[Xen-devel] Re: lmbench lat_mmap slowdown with CONFIG_PARAVIRT

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

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

Attachment: 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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