> > We can do some VMX testing on existing POWER6 machines. The VSX
> > instruction set hasn't been fully implemented in GCC yet so we'll need
> > to wait a bit for that. Does anyone have an idea for a good VMX/Altivec
> > benchmark?
>
> Note that there are two aspects to the problem:
>
> - Lazy save/restore on SMP. This would avoid both the save and restore
> phases, thus is where the most potential gain is to be made. At the
> expense of some tricky IPI work when processes migrate between CPUs.
>
> However, it will only be useful -if- a process using FP/VMX/VSX is
> "interrupted" by another process that isn't using them. For example, a
> kernel thread. So it's unclear whether that's worth it in practice, ie,
> does this happen that often ?
>
> - Always restoring the FP/VMX/VSX state on context switch "in" rather
> than taking a fault. This is reasonably simple, but at the potential
> expense of adding the save/restore overhead to applications that only
> seldomly use these facilities. (Some heuristics might help here).
>
> However, the question here what do this buy us ?
>
> IE, In the worst case scenario, which is HZ=1000, so every 1ms, the
> process would have the overhead of an interrupt to do the restore of the
> state. IE. The restore state itself doesn't count since it would be done
> either way (at context switch vs. in the unavailable interrupt), so all
> we win here is the overhead of the actual interrupt, which is
> implemented as a fast interrupt in assembly. So we have what here ? 1000
> cycles to be pessimistic ? On a 1Ghz CPU, that is 1/1000 of the time
> slice, and both of these are rather pessimistic numbers.
>
> So that leaves us with the possible case of 2 tasks using the facility
> and running a fraction of the timeslice each, for example because they
> are ping-ponging with each other.
>
> Is that something that happens in practice to make it noticeable ?
I hacked up the below to put stats in /proc/self/sched.
A quick grep through /proc on a rhel5.2 machine (egrep
'(fp_count|switch_count)' /proc/*/sched) shows a few apps use fp a few
dozen times but then stop. This is only init apps like hald, so need to
check some real world apps too.
Ryan: let me know if this allows you to collect some useful stats.
Subject: [PATCH] powerpc: add context switch, fpr & vr stats to
/proc/self/sched.
Add a counter for every task switch, fp and vr exception to
/proc/self/sched.
[r...@p5-20-p6-e0 ~]# cat /proc/3422/sched |tail -3
switch_count : 559
fp_count : 317
vr_count : 0
[r...@p5-20-p6-e0 ~]#
Signed-off-by: Michael Neuling <mi...@neuling.org>
---
arch/powerpc/include/asm/processor.h | 3 +++
arch/powerpc/kernel/asm-offsets.c | 3 +++
arch/powerpc/kernel/fpu.S | 3 +++
arch/powerpc/kernel/process.c | 3 +++
arch/powerpc/kernel/setup-common.c | 10 ++++++++++
include/linux/seq_file.h | 12 ++++++++++++
kernel/sched_debug.c | 16 ++++------------
7 files changed, 38 insertions(+), 12 deletions(-)
Index: linux-2.6-ozlabs/arch/powerpc/include/asm/processor.h
===================================================================
--- linux-2.6-ozlabs.orig/arch/powerpc/include/asm/processor.h
+++ linux-2.6-ozlabs/arch/powerpc/include/asm/processor.h
@@ -174,11 +174,13 @@ struct thread_struct {
} fpscr;
int fpexc_mode; /* floating-point exception mode */
unsigned int align_ctl; /* alignment handling control */
+ unsigned long fp_count; /* FP restore count */
#ifdef CONFIG_PPC64
unsigned long start_tb; /* Start purr when proc switched in */
unsigned long accum_tb; /* Total accumilated purr for process */
#endif
unsigned long dabr; /* Data address breakpoint register */
+ unsigned long switch_count; /* switch count */
#ifdef CONFIG_ALTIVEC
/* Complete AltiVec register set */
vector128 vr[32] __attribute__((aligned(16)));
@@ -186,6 +188,7 @@ struct thread_struct {
vector128 vscr __attribute__((aligned(16)));
unsigned long vrsave;
int used_vr; /* set if process has used altivec */
+ unsigned long vr_count; /* VSX restore count */
#endif /* CONFIG_ALTIVEC */
#ifdef CONFIG_VSX
/* VSR status */
Index: linux-2.6-ozlabs/arch/powerpc/kernel/asm-offsets.c
===================================================================
--- linux-2.6-ozlabs.orig/arch/powerpc/kernel/asm-offsets.c
+++ linux-2.6-ozlabs/arch/powerpc/kernel/asm-offsets.c
@@ -74,14 +74,17 @@ int main(void)
DEFINE(KSP, offsetof(struct thread_struct, ksp));
DEFINE(KSP_LIMIT, offsetof(struct thread_struct, ksp_limit));
DEFINE(PT_REGS, offsetof(struct thread_struct, regs));
+ DEFINE(THREAD_SWITCHCOUNT, offsetof(struct thread_struct,
switch_count));
DEFINE(THREAD_FPEXC_MODE, offsetof(struct thread_struct, fpexc_mode));
DEFINE(THREAD_FPR0, offsetof(struct thread_struct, fpr[0]));
DEFINE(THREAD_FPSCR, offsetof(struct thread_struct, fpscr));
+ DEFINE(THREAD_FPCOUNT, offsetof(struct thread_struct, fp_count));
#ifdef CONFIG_ALTIVEC
DEFINE(THREAD_VR0, offsetof(struct thread_struct, vr[0]));
DEFINE(THREAD_VRSAVE, offsetof(struct thread_struct, vrsave));
DEFINE(THREAD_VSCR, offsetof(struct thread_struct, vscr));
DEFINE(THREAD_USED_VR, offsetof(struct thread_struct, used_vr));
+ DEFINE(THREAD_VRCOUNT, offsetof(struct thread_struct, vr_count));
#endif /* CONFIG_ALTIVEC */
#ifdef CONFIG_VSX
DEFINE(THREAD_VSR0, offsetof(struct thread_struct, fpr));
Index: linux-2.6-ozlabs/arch/powerpc/kernel/fpu.S
===================================================================
--- linux-2.6-ozlabs.orig/arch/powerpc/kernel/fpu.S
+++ linux-2.6-ozlabs/arch/powerpc/kernel/fpu.S
@@ -102,6 +102,9 @@ END_FTR_SECTION_IFSET(CPU_FTR_VSX)
ori r12,r12,MSR_FP
or r12,r12,r4
std r12,_MSR(r1)
+ ld r4,THREAD_FPCOUNT(r5)
+ addi r4, r4, 1
+ std r4,THREAD_FPCOUNT(r5)
#endif
lfd fr0,THREAD_FPSCR(r5)
MTFSF_L(fr0)
Index: linux-2.6-ozlabs/arch/powerpc/kernel/process.c
===================================================================
--- linux-2.6-ozlabs.orig/arch/powerpc/kernel/process.c
+++ linux-2.6-ozlabs/arch/powerpc/kernel/process.c
@@ -744,17 +744,20 @@ void start_thread(struct pt_regs *regs,
#endif
discard_lazy_cpu_state();
+ current->thread.switch_count = 0;
#ifdef CONFIG_VSX
current->thread.used_vsr = 0;
#endif
memset(current->thread.fpr, 0, sizeof(current->thread.fpr));
current->thread.fpscr.val = 0;
+ current->thread.fp_count = 0;
#ifdef CONFIG_ALTIVEC
memset(current->thread.vr, 0, sizeof(current->thread.vr));
memset(¤t->thread.vscr, 0, sizeof(current->thread.vscr));
current->thread.vscr.u[3] = 0x00010000; /* Java mode disabled */
current->thread.vrsave = 0;
current->thread.used_vr = 0;
+ current->thread.vr_count = 0;
#endif /* CONFIG_ALTIVEC */
#ifdef CONFIG_SPE
memset(current->thread.evr, 0, sizeof(current->thread.evr));
Index: linux-2.6-ozlabs/arch/powerpc/kernel/setup-common.c
===================================================================
--- linux-2.6-ozlabs.orig/arch/powerpc/kernel/setup-common.c
+++ linux-2.6-ozlabs/arch/powerpc/kernel/setup-common.c
@@ -669,3 +669,13 @@ static int powerpc_debugfs_init(void)
}
arch_initcall(powerpc_debugfs_init);
#endif
+
+void arch_proc_sched_show_task(struct task_struct *p, struct seq_file *m) {
+ SEQ_printf(m, "%-35s:%21Ld\n",
+ "switch_count", (long long)p->thread.switch_count);
+ SEQ_printf(m, "%-35s:%21Ld\n",
+ "fp_count", (long long)p->thread.fp_count);
+ SEQ_printf(m, "%-35s:%21Ld\n",
+ "vr_count", (long long)p->thread.vr_count);
+}
+
Index: linux-2.6-ozlabs/include/linux/seq_file.h
===================================================================
--- linux-2.6-ozlabs.orig/include/linux/seq_file.h
+++ linux-2.6-ozlabs/include/linux/seq_file.h
@@ -95,4 +95,16 @@ extern struct list_head *seq_list_start_
extern struct list_head *seq_list_next(void *v, struct list_head *head,
loff_t *ppos);
+/*
+ * This allows printing both to /proc/sched_debug and
+ * to the console
+ */
+#define SEQ_printf(m, x...) \
+ do { \
+ if (m) \
+ seq_printf(m, x); \
+ else \
+ printk(x); \
+ } while (0)
+
#endif
Index: linux-2.6-ozlabs/kernel/sched_debug.c
===================================================================
--- linux-2.6-ozlabs.orig/kernel/sched_debug.c
+++ linux-2.6-ozlabs/kernel/sched_debug.c
@@ -17,18 +17,6 @@
#include <linux/utsname.h>
/*
- * This allows printing both to /proc/sched_debug and
- * to the console
- */
-#define SEQ_printf(m, x...) \
- do { \
- if (m) \
- seq_printf(m, x); \
- else \
- printk(x); \
- } while (0)
-
-/*
* Ease the printing of nsec fields:
*/
static long long nsec_high(unsigned long long nsec)
@@ -370,6 +358,9 @@ static int __init init_sched_debug_procf
__initcall(init_sched_debug_procfs);
+void __attribute__ ((weak))
+arch_proc_sched_show_task(struct task_struct *p, struct seq_file *m) {}
+
void proc_sched_show_task(struct task_struct *p, struct seq_file *m)
{
unsigned long nr_switches;
@@ -473,6 +464,7 @@ void proc_sched_show_task(struct task_st
SEQ_printf(m, "%-35s:%21Ld\n",
"clock-delta", (long long)(t1-t0));
}
+ arch_proc_sched_show_task(p, m);
}
void proc_sched_set_task(struct task_struct *p)
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