Re: [PATCH 06/13] mm: Preemptible mmu_gather

Peter Zijlstra
2010-04-09T20:36:33Z

On Fri, 2010-04-09 at 13:25 +1000, Nick Piggin wrote:
> Have you done some profiling on this? What I would like to see, if
> it's not too much complexity, is to have a small set of pages to
> handle common size frees, and then use them up first by default
> before attempting to allocate more.
>
> Also, it would be cool to be able to chain allocations to avoid
> TLB flushes even on big frees (overridable by arch of course, in
> case they're doing some non-preeemptible work or you wish to break
> up lock hold times). But that might be just getting over engineered.
>
Measuring ITLB_FLUSH on Intel nehalem using:

perf stat -a -e r01ae make O=defconfig-build/ -j48 bzImage

-linus 5825850 +- 2545 (100%)
+patches 5891341 +- 6045 (101%)
+below 5783991 +- 4725 ( 99%)

(No slab allocations yet)

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
include/asm-generic/tlb.h | 122 ++++++++++++++++++++++++++++++----------------
1 file changed, 82 insertions(+), 40 deletions(-)

Index: linux-2.6/include/asm-generic/tlb.h
===================================================================
--- linux-2.6.orig/include/asm-generic/tlb.h
+++ linux-2.6/include/asm-generic/tlb.h
@@ -17,16 +17,6 @@
#include <asm/pgalloc.h>
#include <asm/tlbflush.h>

-/*
- * For UP we don't need to worry about TLB flush
- * and page free order so much..
- */
-#ifdef CONFIG_SMP
- #define tlb_fast_mode(tlb) ((tlb)->nr == ~0U)
-#else
- #define tlb_fast_mode(tlb) 1
-#endif
-
#ifdef HAVE_ARCH_RCU_TABLE_FREE
/*
* Semi RCU freeing of the page directories.
@@ -70,31 +60,66 @@ extern void tlb_remove_table(struct mmu_

#endif

+struct mmu_gather_batch {
+ struct mmu_gather_batch *next;
+ unsigned int nr;
+ unsigned int max;
+ struct page *pages[0];
+};
+
+#define MAX_GATHER_BATCH \
+ ((PAGE_SIZE - sizeof(struct mmu_gather_batch)) / sizeof(unsigned long))
+
/* struct mmu_gather is an opaque type used by the mm code for passing around
* any data needed by arch specific code for tlb_remove_page.
*/
struct mmu_gather {
struct mm_struct *mm;
- unsigned int nr; /* set to ~0U means fast mode */
- unsigned int max; /* nr < max */
- unsigned int need_flush;/* Really unmapped some ptes? */
- unsigned int fullmm; /* non-zero means full mm flush */
- struct page **pages;
- struct page *local[8];
+ unsigned int need_flush : 1, /* Did free PTEs */
+ fast_mode : 1; /* No batching */
+ unsigned int fullmm; /* Flush full mm */
+
+ struct mmu_gather_batch *active;
+ struct mmu_gather_batch local;
+ struct page *__pages[8];

#ifdef HAVE_ARCH_RCU_TABLE_FREE
struct mmu_table_batch *batch;
#endif
};

-static inline void __tlb_alloc_pages(struct mmu_gather *tlb)
+/*
+ * For UP we don't need to worry about TLB flush
+ * and page free order so much..
+ */
+#ifdef CONFIG_SMP
+ #define tlb_fast_mode(tlb) (tlb->fast_mode)
+#else
+ #define tlb_fast_mode(tlb) 1
+#endif
+
+static inline int tlb_next_batch(struct mmu_gather *tlb)
{
- unsigned long addr = __get_free_pages(GFP_ATOMIC, 0);
+ struct mmu_gather_batch *batch;

- if (addr) {
- tlb->pages = (void *)addr;
- tlb->max = PAGE_SIZE / sizeof(struct page *);
+ batch = tlb->active;
+ if (batch->next) {
+ tlb->active = batch->next;
+ return 1;
}
+
+ batch = (void *)__get_free_pages(GFP_ATOMIC, 0);
+ if (!batch)
+ return 0;
+
+ batch->next = NULL;
+ batch->nr = 0;
+ batch->max = MAX_GATHER_BATCH;
+
+ tlb->active->next = batch;
+ tlb->active = batch;
+
+ return 1;
}

/* tlb_gather_mmu
@@ -105,17 +130,16 @@ tlb_gather_mmu(struct mmu_gather *tlb, s
{
tlb->mm = mm;

- tlb->max = ARRAY_SIZE(tlb->local);
- tlb->pages = tlb->local;
-
- if (num_online_cpus() > 1) {
- tlb->nr = 0;
- __tlb_alloc_pages(tlb);
- } else /* Use fast mode if only one CPU is online */
- tlb->nr = ~0U;
-
+ tlb->need_flush = 0;
+ if (num_online_cpus() == 1)
+ tlb->fast_mode = 1;
tlb->fullmm = full_mm_flush;

+ tlb->local.next = NULL;
+ tlb->local.nr = 0;
+ tlb->local.max = ARRAY_SIZE(tlb->__pages);
+ tlb->active = &tlb->local;
+
#ifdef HAVE_ARCH_RCU_TABLE_FREE
tlb->batch = NULL;
#endif
@@ -124,6 +148,8 @@ tlb_gather_mmu(struct mmu_gather *tlb, s
static inline void
tlb_flush_mmu(struct mmu_gather *tlb, unsigned long start, unsigned long end)
{
+ struct mmu_gather_batch *batch;
+
if (!tlb->need_flush)
return;
tlb->need_flush = 0;
@@ -131,12 +157,14 @@ tlb_flush_mmu(struct mmu_gather *tlb, un
#ifdef HAVE_ARCH_RCU_TABLE_FREE
tlb_table_flush(tlb);
#endif
- if (!tlb_fast_mode(tlb)) {
- free_pages_and_swap_cache(tlb->pages, tlb->nr);
- tlb->nr = 0;
- if (tlb->pages == tlb->local)
- __tlb_alloc_pages(tlb);
+ if (tlb_fast_mode(tlb))
+ return;
+
+ for (batch = &tlb->local; batch; batch = batch->next) {
+ free_pages_and_swap_cache(batch->pages, batch->nr);
+ batch->nr = 0;
}
+ tlb->active = &tlb->local;
}

/* tlb_finish_mmu
@@ -146,13 +174,18 @@ tlb_flush_mmu(struct mmu_gather *tlb, un
static inline void
tlb_finish_mmu(struct mmu_gather *tlb, unsigned long start, unsigned long end)
{
+ struct mmu_gather_batch *batch, *next;
+
tlb_flush_mmu(tlb, start, end);

/* keep the page table cache within bounds */
check_pgt_cache();

- if (tlb->pages != tlb->local)
- free_pages((unsigned long)tlb->pages, 0);
+ for (batch = tlb->local.next; batch; batch = next) {
+ next = batch->next;
+ free_pages((unsigned long)batch, 0);
+ }
+ tlb->local.next = NULL;
}

/* tlb_remove_page
@@ -162,14 +195,23 @@ tlb_finish_mmu(struct mmu_gather *tlb, u
*/
static inline void tlb_remove_page(struct mmu_gather *tlb, struct page *page)
{
+ struct mmu_gather_batch *batch;
+
tlb->need_flush = 1;
+
if (tlb_fast_mode(tlb)) {
free_page_and_swap_cache(page);
return;
}
- tlb->pages[tlb->nr++] = page;
- if (tlb->nr >= tlb->max)
- tlb_flush_mmu(tlb, 0, 0);
+
+ batch = tlb->active;
+ if (batch->nr == batch->max) {
+ if (!tlb_next_batch(tlb))
+ tlb_flush_mmu(tlb, 0, 0);
+ batch = tlb->active;
+ }
+
+ batch->pages[batch->nr++] = page;
}

/**