[RFC PATCH] riscv: Optimize gcd() performance by selecting CPU_NO_EFFICIENT_FFS

[Kuan-Wei Chiu]

+Cc Andrew, since this might touch lib/math/gcd.c

On Fri, Mar 28, 2025 at 03:07:36PM +0100, Alexandre Ghiti wrote:
> Hi Kuan-Wei,
> 
> First sorry for the late review.
> 
> On 17/02/2025 02:37, Kuan-Wei Chiu wrote:
> > When the Zbb extension is not supported, ffs() falls back to a software
> > implementation instead of leveraging the hardware ctz instruction for
> > fast computation. In such cases, selecting CPU_NO_EFFICIENT_FFS
> > optimizes the efficiency of gcd().
> > 
> > The implementation of gcd() depends on the CPU_NO_EFFICIENT_FFS option.
> > With hardware support for ffs, the binary GCD algorithm is used.
> > Without it, the odd-even GCD algorithm is employed for better
> > performance.
> > 
> > Co-developed-by: Yu-Chun Lin <eleanor15x at gmail.com>
> > Signed-off-by: Yu-Chun Lin <eleanor15x at gmail.com>
> > Signed-off-by: Kuan-Wei Chiu <visitorckw at gmail.com>
> > ---
> > Although selecting NO_EFFICIENT_FFS seems reasonable without ctz
> > instructions, this patch hasn't been tested on real hardware. We'd
> > greatly appreciate it if someone could help test and provide
> > performance numbers!
> > 
> >   arch/riscv/Kconfig | 1 +
> >   1 file changed, 1 insertion(+)
> > 
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index 7612c52e9b1e..2dd3699ad09b 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -91,6 +91,7 @@ config RISCV
> >   	select CLINT_TIMER if RISCV_M_MODE
> >   	select CLONE_BACKWARDS
> >   	select COMMON_CLK
> > +	select CPU_NO_EFFICIENT_FFS if !RISCV_ISA_ZBB
> >   	select CPU_PM if CPU_IDLE || HIBERNATION || SUSPEND
> >   	select EDAC_SUPPORT
> >   	select FRAME_POINTER if PERF_EVENTS || (FUNCTION_TRACER && !DYNAMIC_FTRACE)
> 
> 
> So your patch is correct. But a kernel built with RISCV_ISA_ZBB does not
> mean the platform supports zbb and in that case, we'd still use the slow
> version of gcd().
> 
> Then I would use static keys instead, can you try to come up with a patch
> that does that?
> 
Here's my current thought: I'd like to add a static key named
efficient_ffs_key in gcd.c, and possibly call
static_branch_disable(&efficient_ffs_key) somewhere under arch/riscv/
when RISCV_ISA_ZBB is enabled but the Zbb extension is not detected at
runtime.

However, I'm new to the RISC-V kernel code and not sure where would be
the most appropriate place to insert that static_branch_disable() call.
Suggestions are very welcome!

Below is the diff for context.

Regards,
Kuan-Wei

diff --git a/lib/math/gcd.c b/lib/math/gcd.c
index e3b042214d1b..514b8a86b461 100644
--- a/lib/math/gcd.c
+++ b/lib/math/gcd.c
@@ -2,6 +2,7 @@
 #include <linux/kernel.h>
 #include <linux/gcd.h>
 #include <linux/export.h>
+#include <linux/jump_label.h>

 /*
  * This implements the binary GCD algorithm. (Often attributed to Stein,
@@ -11,6 +12,8 @@
  * has decent hardware division.
  */

+DEFINE_STATIC_KEY_TRUE(efficient_ffs_key);
+
 #if !defined(CONFIG_CPU_NO_EFFICIENT_FFS)

 /* If __ffs is available, the even/odd algorithm benchmarks slower. */
@@ -20,7 +23,7 @@
  * @a: first value
  * @b: second value
  */
-unsigned long gcd(unsigned long a, unsigned long b)
+static unsigned long gcd_binary(unsigned long a, unsigned long b)
 {
 	unsigned long r = a | b;

@@ -44,7 +47,7 @@ unsigned long gcd(unsigned long a, unsigned long b)
 	}
 }

-#else
+#endif

 /* If normalization is done by loops, the even/odd algorithm is a win. */
 unsigned long gcd(unsigned long a, unsigned long b)
@@ -54,6 +57,11 @@ unsigned long gcd(unsigned long a, unsigned long b)
 	if (!a || !b)
 		return r;

+#if !defined(CONFIG_CPU_NO_EFFICIENT_FFS)
+	if (static_branch_likely(&efficient_ffs_key))
+		return binary_gcd(a, b);
+#endif
+
 	/* Isolate lsbit of r */
 	r &= -r;

@@ -80,6 +88,4 @@ unsigned long gcd(unsigned long a, unsigned long b)
 	}
 }

-#endif
-
 EXPORT_SYMBOL_GPL(gcd);