_region' [-Werror,-Wframe-larger-than=]

Restructuring cudbg_collect_mem_region() lets clang do the same
optimization that gcc does and reuse the stack slots as it can
see that the large variables are never used together.

A better fix might be to avoid using cudbg_meminfo on the stack
altogether, but that requires a larger rewrite.

Fixes: a1c69520f785 ("cxgb4: collect MC memory dump")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 .../net/ethernet/chelsio/cxgb4/cudbg_lib.c    | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/drivers/net/ethernet/chelsio/cxgb4/cudbg_lib.c b/drivers/net/ethernet/chelsio/cxgb4/cudbg_lib.c
index a76529a7662d..c2e92786608b 100644
--- a/drivers/net/ethernet/chelsio/cxgb4/cudbg_lib.c
+++ b/drivers/net/ethernet/chelsio/cxgb4/cudbg_lib.c
@@ -1054,14 +1054,12 @@ static void cudbg_t4_fwcache(struct cudbg_init *pdbg_init,
 	}
 }
 
-static int cudbg_collect_mem_region(struct cudbg_init *pdbg_init,
-				    struct cudbg_buffer *dbg_buff,
-				    struct cudbg_error *cudbg_err,
-				    u8 mem_type)
+static unsigned long cudbg_mem_region_size(struct cudbg_init *pdbg_init,
+					   struct cudbg_error *cudbg_err,
+					   u8 mem_type)
 {
 	struct adapter *padap = pdbg_init->adap;
 	struct cudbg_meminfo mem_info;
-	unsigned long size;
 	u8 mc_idx;
 	int rc;
 
@@ -1075,7 +1073,16 @@ static int cudbg_collect_mem_region(struct cudbg_init *pdbg_init,
 	if (rc)
 		return rc;
 
-	size = mem_info.avail[mc_idx].limit - mem_info.avail[mc_idx].base;
+	return mem_info.avail[mc_idx].limit - mem_info.avail[mc_idx].base;
+}
+
+static int cudbg_collect_mem_region(struct cudbg_init *pdbg_init,
+				    struct cudbg_buffer *dbg_buff,
+				    struct cudbg_error *cudbg_err,
+				    u8 mem_type)
+{
+	unsigned long size = cudbg_mem_region_size(pdbg_init, cudbg_err, mem_type);
+
 	return cudbg_read_fw_mem(pdbg_init, dbg_buff, mem_type, size,
 				 cudbg_err);
 }
-- 
2.20.0

Re: [PATCH] [net-next] cxgb4: reduce kernel stack usage in cudbg_collect_mem_region()

[David Miller]

From: Arnd Bergmann <arnd@arndb.de>
Date: Fri, 12 Jul 2019 11:06:33 +0200

> The cudbg_collect_mem_region() and cudbg_read_fw_mem() both use several
> hundred kilobytes of kernel stack space. One gets inlined into the other,
> which causes the stack usage to be combined beyond the warning limit
> when building with clang:
> 
> drivers/net/ethernet/chelsio/cxgb4/cudbg_lib.c:1057:12: error: stack frame size of 1244 bytes in function 'cudbg_collect_mem_region' [-Werror,-Wframe-larger-than=]
> 
> Restructuring cudbg_collect_mem_region() lets clang do the same
> optimization that gcc does and reuse the stack slots as it can
> see that the large variables are never used together.
> 
> A better fix might be to avoid using cudbg_meminfo on the stack
> altogether, but that requires a larger rewrite.
> 
> Fixes: a1c69520f785 ("cxgb4: collect MC memory dump")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Applied.

Re: [PATCH] [net-next] cxgb4: reduce kernel stack usage in cudbg_collect_mem_region()

[Joe Perches]

On Fri, 2019-07-12 at 15:36 -0700, David Miller wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> Date: Fri, 12 Jul 2019 11:06:33 +0200
> 
> > The cudbg_collect_mem_region() and cudbg_read_fw_mem() both use several
> > hundred kilobytes of kernel stack space.

Several hundred 'kilo' bytes?
I hope not.

Re: [PATCH] [net-next] cxgb4: reduce kernel stack usage in cudbg_collect_mem_region()

[Arnd Bergmann]

On Sat, Jul 13, 2019 at 2:14 AM Joe Perches <joe@perches.com> wrote:
>
> On Fri, 2019-07-12 at 15:36 -0700, David Miller wrote:
> > From: Arnd Bergmann <arnd@arndb.de>
> > Date: Fri, 12 Jul 2019 11:06:33 +0200
> >
> > > The cudbg_collect_mem_region() and cudbg_read_fw_mem() both use several
> > > hundred kilobytes of kernel stack space.
>
> Several hundred 'kilo' bytes?
> I hope not.

Right, my mistake.

       Arnd