Ingo Molnar wrote:
ping?
This is a very serious paravirt_ops slowdown affecting the native kernel's
performance to the tune of 5-10% in certain workloads.
It's been about 2 years ago that paravirt_ops went upstream, when you told
us that something like this would never happen, that paravirt_ops is
designed so flexibly that it will never hinder the native kernel - and if
it does it will be easy to fix it. Now is the time to fulfill that
promise.
I couldn't exactly reproduce your results, but I guess they're similar
in shape. Comparing 2.6.29-rc2-nopv with -pvops, I saw this ratio (pass
1-5). Interestingly I'm seeing identical instruction counts for pvops
vs non-pvops, and a lower cycle count. The cache references are way up
and the miss rate is up a bit, which I guess is the source of the slowdown.
With the attached patch, I get a clear improvement; it replaces the
do-nothing pte_val/make_pte functions with inlined movs to move the
argument to return, overpatching the 6-byte indirect call (on i386 it
would just be all nopped out). CPU cycles and cache misses are way
down, and the tick count is down from ~5% worse to ~2%. But the cache
reference rate is even higher, which really doesn't make sense to me.
But the patch is a clear improvement, and its hard to see how it could
make anything worse (its always going to replace an indirect call with
simple inlined code).
(Full numbers in spreadsheet.)
I have a couple of other patches to reduce the register pressure of the
pvops calls, but I'm trying to work out how to make sure its not all to
complex and/or fragile.
J
pvops-mmap-measurements.ods
Description: application/vnd.oasis.opendocument.spreadsheet
Subject: x86/pvops: add a paravirt_indent functions to allow special patching
Several paravirt ops implementations simply return their arguments,
the most obvious being the make_pte/pte_val class of operations on
native.
On 32-bit, the identity function is literally a no-op, as the calling
convention uses the same registers for the first argument and return.
On 64-bit, it can be implemented with a single "mov".
This patch adds special identity functions for 32 and 64 bit argument,
and machinery to recognize them and replace them with either nops or a
mov as appropriate.
At the moment, the only users for the identity functions are the
pagetable entry conversion functions.
The result is a measureable improvement on pagetable-heavy benchmarks
(2-3%, reducing the pvops overhead from 5 to 2%).
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
---
arch/x86/include/asm/paravirt.h | 5 ++
arch/x86/kernel/paravirt.c | 75 ++++++++++++++++++++++++++++++-----
arch/x86/kernel/paravirt_patch_32.c | 12 +++++
arch/x86/kernel/paravirt_patch_64.c | 15 +++++++
4 files changed, 98 insertions(+), 9 deletions(-)
===================================================================
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -390,6 +390,8 @@
asm("start_" #ops "_" #name ": " code "; end_" #ops "_" #name ":")
unsigned paravirt_patch_nop(void);
+unsigned paravirt_patch_ident_32(void *insnbuf, unsigned len);
+unsigned paravirt_patch_ident_64(void *insnbuf, unsigned len);
unsigned paravirt_patch_ignore(unsigned len);
unsigned paravirt_patch_call(void *insnbuf,
const void *target, u16 tgt_clobbers,
@@ -1378,6 +1380,9 @@
}
void _paravirt_nop(void);
+u32 _paravirt_ident_32(u32);
+u64 _paravirt_ident_64(u64);
+
#define paravirt_nop ((void *)_paravirt_nop)
void paravirt_use_bytelocks(void);
===================================================================
--- a/arch/x86/kernel/paravirt.c
+++ b/arch/x86/kernel/paravirt.c
@@ -44,6 +44,17 @@
{
}
+/* identity function, which can be inlined */
+u32 _paravirt_ident_32(u32 x)
+{
+ return x;
+}
+
+u64 _paravirt_ident_64(u64 x)
+{
+ return x;
+}
+
static void __init default_banner(void)
{
printk(KERN_INFO "Booting paravirtualized kernel on %s\n",
@@ -138,9 +149,16 @@
if (opfunc == NULL)
/* If there's no function, patch it with a ud2a (BUG) */
ret = paravirt_patch_insns(insnbuf, len, ud2a,
ud2a+sizeof(ud2a));
- else if (opfunc == paravirt_nop)
+ else if (opfunc == _paravirt_nop)
/* If the operation is a nop, then nop the callsite */
ret = paravirt_patch_nop();
+
+ /* identity functions just return their single argument */
+ else if (opfunc == _paravirt_ident_32)
+ ret = paravirt_patch_ident_32(insnbuf, len);
+ else if (opfunc == _paravirt_ident_64)
+ ret = paravirt_patch_ident_64(insnbuf, len);
+
else if (type == PARAVIRT_PATCH(pv_cpu_ops.iret) ||
type == PARAVIRT_PATCH(pv_cpu_ops.irq_enable_sysexit) ||
type == PARAVIRT_PATCH(pv_cpu_ops.usergs_sysret32) ||
@@ -373,6 +391,45 @@
#endif
};
+typedef pte_t make_pte_t(pteval_t);
+typedef pmd_t make_pmd_t(pmdval_t);
+typedef pud_t make_pud_t(pudval_t);
+typedef pgd_t make_pgd_t(pgdval_t);
+
+typedef pteval_t pte_val_t(pte_t);
+typedef pmdval_t pmd_val_t(pmd_t);
+typedef pudval_t pud_val_t(pud_t);
+typedef pgdval_t pgd_val_t(pgd_t);
+
+
+#if defined(CONFIG_X86_32) && !defined(CONFIG_X86_PAE)
+/* 32-bit pagetable entries */
+#define paravirt_native_make_pte (make_pte_t *)_paravirt_ident_32
+#define paravirt_native_pte_val (pte_val_t *)_paravirt_ident_32
+
+#define paravirt_native_make_pmd (make_pmd_t *)_paravirt_ident_32
+#define paravirt_native_pmd_val (pmd_val_t *)_paravirt_ident_32
+
+#define paravirt_native_make_pud (make_pud_t *)_paravirt_ident_32
+#define paravirt_native_pud_val (pud_val_t *)_paravirt_ident_32
+
+#define paravirt_native_make_pgd (make_pgd_t *)_paravirt_ident_32
+#define paravirt_native_pgd_val (pgd_val_t *)_paravirt_ident_32
+#else
+/* 64-bit pagetable entries */
+#define paravirt_native_make_pte (make_pte_t *)_paravirt_ident_64
+#define paravirt_native_pte_val (pte_val_t *)_paravirt_ident_64
+
+#define paravirt_native_make_pmd (make_pmd_t *)_paravirt_ident_64
+#define paravirt_native_pmd_val (pmd_val_t *)_paravirt_ident_64
+
+#define paravirt_native_make_pud (make_pud_t *)_paravirt_ident_64
+#define paravirt_native_pud_val (pud_val_t *)_paravirt_ident_64
+
+#define paravirt_native_make_pgd (make_pgd_t *)_paravirt_ident_64
+#define paravirt_native_pgd_val (pgd_val_t *)_paravirt_ident_64
+#endif
+
struct pv_mmu_ops pv_mmu_ops = {
#ifndef CONFIG_X86_64
.pagetable_setup_start = native_pagetable_setup_start,
@@ -424,21 +481,21 @@
.pmd_clear = native_pmd_clear,
#endif
.set_pud = native_set_pud,
- .pmd_val = native_pmd_val,
- .make_pmd = native_make_pmd,
+ .pmd_val = paravirt_native_pmd_val,
+ .make_pmd = paravirt_native_make_pmd,
#if PAGETABLE_LEVELS == 4
- .pud_val = native_pud_val,
- .make_pud = native_make_pud,
+ .pud_val = paravirt_native_pud_val,
+ .make_pud = paravirt_native_make_pud,
.set_pgd = native_set_pgd,
#endif
#endif /* PAGETABLE_LEVELS >= 3 */
- .pte_val = native_pte_val,
- .pgd_val = native_pgd_val,
+ .pte_val = paravirt_native_pte_val,
+ .pgd_val = paravirt_native_pgd_val,
- .make_pte = native_make_pte,
- .make_pgd = native_make_pgd,
+ .make_pte = paravirt_native_make_pte,
+ .make_pgd = paravirt_native_make_pgd,
.dup_mmap = paravirt_nop,
.exit_mmap = paravirt_nop,
===================================================================
--- a/arch/x86/kernel/paravirt_patch_32.c
+++ b/arch/x86/kernel/paravirt_patch_32.c
@@ -12,6 +12,18 @@
DEF_NATIVE(pv_cpu_ops, clts, "clts");
DEF_NATIVE(pv_cpu_ops, read_tsc, "rdtsc");
+unsigned paravirt_patch_ident_32(void *insnbuf, unsigned len)
+{
+ /* arg in %eax, return in %eax */
+ return 0;
+}
+
+unsigned paravirt_patch_ident_64(void *insnbuf, unsigned len)
+{
+ /* arg in %edx:%eax, return in %edx:%eax */
+ return 0;
+}
+
unsigned native_patch(u8 type, u16 clobbers, void *ibuf,
unsigned long addr, unsigned len)
{
===================================================================
--- a/arch/x86/kernel/paravirt_patch_64.c
+++ b/arch/x86/kernel/paravirt_patch_64.c
@@ -19,6 +19,21 @@
DEF_NATIVE(pv_cpu_ops, usergs_sysret32, "swapgs; sysretl");
DEF_NATIVE(pv_cpu_ops, swapgs, "swapgs");
+DEF_NATIVE(, mov32, "mov %edi, %eax");
+DEF_NATIVE(, mov64, "mov %rdi, %rax");
+
+unsigned paravirt_patch_ident_32(void *insnbuf, unsigned len)
+{
+ return paravirt_patch_insns(insnbuf, len,
+ start__mov32, end__mov32);
+}
+
+unsigned paravirt_patch_ident_64(void *insnbuf, unsigned len)
+{
+ return paravirt_patch_insns(insnbuf, len,
+ start__mov64, end__mov64);
+}
+
unsigned native_patch(u8 type, u16 clobbers, void *ibuf,
unsigned long addr, unsigned len)
{
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