diff mbox

[guv,v2,04/31] net: Replace __get_cpu_var uses

Message ID 00000140bc5ef9df-d7f75909-a25e-4471-84b2-dfbec07117b0-000000@email.amazonses.com
State Not Applicable, archived
Delegated to: David Miller
Headers show

Commit Message

Christoph Lameter (Ampere) Aug. 26, 2013, 8:44 p.m. UTC 
__get_cpu_var() is used for multiple purposes in the kernel source. One of them is
address calculation via the form &__get_cpu_var(x). This calculates the address for
the instance of the percpu variable of the current processor based on an offset.

Others usage cases are for storing and retrieving data from the current processors percpu area.
__get_cpu_var() can be used as an lvalue when writing data or on the right side of an assignment.

__get_cpu_var() is defined as :


#define __get_cpu_var(var) (*this_cpu_ptr(&(var)))



__get_cpu_var() always only does a address determination. However, store and retrieve operations
could use a segment prefix (or global register on other platforms) to avoid the address calculation.

this_cpu_write() and this_cpu_read() can directly take an offset into a percpu area and use
optimized assembly code to read and write per cpu variables.


This patch converts __get_cpu_var into either and explicit address calculation using this_cpu_ptr()
or into a use of this_cpu operations that use the offset. Thereby address calcualtions are avoided
and less registers are used when code is generated.

At the end of the patchset all uses of __get_cpu_var have been removed so the macro is removed too.

The patchset includes passes over all arches as well. Once these operations are used throughout then
specialized macros can be defined in non -x86 arches as well in order to optimize per cpu access by
f.e. using a global register that may be set to the per cpu base.




Transformations done to __get_cpu_var()


1. Determine the address of the percpu instance of the current processor.

	DEFINE_PER_CPU(int, y);
	int *x = &__get_cpu_var(y);

    Converts to

	int *x = this_cpu_ptr(&y);


2. Same as #1 but this time an array structure is involved.

	DEFINE_PER_CPU(int, y[20]);
	int *x = __get_cpu_var(y);

    Converts to

	int *x = this_cpu_ptr(y);


3. Retrieve the content of the current processors instance of a per cpu variable.

	DEFINE_PER_CPU(int, u);
	int x = __get_cpu_var(y)

   Converts to

	int x = __this_cpu_read(y);


4. Retrieve the content of a percpu struct

	DEFINE_PER_CPU(struct mystruct, y);
	struct mystruct x = __get_cpu_var(y);

   Converts to

	memcpy(this_cpu_ptr(&y), x, sizeof(x));


5. Assignment to a per cpu variable

	DEFINE_PER_CPU(int, y)
	__get_cpu_var(y) = x;

   Converts to

	this_cpu_write(y, x);


6. Increment/Decrement etc of a per cpu variable

	DEFINE_PER_CPU(int, y);
	__get_cpu_var(y)++

   Converts to

	this_cpu_inc(y)

Signed-off-by: Christoph Lameter <cl@linux.com>


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Comments

David Howells Sept. 2, 2013, 9:35 p.m. UTC | #1 
Would it be possible to use __thread annotations for per-CPU variables, I
wonder?

David
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David Miller Sept. 3, 2013, 3:33 a.m. UTC | #2 
From: David Howells <dhowells@redhat.com>
Date: Mon, 02 Sep 2013 22:35:06 +0100

> Would it be possible to use __thread annotations for per-CPU
> variables, I wonder?

Paul Mackerras tried it on powerpc and you can't do it.

The problem is that there is no way to tell the compiler that sched()
and similar (potentially) change the thread pointer base.

It really will cache pre-computed __thread pointer calculations across
sched().
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Christoph Lameter (Ampere) Sept. 3, 2013, 2:20 p.m. UTC | #3 
On Mon, 2 Sep 2013, David Howells wrote:

> Would it be possible to use __thread annotations for per-CPU variables, I
> wonder?

We already have a __percpu annotation. Looked at __thread a couple of
years ago but found that support for the kernel segment register was not
available. If we had __thread then most of the operations could simply be
provided by the compiler.



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Christoph Lameter (Ampere) Sept. 3, 2013, 2:21 p.m. UTC | #4 
On Mon, 2 Sep 2013, David Miller wrote:

> It really will cache pre-computed __thread pointer calculations across
> sched().

On x86 the compiler could use the segment prefix and then it may work.

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David Howells Sept. 3, 2013, 8:39 p.m. UTC | #5 
David Miller <davem@davemloft.net> wrote:

> > Would it be possible to use __thread annotations for per-CPU
> > variables, I wonder?
> 
> Paul Mackerras tried it on powerpc and you can't do it.
> 
> The problem is that there is no way to tell the compiler that sched()
> and similar (potentially) change the thread pointer base.
> 
> It really will cache pre-computed __thread pointer calculations across
> sched().

Yeah...  I thought there might be something like that - but I could see x86_64
using %fs as a base.

David
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diff mbox

Patch

Index: linux/net/core/dev.c
===================================================================
--- linux.orig/net/core/dev.c	2013-08-26 14:16:59.000000000 -0500
+++ linux/net/core/dev.c	2013-08-26 14:18:37.214005168 -0500
@@ -2129,7 +2129,7 @@  static inline void __netif_reschedule(st
 	unsigned long flags;
 
 	local_irq_save(flags);
-	sd = &__get_cpu_var(softnet_data);
+	sd = this_cpu_ptr(&softnet_data);
 	q->next_sched = NULL;
 	*sd->output_queue_tailp = q;
 	sd->output_queue_tailp = &q->next_sched;
@@ -2151,7 +2151,7 @@  void dev_kfree_skb_irq(struct sk_buff *s
 		unsigned long flags;
 
 		local_irq_save(flags);
-		sd = &__get_cpu_var(softnet_data);
+		sd = this_cpu_ptr(&softnet_data);
 		skb->next = sd->completion_queue;
 		sd->completion_queue = skb;
 		raise_softirq_irqoff(NET_TX_SOFTIRQ);
@@ -3111,7 +3111,7 @@  static void rps_trigger_softirq(void *da
 static int rps_ipi_queued(struct softnet_data *sd)
 {
 #ifdef CONFIG_RPS
-	struct softnet_data *mysd = &__get_cpu_var(softnet_data);
+	struct softnet_data *mysd = this_cpu_ptr(&softnet_data);
 
 	if (sd != mysd) {
 		sd->rps_ipi_next = mysd->rps_ipi_list;
@@ -3138,7 +3138,7 @@  static bool skb_flow_limit(struct sk_buf
 	if (qlen < (netdev_max_backlog >> 1))
 		return false;
 
-	sd = &__get_cpu_var(softnet_data);
+	sd = this_cpu_ptr(&softnet_data);
 
 	rcu_read_lock();
 	fl = rcu_dereference(sd->flow_limit);
@@ -3280,7 +3280,7 @@  EXPORT_SYMBOL(netif_rx_ni);
 
 static void net_tx_action(struct softirq_action *h)
 {
-	struct softnet_data *sd = &__get_cpu_var(softnet_data);
+	struct softnet_data *sd = this_cpu_ptr(&softnet_data);
 
 	if (sd->completion_queue) {
 		struct sk_buff *clist;
@@ -3700,7 +3700,7 @@  EXPORT_SYMBOL(netif_receive_skb);
 static void flush_backlog(void *arg)
 {
 	struct net_device *dev = arg;
-	struct softnet_data *sd = &__get_cpu_var(softnet_data);
+	struct softnet_data *sd = this_cpu_ptr(&softnet_data);
 	struct sk_buff *skb, *tmp;
 
 	rps_lock(sd);
@@ -4146,7 +4146,7 @@  void __napi_schedule(struct napi_struct
 	unsigned long flags;
 
 	local_irq_save(flags);
-	____napi_schedule(&__get_cpu_var(softnet_data), n);
+	____napi_schedule(this_cpu_ptr(&softnet_data), n);
 	local_irq_restore(flags);
 }
 EXPORT_SYMBOL(__napi_schedule);
@@ -4274,7 +4274,7 @@  EXPORT_SYMBOL(netif_napi_del);
 
 static void net_rx_action(struct softirq_action *h)
 {
-	struct softnet_data *sd = &__get_cpu_var(softnet_data);
+	struct softnet_data *sd = this_cpu_ptr(&softnet_data);
 	unsigned long time_limit = jiffies + 2;
 	int budget = netdev_budget;
 	void *have;
Index: linux/net/core/drop_monitor.c
===================================================================
--- linux.orig/net/core/drop_monitor.c	2013-08-26 14:16:59.000000000 -0500
+++ linux/net/core/drop_monitor.c	2013-08-26 14:18:37.218005126 -0500
@@ -142,7 +142,7 @@  static void trace_drop_common(struct sk_
 	unsigned long flags;
 
 	local_irq_save(flags);
-	data = &__get_cpu_var(dm_cpu_data);
+	data = this_cpu_ptr(&dm_cpu_data);
 	spin_lock(&data->lock);
 	dskb = data->skb;
 
Index: linux/net/core/skbuff.c
===================================================================
--- linux.orig/net/core/skbuff.c	2013-08-26 14:16:59.000000000 -0500
+++ linux/net/core/skbuff.c	2013-08-26 14:18:37.218005126 -0500
@@ -371,7 +371,7 @@  static void *__netdev_alloc_frag(unsigne
 	unsigned long flags;
 
 	local_irq_save(flags);
-	nc = &__get_cpu_var(netdev_alloc_cache);
+	nc = this_cpu_ptr(&netdev_alloc_cache);
 	if (unlikely(!nc->frag.page)) {
 refill:
 		for (order = NETDEV_FRAG_PAGE_MAX_ORDER; ;) {
Index: linux/net/ipv4/syncookies.c
===================================================================
--- linux.orig/net/ipv4/syncookies.c	2013-08-26 14:16:59.000000000 -0500
+++ linux/net/ipv4/syncookies.c	2013-08-26 14:18:37.210005210 -0500
@@ -44,7 +44,7 @@  static DEFINE_PER_CPU(__u32 [16 + 5 + SH
 static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
 		       u32 count, int c)
 {
-	__u32 *tmp = __get_cpu_var(ipv4_cookie_scratch);
+	__u32 *tmp = this_cpu_ptr(ipv4_cookie_scratch);
 
 	memcpy(tmp + 4, syncookie_secret[c], sizeof(syncookie_secret[c]));
 	tmp[0] = (__force u32)saddr;
Index: linux/net/ipv4/tcp_output.c
===================================================================
--- linux.orig/net/ipv4/tcp_output.c	2013-08-26 14:16:59.000000000 -0500
+++ linux/net/ipv4/tcp_output.c	2013-08-26 14:18:37.218005126 -0500
@@ -810,7 +810,7 @@  void tcp_wfree(struct sk_buff *skb)
 
 		/* queue this socket to tasklet queue */
 		local_irq_save(flags);
-		tsq = &__get_cpu_var(tsq_tasklet);
+		tsq = this_cpu_ptr(&tsq_tasklet);
 		list_add(&tp->tsq_node, &tsq->head);
 		tasklet_schedule(&tsq->tasklet);
 		local_irq_restore(flags);
Index: linux/net/ipv6/syncookies.c
===================================================================
--- linux.orig/net/ipv6/syncookies.c	2013-08-26 14:16:59.000000000 -0500
+++ linux/net/ipv6/syncookies.c	2013-08-26 14:18:37.214005168 -0500
@@ -66,7 +66,7 @@  static DEFINE_PER_CPU(__u32 [16 + 5 + SH
 static u32 cookie_hash(const struct in6_addr *saddr, const struct in6_addr *daddr,
 		       __be16 sport, __be16 dport, u32 count, int c)
 {
-	__u32 *tmp = __get_cpu_var(ipv6_cookie_scratch);
+	__u32 *tmp = this_cpu_ptr(ipv6_cookie_scratch);
 
 	/*
 	 * we have 320 bits of information to hash, copy in the remaining
Index: linux/net/rds/ib_rdma.c
===================================================================
--- linux.orig/net/rds/ib_rdma.c	2013-08-26 14:16:59.000000000 -0500
+++ linux/net/rds/ib_rdma.c	2013-08-26 14:18:37.218005126 -0500
@@ -267,7 +267,7 @@  static inline struct rds_ib_mr *rds_ib_r
 	unsigned long *flag;
 
 	preempt_disable();
-	flag = &__get_cpu_var(clean_list_grace);
+	flag = this_cpu_ptr(&clean_list_grace);
 	set_bit(CLEAN_LIST_BUSY_BIT, flag);
 	ret = llist_del_first(&pool->clean_list);
 	if (ret)
Index: linux/include/net/netfilter/nf_conntrack.h
===================================================================
--- linux.orig/include/net/netfilter/nf_conntrack.h	2013-08-26 14:23:46.000000000 -0500
+++ linux/include/net/netfilter/nf_conntrack.h	2013-08-26 14:24:17.190395755 -0500
@@ -243,7 +243,7 @@  extern s16 (*nf_ct_nat_offset)(const str
 DECLARE_PER_CPU(struct nf_conn, nf_conntrack_untracked);
 static inline struct nf_conn *nf_ct_untracked_get(void)
 {
-	return &__raw_get_cpu_var(nf_conntrack_untracked);
+	return __this_cpu_ptr(&nf_conntrack_untracked);
 }
 extern void nf_ct_untracked_status_or(unsigned long bits);