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net: Use this_cpu_inc() to increment net->core_stats

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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>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/YmbO0pxgtKpCw4SY@linutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Sebastian Andrzej Siewior 2022-04-25 18:39:46 +02:00 committed by Jakub Kicinski
parent acb16b395c
commit 6510ea973d
2 changed files with 14 additions and 21 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

21
include/linux/netdevice.h
View file

@ -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>
@ -3843,15 +3843,15 @@ static __always_inline bool __is_skb_forwardable(const struct net_device *dev,
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)
static inline struct net_device_core_stats __percpu *dev_core_stats(struct net_device *dev)
{
/* This READ_ONCE() pairs with the write in netdev_core_stats_alloc() */
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);
}
@ -3859,14 +3859,11 @@ static inline struct net_device_core_stats *dev_core_stats(struct net_device *de
#define DEV_CORE_STATS_INC(FIELD) \
static inline void dev_core_stats_##FIELD##_inc(struct net_device *dev) \
{ \
struct net_device_core_stats *p; \
struct net_device_core_stats __percpu *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)

14
net/core/dev.c
View file

@ -10304,7 +10304,7 @@ void netdev_stats_to_stats64(struct rtnl_link_stats64 *stats64,
}
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_core_stats_alloc(struct net_device *dev)
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(struct net_device *dev,
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 += READ_ONCE(core_stats->rx_dropped);
storage->tx_dropped += READ_ONCE(core_stats->tx_dropped);
storage->rx_nohandler += READ_ONCE(core_stats->rx_nohandler);
}
}
return storage;