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Message-ID: <878qng7i63.fsf@toke.dk>
Date: Thu, 01 May 2025 12:13:24 +0200
From: Toke Høiland-Jørgensen <toke@...hat.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
netdev@...r.kernel.org, linux-rt-devel@...ts.linux.dev
Cc: "David S. Miller" <davem@...emloft.net>, Eric Dumazet
<edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
<pabeni@...hat.com>, Simon Horman <horms@...nel.org>, Thomas Gleixner
<tglx@...utronix.de>, Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Andrew Lunn <andrew+netdev@...n.ch>, Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>, Jesper Dangaard Brouer
<hawk@...nel.org>, John Fastabend <john.fastabend@...il.com>
Subject: Re: [PATCH net-next v3 05/18] xdp: Use nested-BH locking for
system_page_pool
Sebastian Andrzej Siewior <bigeasy@...utronix.de> writes:
> system_page_pool is a per-CPU variable and relies on disabled BH for its
> locking. Without per-CPU locking in local_bh_disable() on PREEMPT_RT
> this data structure requires explicit locking.
>
> Make a struct with a page_pool member (original system_page_pool) and a
> local_lock_t and use local_lock_nested_bh() for locking. This change
> adds only lockdep coverage and does not alter the functional behaviour
> for !PREEMPT_RT.
>
> Cc: Andrew Lunn <andrew+netdev@...n.ch>
> Cc: Alexei Starovoitov <ast@...nel.org>
> Cc: Daniel Borkmann <daniel@...earbox.net>
> Cc: Jesper Dangaard Brouer <hawk@...nel.org>
> Cc: John Fastabend <john.fastabend@...il.com>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
> include/linux/netdevice.h | 7 ++++++-
> net/core/dev.c | 15 ++++++++++-----
> net/core/xdp.c | 11 +++++++++--
> 3 files changed, 25 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 2d11d013cabed..2018e2432cb56 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -3502,7 +3502,12 @@ struct softnet_data {
> };
>
> DECLARE_PER_CPU_ALIGNED(struct softnet_data, softnet_data);
> -DECLARE_PER_CPU(struct page_pool *, system_page_pool);
> +
> +struct page_pool_bh {
> + struct page_pool *pool;
> + local_lock_t bh_lock;
> +};
> +DECLARE_PER_CPU(struct page_pool_bh, system_page_pool);
>
> #ifndef CONFIG_PREEMPT_RT
> static inline int dev_recursion_level(void)
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 1be7cb73a6024..b56becd070bc7 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -462,7 +462,9 @@ EXPORT_PER_CPU_SYMBOL(softnet_data);
> * PP consumers must pay attention to run APIs in the appropriate context
> * (e.g. NAPI context).
> */
> -DEFINE_PER_CPU(struct page_pool *, system_page_pool);
> +DEFINE_PER_CPU(struct page_pool_bh, system_page_pool) = {
> + .bh_lock = INIT_LOCAL_LOCK(bh_lock),
> +};
I'm a little fuzzy on how DEFINE_PER_CPU() works, but does this
initialisation automatically do the right thing with the multiple
per-CPU instances?
> #ifdef CONFIG_LOCKDEP
> /*
> @@ -5238,7 +5240,10 @@ netif_skb_check_for_xdp(struct sk_buff **pskb, const struct bpf_prog *prog)
> struct sk_buff *skb = *pskb;
> int err, hroom, troom;
>
> - if (!skb_cow_data_for_xdp(this_cpu_read(system_page_pool), pskb, prog))
> + local_lock_nested_bh(&system_page_pool.bh_lock);
> + err = skb_cow_data_for_xdp(this_cpu_read(system_page_pool.pool), pskb, prog);
> + local_unlock_nested_bh(&system_page_pool.bh_lock);
> + if (!err)
> return 0;
>
> /* In case we have to go down the path and also linearize,
> @@ -12629,7 +12634,7 @@ static int net_page_pool_create(int cpuid)
> return err;
> }
>
> - per_cpu(system_page_pool, cpuid) = pp_ptr;
> + per_cpu(system_page_pool.pool, cpuid) = pp_ptr;
> #endif
> return 0;
> }
> @@ -12759,13 +12764,13 @@ static int __init net_dev_init(void)
> for_each_possible_cpu(i) {
> struct page_pool *pp_ptr;
>
> - pp_ptr = per_cpu(system_page_pool, i);
> + pp_ptr = per_cpu(system_page_pool.pool, i);
> if (!pp_ptr)
> continue;
>
> xdp_unreg_page_pool(pp_ptr);
> page_pool_destroy(pp_ptr);
> - per_cpu(system_page_pool, i) = NULL;
> + per_cpu(system_page_pool.pool, i) = NULL;
> }
> }
>
> diff --git a/net/core/xdp.c b/net/core/xdp.c
> index f86eedad586a7..b2a5c934fe7b7 100644
> --- a/net/core/xdp.c
> +++ b/net/core/xdp.c
> @@ -737,10 +737,10 @@ static noinline bool xdp_copy_frags_from_zc(struct sk_buff *skb,
> */
> struct sk_buff *xdp_build_skb_from_zc(struct xdp_buff *xdp)
> {
> - struct page_pool *pp = this_cpu_read(system_page_pool);
> const struct xdp_rxq_info *rxq = xdp->rxq;
> u32 len = xdp->data_end - xdp->data_meta;
> u32 truesize = xdp->frame_sz;
> + struct page_pool *pp;
> struct sk_buff *skb;
> int metalen;
> void *data;
> @@ -748,13 +748,18 @@ struct sk_buff *xdp_build_skb_from_zc(struct xdp_buff *xdp)
> if (!IS_ENABLED(CONFIG_PAGE_POOL))
> return NULL;
>
> + local_lock_nested_bh(&system_page_pool.bh_lock);
> + pp = this_cpu_read(system_page_pool.pool);
> data = page_pool_dev_alloc_va(pp, &truesize);
> - if (unlikely(!data))
> + if (unlikely(!data)) {
> + local_unlock_nested_bh(&system_page_pool.bh_lock);
> return NULL;
> + }
>
> skb = napi_build_skb(data, truesize);
> if (unlikely(!skb)) {
> page_pool_free_va(pp, data, true);
> + local_unlock_nested_bh(&system_page_pool.bh_lock);
> return NULL;
> }
>
> @@ -773,9 +778,11 @@ struct sk_buff *xdp_build_skb_from_zc(struct xdp_buff *xdp)
>
> if (unlikely(xdp_buff_has_frags(xdp)) &&
> unlikely(!xdp_copy_frags_from_zc(skb, xdp, pp))) {
> + local_unlock_nested_bh(&system_page_pool.bh_lock);
> napi_consume_skb(skb, true);
> return NULL;
> }
> + local_unlock_nested_bh(&system_page_pool.bh_lock);
Hmm, instead of having four separate unlock calls in this function, how
about initialising skb = NULL, and having the unlock call just above
'return skb' with an out: label?
Then the three topmost 'return NULL' can just straight-forwardly be
replaced with 'goto out', while the last one becomes 'skb = NULL; goto
out;'. I think that would be more readable than this repetition.
-Toke
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