Re: [RFC PATCH] x86, fpu: Use eagerfpu by default on all CPUs

[Borislav Petkov]

On Sun, Feb 22, 2015 at 09:18:40AM +0100, Ingo Molnar wrote:
> So am I interpreting the older and your latest numbers 
> correctly in stating that the cost observation has flipped 
> around 180 degrees: the first measurement showed eager FPU 
> to be a win, but now that we can do more precise 
> measurements, eager FPU has actually slowed down the kernel 
> build by ~0.5%?

Well, I wouldn't take the latest numbers too seriously - that was a
single run without --repeat.

> That's not good, and kernel builds are just a random load 
> that isn't even that FPU or context switch heavy - there 
> will certainly be other loads that would be hurt even more.

That is my fear.

> So just before we base wide reaching decisions based on any 
> of these measurements, would you mind help us increase our 
> confidence in the numbers some more:
> 
>   - It might make sense to do a 'perf stat --null --repeat'
>     measurement as well [without any -e arguments], to make 
>     sure the rich PMU stats you are gathering are not 
>     interfering?
> 
>     With 'perf stat --null --repeat' perf acts essenially 
>     as a /usr/bin/time replacement, but can measure down to
>     microseconds and will calculate noise/sttdev properly.

Cool, let me do that.

>   - Perhaps also double check the debug switch: is it
>     really properly switching FPU handling mode?

I've changed the use_eager_fpu() test to do:

 static __always_inline __pure bool use_eager_fpu(void)
 {
       return boot_cpu_has(X86_FEATURE_EAGER_FPU);
 }

and I'm clearing/setting eager FPU with
setup_force_cpu_cap/setup_clear_cpu_cap, see full diff below.

>   - Do you have enough RAM that there's essentially no IO
>     in the system worth speaking of? Do you have enough RAM
>     to copy a whole kernel tree to /tmp/linux/ and do the
>     measurement there, on ramfs?

/proc/meminfo says "MemTotal: 4011860 kB" which is probably not enough.
But I could find one somewhere :-)

---
 arch/x86/include/asm/fpu-internal.h |  6 +++-
 arch/x86/kernel/xsave.c             | 57 ++++++++++++++++++++++++++++++++++++-
 2 files changed, 61 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/fpu-internal.h b/arch/x86/include/asm/fpu-internal.h
index e97622f57722..c8a161d02056 100644
--- a/arch/x86/include/asm/fpu-internal.h
+++ b/arch/x86/include/asm/fpu-internal.h
@@ -38,6 +38,8 @@ int ia32_setup_frame(int sig, struct ksignal *ksig,
 # define ia32_setup_rt_frame	__setup_rt_frame
 #endif
 
+
+extern unsigned long fpu_saved;
 extern unsigned int mxcsr_feature_mask;
 extern void fpu_init(void);
 extern void eager_fpu_init(void);
@@ -87,7 +89,7 @@ static inline int is_x32_frame(void)
 
 static __always_inline __pure bool use_eager_fpu(void)
 {
-	return static_cpu_has_safe(X86_FEATURE_EAGER_FPU);
+	return boot_cpu_has(X86_FEATURE_EAGER_FPU);
 }
 
 static __always_inline __pure bool use_xsaveopt(void)
@@ -242,6 +244,8 @@ static inline void fpu_fxsave(struct fpu *fpu)
  */
 static inline int fpu_save_init(struct fpu *fpu)
 {
+	fpu_saved++;
+
 	if (use_xsave()) {
 		fpu_xsave(fpu);
 
diff --git a/arch/x86/kernel/xsave.c b/arch/x86/kernel/xsave.c
index 0de1fae2bdf0..943af0adacff 100644
--- a/arch/x86/kernel/xsave.c
+++ b/arch/x86/kernel/xsave.c
@@ -14,6 +14,8 @@
 #include <asm/sigframe.h>
 #include <asm/xcr.h>
 
+#include <linux/debugfs.h>
+
 /*
  * Supported feature mask by the CPU and the kernel.
  */
@@ -638,7 +640,7 @@ static void __init xstate_enable_boot_cpu(void)
 	setup_init_fpu_buf();
 
 	/* Auto enable eagerfpu for xsaveopt */
-	if (cpu_has_xsaveopt && eagerfpu != DISABLE)
+	if (eagerfpu != DISABLE)
 		eagerfpu = ENABLE;
 
 	if (pcntxt_mask & XSTATE_EAGER) {
@@ -739,3 +741,56 @@ void *get_xsave_addr(struct xsave_struct *xsave, int xstate)
 	return (void *)xsave + xstate_comp_offsets[feature];
 }
 EXPORT_SYMBOL_GPL(get_xsave_addr);
+
+unsigned long fpu_saved;
+
+static void my_clts(void *arg)
+{
+	asm volatile("clts");
+}
+
+static int eager_get(void *data, u64 *val)
+{
+	*val = fpu_saved;
+
+	return 0;
+}
+
+static int eager_set(void *data, u64 val)
+{
+	preempt_disable();
+	if (val) {
+		on_each_cpu(my_clts, NULL, 1);
+		setup_force_cpu_cap(X86_FEATURE_EAGER_FPU);
+	} else {
+		setup_clear_cpu_cap(X86_FEATURE_EAGER_FPU);
+		stts();
+	}
+	preempt_enable();
+
+	fpu_saved = 0;
+
+	return 0;
+}
+
+DEFINE_SIMPLE_ATTRIBUTE(eager_fops, eager_get, eager_set, "%llu\n");
+
+static int __init setup_eagerfpu_knob(void)
+{
+	static struct dentry *d_eager, *f_eager;
+
+	d_eager = debugfs_create_dir("fpu", NULL);
+	if (!d_eager) {
+		pr_err("Error creating fpu debugfs dir\n");
+		return -ENOMEM;
+	}
+
+	f_eager = debugfs_create_file("eager", 0644, d_eager, NULL, &eager_fops);
+	if (!f_eager) {
+		pr_err("Error creating fpu debugfs node\n");
+		return -ENOMEM;
+	}
+
+	return 0;
+}
+late_initcall(setup_eagerfpu_knob);
-- 
2.2.0.33.gc18b867


-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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