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Date: Sat, 21 Feb 2015 22:36:25 +0100
From: Borislav Petkov <bp@...en8.de>
To: Ingo Molnar <mingo@...nel.org>
Cc: Andy Lutomirski <luto@...capital.net>,
Oleg Nesterov <oleg@...hat.com>,
Rik van Riel <riel@...hat.com>, x86@...nel.org,
linux-kernel@...r.kernel.org,
Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC PATCH] x86, fpu: Use eagerfpu by default on all CPUs
On Sat, Feb 21, 2015 at 08:23:52PM +0100, Ingo Molnar wrote:
> to switch between the modes?
I went all out and did a debugfs file, see patch at the end, which
counts FPU saves. Then I ran this script:
---
#!/bin/bash
D="/sys/kernel/debug/fpu/eager"
echo "Lazy FPU: "
echo 0 > $D
echo -n " FPU saves before: "; cat $D
perf stat -a -e task-clock,cycles,instructions,branch-misses,cache-misses,faults,context-switches,migrations --sync --pre ~/bin/pre-build-kernel.sh make -s -j12
echo -n " FPU saves after: "; cat $D
echo ""
echo "Eager FPU: "
echo 1 > $D
echo -n " FPU saves before: "; cat $D
perf stat -a -e task-clock,cycles,instructions,branch-misses,cache-misses,faults,context-switches,migrations --sync --pre ~/bin/pre-build-kernel.sh make -s -j12
echo -n " FPU saves after: "; cat $D
---
which spit this:
Lazy FPU:
FPU saves before: 3
Setup is 16252 bytes (padded to 16384 bytes).
System is 4222 kB
CRC c79a13ab
Kernel: arch/x86/boot/bzImage is ready (#41)
Performance counter stats for 'system wide':
1315527.989020 task-clock (msec) # 6.003 CPUs utilized [100.00%]
3,042,312,057,208 cycles # 2.313 GHz [100.00%]
2,790,807,863,402 instructions # 0.92 insns per cycle [100.00%]
31,658,299,111 branch-misses # 24.065 M/sec [100.00%]
27,504,255,277 cache-misses # 20.907 M/sec
26,802,015 faults # 0.020 M/sec [100.00%]
1,248,899 context-switches # 0.949 K/sec [100.00%]
69,553 migrations # 0.053 K/sec
219.127929718 seconds time elapsed
FPU saves after: 704186
Eager FPU:
FPU saves before: 4
Setup is 16252 bytes (padded to 16384 bytes).
System is 4222 kB
CRC 6767bb2e
Kernel: arch/x86/boot/bzImage is ready (#42)
Performance counter stats for 'system wide':
1321651.543922 task-clock (msec) # 6.003 CPUs utilized [100.00%]
3,044,403,437,364 cycles # 2.303 GHz [100.00%]
2,790,835,886,565 instructions # 0.92 insns per cycle [100.00%]
31,638,090,259 branch-misses # 23.938 M/sec [100.00%]
27,491,643,095 cache-misses # 20.801 M/sec
26,869,732 faults # 0.020 M/sec [100.00%]
1,252,034 context-switches # 0.947 K/sec [100.00%]
69,247 migrations # 0.052 K/sec
220.148034331 seconds time elapsed
FPU saves after: 901638
---
so we have a second slowdown and 200K FPU saves more in eager mode.
Provided I've not done a mistake, looks like the increase in cycles gets
mirrored in 1 second time longer. I've not done the --repeat 10 thing
again, maybe I should do it too, just to be fair as this is a single
run.
---
arch/x86/include/asm/fpu-internal.h | 4 ++++
arch/x86/kernel/xsave.c | 47 ++++++++++++++++++++++++++++++++++++-
2 files changed, 50 insertions(+), 1 deletion(-)
diff --git a/arch/x86/include/asm/fpu-internal.h b/arch/x86/include/asm/fpu-internal.h
index e97622f57722..7141f353e960 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);
@@ -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..029de8b629d0 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 +741,46 @@ 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 int eager_get(void *data, u64 *val)
+{
+ *val = fpu_saved;
+
+ return 0;
+}
+
+static int eager_set(void *data, u64 val)
+{
+ if (val)
+ setup_force_cpu_cap(X86_FEATURE_EAGER_FPU);
+ else
+ setup_clear_cpu_cap(X86_FEATURE_EAGER_FPU);
+
+ 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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