Commit 6510ea97 authored by Sebastian Andrzej Siewior's avatar Sebastian Andrzej Siewior Committed by Jakub Kicinski
Browse files

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: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: default avatarEric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/YmbO0pxgtKpCw4SY@linutronix.de


Signed-off-by: default avatarJakub Kicinski <kuba@kernel.org>
parent acb16b39

include/linux/netdevice.h

+9 −12
Original line number Diff line number Diff line
@@ -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)

net/core/dev.c

+5 −9
Original line number Diff line number Diff line
@@ -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;