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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Thu, 21 Apr 2022 13:20:32 +0200
Subject: [PATCH] net: Use this_cpu_inc() to increment net->core_stats
The macro dev_core_stats_##FIELD##_inc() disables preemption and invokes
netdev_core_stats_alloc() to return a per-CPU pointer.
netdev_core_stats_alloc() will allocate memory on its first invocation
which breaks on PREEMPT_RT because it requires non-atomic context for
memory allocation.
This can be avoided by enabling preemption in netdev_core_stats_alloc()
assuming the caller always disables preemption.
It might be better to replace local_inc() with this_cpu_inc() now that
dev_core_stats_##FIELD##_inc() gained a preempt-disable section and does
not rely on already disabled preemption. This results in less
instructions on x86-64:
local_inc:
| incl %gs:__preempt_count(%rip) # __preempt_count
| movq 488(%rdi), %rax # _1->core_stats, _22
| testq %rax, %rax # _22
| je .L585 #,
| add %gs:this_cpu_off(%rip), %rax # this_cpu_off, tcp_ptr__
| .L586:
| testq %rax, %rax # _27
| je .L587 #,
| incq (%rax) # _6->a.counter
| .L587:
| decl %gs:__preempt_count(%rip) # __preempt_count
this_cpu_inc(), this patch:
| movq 488(%rdi), %rax # _1->core_stats, _5
| testq %rax, %rax # _5
| je .L591 #,
| .L585:
| incq %gs:(%rax) # _18->rx_dropped
Use unsigned long as type for the counter. Use this_cpu_inc() to
increment the counter. Use a plain read of the counter.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
include/linux/netdevice.h | 17 +++++++----------
net/core/dev.c | 14 +++++---------
2 files changed, 12 insertions(+), 19 deletions(-)
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -199,10 +199,10 @@ struct net_device_stats {
* Try to fit them in a single cache line, for dev_get_stats() sake.
*/
struct net_device_core_stats {
- local_t rx_dropped;
- local_t tx_dropped;
- local_t rx_nohandler;
-} __aligned(4 * sizeof(local_t));
+ unsigned long rx_dropped;
+ unsigned long tx_dropped;
+ unsigned long rx_nohandler;
+} __aligned(4 * sizeof(unsigned long));
#include <linux/cache.h>
#include <linux/skbuff.h>
@@ -3843,7 +3843,7 @@ static __always_inline bool __is_skb_for
return false;
}
-struct net_device_core_stats *netdev_core_stats_alloc(struct net_device *dev);
+struct net_device_core_stats __percpu *netdev_core_stats_alloc(struct net_device *dev);
static inline struct net_device_core_stats *dev_core_stats(struct net_device *dev)
{
@@ -3851,7 +3851,7 @@ static inline struct net_device_core_sta
struct net_device_core_stats __percpu *p = READ_ONCE(dev->core_stats);
if (likely(p))
- return this_cpu_ptr(p);
+ return p;
return netdev_core_stats_alloc(dev);
}
@@ -3861,12 +3861,9 @@ static inline void dev_core_stats_##FIEL
{ \
struct net_device_core_stats *p; \
\
- preempt_disable(); \
p = dev_core_stats(dev); \
- \
if (p) \
- local_inc(&p->FIELD); \
- preempt_enable(); \
+ this_cpu_inc(p->FIELD); \
}
DEV_CORE_STATS_INC(rx_dropped)
DEV_CORE_STATS_INC(tx_dropped)
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -10304,7 +10304,7 @@ void netdev_stats_to_stats64(struct rtnl
}
EXPORT_SYMBOL(netdev_stats_to_stats64);
-struct net_device_core_stats *netdev_core_stats_alloc(struct net_device *dev)
+struct net_device_core_stats __percpu *netdev_core_stats_alloc(struct net_device *dev)
{
struct net_device_core_stats __percpu *p;
@@ -10315,11 +10315,7 @@ struct net_device_core_stats *netdev_cor
free_percpu(p);
/* This READ_ONCE() pairs with the cmpxchg() above */
- p = READ_ONCE(dev->core_stats);
- if (!p)
- return NULL;
-
- return this_cpu_ptr(p);
+ return READ_ONCE(dev->core_stats);
}
EXPORT_SYMBOL(netdev_core_stats_alloc);
@@ -10356,9 +10352,9 @@ struct rtnl_link_stats64 *dev_get_stats(
for_each_possible_cpu(i) {
core_stats = per_cpu_ptr(p, i);
- storage->rx_dropped += local_read(&core_stats->rx_dropped);
- storage->tx_dropped += local_read(&core_stats->tx_dropped);
- storage->rx_nohandler += local_read(&core_stats->rx_nohandler);
+ storage->rx_dropped += core_stats->rx_dropped;
+ storage->tx_dropped += core_stats->tx_dropped;
+ storage->rx_nohandler += core_stats->rx_nohandler;
}
}
return storage;
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