From: Christopher Yeoh <cyeoh@au1.ibm.com>
Date: Wed, 24 Aug 2011 23:46:42 +0000 (+1000)
Subject: > You might get some speed benefit by optimising for the small copies
X-Git-Tag: next-20110905~1^2~97
X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=1cf69cf86c72614dc93e91cd092b15c1fd32a332;p=karo-tx-linux.git

> You might get some speed benefit by optimising for the small copies
> here.  Define a local on-stack array of N page*'s and point
> process_pages at that if the number of pages is <= N.  Saves a
> malloc/free and is more cache-friendly.  But only if the result is
> measurable!

I have done some benchmarking on this, and it gains about 5-7% on a
microbenchmark with 4kb size copies and about a 1% gain with a more
realistic (but modified for smaller copies) hpcc benchmark. The
performance gain disappears into the noise by about 64kb sized copies.
No measurable overhead for larger copies. So I think its worth including

Included below is the patch (based on v4) - for ease of review the first diff
is just against the latest version of CMA which has been posted here previously.
The second is the entire CMA patch.

Signed-off-by: Chris Yeoh <cyeoh@au1.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: David Howells <dhowells@redhat.com>
Cc: James Morris <jmorris@namei.org>
Cc: <linux-man@vger.kernel.org>
Cc: <linux-arch@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

diff --git a/mm/process_vm_access.c b/mm/process_vm_access.c
index 5ac8abc0d09b..e920aa3ce104 100644
--- a/mm/process_vm_access.c
+++ b/mm/process_vm_access.c
@@ -221,6 +221,10 @@ static int process_vm_rw_single_vec(unsigned long addr,
 	return rc;
 }
 
+/* Maximum number of entries for process pages array
+   which lives on stack */
+#define PVM_MAX_PP_ARRAY_COUNT 16
+
 /**
  * process_vm_rw_core - core of reading/writing pages from task specified
  * @pid: PID of process to read/write from/to
@@ -241,7 +245,8 @@ static ssize_t process_vm_rw_core(pid_t pid, const struct iovec *lvec,
 				  unsigned long flags, int vm_write)
 {
 	struct task_struct *task;
-	struct page **process_pages = NULL;
+	struct page *pp_stack[PVM_MAX_PP_ARRAY_COUNT];
+	struct page **process_pages = pp_stack;
 	struct mm_struct *mm;
 	unsigned long i;
 	ssize_t rc = 0;
@@ -271,13 +276,16 @@ static ssize_t process_vm_rw_core(pid_t pid, const struct iovec *lvec,
 	if (nr_pages == 0)
 		return 0;
 
-	/* For reliability don't try to kmalloc more than 2 pages worth */
-	process_pages = kmalloc(min_t(size_t, PVM_MAX_KMALLOC_PAGES,
-				      sizeof(struct pages *)*nr_pages),
-				GFP_KERNEL);
+	if (nr_pages > PVM_MAX_PP_ARRAY_COUNT) {
+		/* For reliability don't try to kmalloc more than
+		   2 pages worth */
+		process_pages = kmalloc(min_t(size_t, PVM_MAX_KMALLOC_PAGES,
+					      sizeof(struct pages *)*nr_pages),
+					GFP_KERNEL);
 
-	if (!process_pages)
-		return -ENOMEM;
+		if (!process_pages)
+			return -ENOMEM;
+	}
 
 	/* Get process information */
 	rcu_read_lock();
@@ -331,7 +339,8 @@ put_task_struct:
 	put_task_struct(task);
 
 free_proc_pages:
-	kfree(process_pages);
+	if (process_pages != pp_stack)
+		kfree(process_pages);
 	return rc;
 }