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linux/arch/ppc64/mm/hash_low.S
Olof Johansson d03853d566 [PATCH] PPC64: Remove hot busy-wait loop in __hash_page 
It turns out that our current __hash_page code will do a very hot busy-wait
loop waiting on _PAGE_BUSY to be cleared.  It even does ldarx/stdcx in the
loop, which will bounce reservations around like crazy if there's more than
one CPU spinning on the same PTE (or even another PTE in the same
reservation granule).  The end result is that each fault takes longer when
there's contention, which in turn increases the chance of another thread
hitting the same fault and also piling up.  Not pretty.

There's two options here:
1. Do an out-of-line busy loop a'la spinlocks with just loads (no
   reserves)
2. Just bail and refault if needed.

(2) makes sense here: If the PTE is busy, chances are it's in flux anyway
and the other code path making a change might just be ready to hash it.

This fixes a stampede seen on a large-ish system where a multithreaded
HPC app faults in the same text pages on several cpus at the same time.

Signed-off-by: Olof Johansson <olof@lixom.net>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2005-05-01 08:58:45 -07:00

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/*
* ppc64 MMU hashtable management routines
*
* (c) Copyright IBM Corp. 2003
*
* Maintained by: Benjamin Herrenschmidt
* <benh@kernel.crashing.org>
*
* This file is covered by the GNU Public Licence v2 as
* described in the kernel's COPYING file.
*/
#include <asm/processor.h>
#include <asm/pgtable.h>
#include <asm/mmu.h>
#include <asm/page.h>
#include <asm/types.h>
#include <asm/ppc_asm.h>
#include <asm/offsets.h>
#include <asm/cputable.h>
.text
/*
* Stackframe:
*
* +-> Back chain (SP + 256)
* | General register save area (SP + 112)
* | Parameter save area (SP + 48)
* | TOC save area (SP + 40)
* | link editor doubleword (SP + 32)
* | compiler doubleword (SP + 24)
* | LR save area (SP + 16)
* | CR save area (SP + 8)
* SP ---> +-- Back chain (SP + 0)
*/
#define STACKFRAMESIZE 256
/* Save parameters offsets */
#define STK_PARM(i) (STACKFRAMESIZE + 48 + ((i)-3)*8)
/* Save non-volatile offsets */
#define STK_REG(i) (112 + ((i)-14)*8)
/*
* _hash_page(unsigned long ea, unsigned long access, unsigned long vsid,
* pte_t *ptep, unsigned long trap, int local)
*
* Adds a page to the hash table. This is the non-LPAR version for now
*/
_GLOBAL(__hash_page)
mflr r0
std r0,16(r1)
stdu r1,-STACKFRAMESIZE(r1)
/* Save all params that we need after a function call */
std r6,STK_PARM(r6)(r1)
std r8,STK_PARM(r8)(r1)
/* Add _PAGE_PRESENT to access */
ori r4,r4,_PAGE_PRESENT
/* Save non-volatile registers.
* r31 will hold "old PTE"
* r30 is "new PTE"
* r29 is "va"
* r28 is a hash value
* r27 is hashtab mask (maybe dynamic patched instead ?)
*/
std r27,STK_REG(r27)(r1)
std r28,STK_REG(r28)(r1)
std r29,STK_REG(r29)(r1)
std r30,STK_REG(r30)(r1)
std r31,STK_REG(r31)(r1)
/* Step 1:
*
* Check permissions, atomically mark the linux PTE busy
* and hashed.
*/
1:
ldarx r31,0,r6
/* Check access rights (access & ~(pte_val(*ptep))) */
andc. r0,r4,r31
bne- htab_wrong_access
/* Check if PTE is busy */
andi. r0,r31,_PAGE_BUSY
/* If so, just bail out and refault if needed. Someone else
* is changing this PTE anyway and might hash it.
*/
bne- bail_ok
/* Prepare new PTE value (turn access RW into DIRTY, then
* add BUSY,HASHPTE and ACCESSED)
*/
rlwinm r30,r4,32-9+7,31-7,31-7 /* _PAGE_RW -> _PAGE_DIRTY */
or r30,r30,r31
ori r30,r30,_PAGE_BUSY | _PAGE_ACCESSED | _PAGE_HASHPTE
/* Write the linux PTE atomically (setting busy) */
stdcx. r30,0,r6
bne- 1b
isync
/* Step 2:
*
* Insert/Update the HPTE in the hash table. At this point,
* r4 (access) is re-useable, we use it for the new HPTE flags
*/
/* Calc va and put it in r29 */
rldicr r29,r5,28,63-28
rldicl r3,r3,0,36
or r29,r3,r29
/* Calculate hash value for primary slot and store it in r28 */
rldicl r5,r5,0,25 /* vsid & 0x0000007fffffffff */
rldicl r0,r3,64-12,48 /* (ea >> 12) & 0xffff */
xor r28,r5,r0
/* Convert linux PTE bits into HW equivalents */
andi. r3,r30,0x1fe /* Get basic set of flags */
xori r3,r3,HW_NO_EXEC /* _PAGE_EXEC -> NOEXEC */
rlwinm r0,r30,32-9+1,30,30 /* _PAGE_RW -> _PAGE_USER (r0) */
rlwinm r4,r30,32-7+1,30,30 /* _PAGE_DIRTY -> _PAGE_USER (r4) */
and r0,r0,r4 /* _PAGE_RW & _PAGE_DIRTY -> r0 bit 30 */
andc r0,r30,r0 /* r0 = pte & ~r0 */
rlwimi r3,r0,32-1,31,31 /* Insert result into PP lsb */
/* We eventually do the icache sync here (maybe inline that
* code rather than call a C function...)
*/
BEGIN_FTR_SECTION
BEGIN_FTR_SECTION
mr r4,r30
mr r5,r7
bl .hash_page_do_lazy_icache
END_FTR_SECTION_IFSET(CPU_FTR_NOEXECUTE)
END_FTR_SECTION_IFCLR(CPU_FTR_COHERENT_ICACHE)
/* At this point, r3 contains new PP bits, save them in
* place of "access" in the param area (sic)
*/
std r3,STK_PARM(r4)(r1)
/* Get htab_hash_mask */
ld r4,htab_hash_mask@got(2)
ld r27,0(r4) /* htab_hash_mask -> r27 */
/* Check if we may already be in the hashtable, in this case, we
* go to out-of-line code to try to modify the HPTE
*/
andi. r0,r31,_PAGE_HASHPTE
bne htab_modify_pte
htab_insert_pte:
/* Clear hpte bits in new pte (we also clear BUSY btw) and
* add _PAGE_HASHPTE
*/
lis r0,_PAGE_HPTEFLAGS@h
ori r0,r0,_PAGE_HPTEFLAGS@l
andc r30,r30,r0
ori r30,r30,_PAGE_HASHPTE
/* page number in r5 */
rldicl r5,r31,64-PTE_SHIFT,PTE_SHIFT
/* Calculate primary group hash */
and r0,r28,r27
rldicr r3,r0,3,63-3 /* r0 = (hash & mask) << 3 */
/* Call ppc_md.hpte_insert */
ld r7,STK_PARM(r4)(r1) /* Retreive new pp bits */
mr r4,r29 /* Retreive va */
li r6,0 /* primary slot */
li r8,0 /* not bolted and not large */
li r9,0
_GLOBAL(htab_call_hpte_insert1)
bl . /* Will be patched by htab_finish_init() */
cmpdi 0,r3,0
bge htab_pte_insert_ok /* Insertion successful */
cmpdi 0,r3,-2 /* Critical failure */
beq- htab_pte_insert_failure
/* Now try secondary slot */
/* page number in r5 */
rldicl r5,r31,64-PTE_SHIFT,PTE_SHIFT
/* Calculate secondary group hash */
andc r0,r27,r28
rldicr r3,r0,3,63-3 /* r0 = (~hash & mask) << 3 */
/* Call ppc_md.hpte_insert */
ld r7,STK_PARM(r4)(r1) /* Retreive new pp bits */
mr r4,r29 /* Retreive va */
li r6,1 /* secondary slot */
li r8,0 /* not bolted and not large */
li r9,0
_GLOBAL(htab_call_hpte_insert2)
bl . /* Will be patched by htab_finish_init() */
cmpdi 0,r3,0
bge+ htab_pte_insert_ok /* Insertion successful */
cmpdi 0,r3,-2 /* Critical failure */
beq- htab_pte_insert_failure
/* Both are full, we need to evict something */
mftb r0
/* Pick a random group based on TB */
andi. r0,r0,1
mr r5,r28
bne 2f
not r5,r5
2: and r0,r5,r27
rldicr r3,r0,3,63-3 /* r0 = (hash & mask) << 3 */
/* Call ppc_md.hpte_remove */
_GLOBAL(htab_call_hpte_remove)
bl . /* Will be patched by htab_finish_init() */
/* Try all again */
b htab_insert_pte
bail_ok:
li r3,0
b bail
htab_pte_insert_ok:
/* Insert slot number & secondary bit in PTE */
rldimi r30,r3,12,63-15
/* Write out the PTE with a normal write
* (maybe add eieio may be good still ?)
*/
htab_write_out_pte:
ld r6,STK_PARM(r6)(r1)
std r30,0(r6)
li r3, 0
bail:
ld r27,STK_REG(r27)(r1)
ld r28,STK_REG(r28)(r1)
ld r29,STK_REG(r29)(r1)
ld r30,STK_REG(r30)(r1)
ld r31,STK_REG(r31)(r1)
addi r1,r1,STACKFRAMESIZE
ld r0,16(r1)
mtlr r0
blr
htab_modify_pte:
/* Keep PP bits in r4 and slot idx from the PTE around in r3 */
mr r4,r3
rlwinm r3,r31,32-12,29,31
/* Secondary group ? if yes, get a inverted hash value */
mr r5,r28
andi. r0,r31,_PAGE_SECONDARY
beq 1f
not r5,r5
1:
/* Calculate proper slot value for ppc_md.hpte_updatepp */
and r0,r5,r27
rldicr r0,r0,3,63-3 /* r0 = (hash & mask) << 3 */
add r3,r0,r3 /* add slot idx */
/* Call ppc_md.hpte_updatepp */
mr r5,r29 /* va */
li r6,0 /* large is 0 */
ld r7,STK_PARM(r8)(r1) /* get "local" param */
_GLOBAL(htab_call_hpte_updatepp)
bl . /* Will be patched by htab_finish_init() */
/* if we failed because typically the HPTE wasn't really here
* we try an insertion.
*/
cmpdi 0,r3,-1
beq- htab_insert_pte
/* Clear the BUSY bit and Write out the PTE */
li r0,_PAGE_BUSY
andc r30,r30,r0
b htab_write_out_pte
htab_wrong_access:
/* Bail out clearing reservation */
stdcx. r31,0,r6
li r3,1
b bail
htab_pte_insert_failure:
/* Bail out restoring old PTE */
ld r6,STK_PARM(r6)(r1)
std r31,0(r6)
li r3,-1
b bail
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