[PATCH] perf/arm-cmn: reduce stack usage in arm_cmn_probe()

[PATCH] perf/arm-cmn: reduce stack usage in arm_cmn_probe()

[Arnd Bergmann]

From: Arnd Bergmann <arnd@arndb.de>

This function has a rather large stack usage, which triggers the
warning limit with clang if I reduce the default to 1280 bytes:

drivers/perf/arm-cmn.c:2541:12: error: stack frame size (1312) exceeds limit (1280) in 'arm_cmn_probe' [-Werror,-Wframe-larger-than]

This is a combination of two problems:

 - The arm_cmn_discover() function has some large local variables and
   gets inlined here by clang (but not gcc)

 - The (struct pmu) assignment adds an extra copy of the pmu structure
   on the stack and does a memcpy() from that

Address the first one here by marking arm_cmn_discover() as noinline_for_stack,
making clang behave more like gcc here. This gets it under the warning
limit, though the total stack usage does not actually get reduced.

It would be nice to also change the way struct pmu is initialized, but I
see that this is done consistently for all pmu drivers. Ideally the function
pointers should be moved into a 'static const' structure per driver as this
is done in most other subsystems.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 drivers/perf/arm-cmn.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/perf/arm-cmn.c b/drivers/perf/arm-cmn.c
index 031d45d0fe3d..430c89760391 100644
--- a/drivers/perf/arm-cmn.c
+++ b/drivers/perf/arm-cmn.c
@@ -2243,7 +2243,8 @@ static enum cmn_node_type arm_cmn_subtype(enum cmn_node_type type)
 	}
 }
 
-static int arm_cmn_discover(struct arm_cmn *cmn, unsigned int rgn_offset)
+static noinline_for_stack int arm_cmn_discover(struct arm_cmn *cmn,
+					       unsigned int rgn_offset)
 {
 	void __iomem *cfg_region;
 	struct arm_cmn_node cfg, *dn;
-- 
2.39.5

Re: [PATCH] perf/arm-cmn: reduce stack usage in arm_cmn_probe()

[Robin Murphy]

On 20/06/2025 12:51 pm, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> This function has a rather large stack usage, which triggers the
> warning limit with clang if I reduce the default to 1280 bytes:
> 
> drivers/perf/arm-cmn.c:2541:12: error: stack frame size (1312) exceeds limit (1280) in 'arm_cmn_probe' [-Werror,-Wframe-larger-than]
> 
> This is a combination of two problems:
> 
>   - The arm_cmn_discover() function has some large local variables and
>     gets inlined here by clang (but not gcc)
> 
>   - The (struct pmu) assignment adds an extra copy of the pmu structure
>     on the stack and does a memcpy() from that
> 
> Address the first one here by marking arm_cmn_discover() as noinline_for_stack,
> making clang behave more like gcc here. This gets it under the warning
> limit, though the total stack usage does not actually get reduced.

At that point, though, it seems like we may as well just disable the 
warning :/

Fortunately it's not actually that hard to improve matters here, so I've 
just sent that patch:

https://lore.kernel.org/r/e7dd41bf0f1b098e2e4b01ef91318a4b272abff8.1751046159.git.robin.murphy@arm.com/T/#u

> It would be nice to also change the way struct pmu is initialized, but I
> see that this is done consistently for all pmu drivers. Ideally the function
> pointers should be moved into a 'static const' structure per driver as this
> is done in most other subsystems.

Beware that perf_pmu_register() does some further dynamic assignment of 
callbacks based on what the driver provided, so it's not necessarily 
straightforward to change in struct pmu itself. However, FWIW I have 
recently been playing with some ideas for reducing the amount of PMU 
registration boilerplate, and indeed one of them is to have a 
driver-level static template passed to a registration helper, which 
would at least make it easy to avoid the full by-value copies everywhere.

Thanks,
Robin.

> 
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>   drivers/perf/arm-cmn.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/perf/arm-cmn.c b/drivers/perf/arm-cmn.c
> index 031d45d0fe3d..430c89760391 100644
> --- a/drivers/perf/arm-cmn.c
> +++ b/drivers/perf/arm-cmn.c
> @@ -2243,7 +2243,8 @@ static enum cmn_node_type arm_cmn_subtype(enum cmn_node_type type)
>   	}
>   }
>   
> -static int arm_cmn_discover(struct arm_cmn *cmn, unsigned int rgn_offset)
> +static noinline_for_stack int arm_cmn_discover(struct arm_cmn *cmn,
> +					       unsigned int rgn_offset)
>   {
>   	void __iomem *cfg_region;
>   	struct arm_cmn_node cfg, *dn;

Re: [PATCH] perf/arm-cmn: reduce stack usage in arm_cmn_probe()

[Arnd Bergmann]

On Fri, Jun 27, 2025, at 19:57, Robin Murphy wrote:
> On 20/06/2025 12:51 pm, Arnd Bergmann wrote:
>
> At that point, though, it seems like we may as well just disable the 
> warning :/

I had a couple of cases like this, where the same large stack
happens on both gcc and clang, and adding noinline makes clang
behave the same way as gcc, so neither of them warns. Leaving
the warning enabled at this point at least ensures that we catch
it if it ever increases further.

There is a different type of problem (mostly on powerpc and mips,
but sometimes on arm64) where the 'noinline' helps clang to not
produce some really bad object code with unnecessary temporaries
on the stack.

> Fortunately it's not actually that hard to improve matters here, so I've 
> just sent that patch:
>
> https://lore.kernel.org/r/e7dd41bf0f1b098e2e4b01ef91318a4b272abff8.1751046159.git.robin.murphy@arm.com/T/#u

Nice, your version reduces the stack size from 1360 to 784 bytes
for my test randconfig, so that's clearly better than my version.

While attempting to reproduce this now, I also found that another
patch I had avoided the warning: I added -finline-max-stacksize=128
to the KASAN cflags, to work around some overeager inlining on
powerpc, and this had the same effect as my patch, but without
actually having to modify the sources.

     Arnd