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Date: Thu, 13 Apr 2023 15:49:08 +0800
From: Yunsheng Lin <linyunsheng@...wei.com>
To: Jakub Kicinski <kuba@...nel.org>, <davem@...emloft.net>
CC: <netdev@...r.kernel.org>, <edumazet@...gle.com>,
<pabeni@...hat.com>, <hawk@...nel.org>,
<ilias.apalodimas@...aro.org>, <alexander.duyck@...il.com>,
Tariq Toukan <tariqt@...dia.com>
Subject: Re: [PATCH net-next v2 1/3] net: skb: plumb napi state thru skb
freeing paths
On 2023/4/13 12:26, Jakub Kicinski wrote:
> We maintain a NAPI-local cache of skbs which is fed by napi_consume_skb().
> Going forward we will also try to cache head and data pages.
> Plumb the "are we in a normal NAPI context" information thru
> deeper into the freeing path, up to skb_release_data() and
> skb_free_head()/skb_pp_recycle(). The "not normal NAPI context"
> comes from netpoll which passes budget of 0 to try to reap
> the Tx completions but not perform any Rx.
Maybe I missed something obvious about netpoll here.
Does that mean the "normal NAPI context" and "not normal NAPI context"
will call napi->poll() concurrently with different budget? Doesn't
that mean two different contexts may do the tx completion concurrently?
Does it break the single-producer single-consumer assumption of tx queue?
>
> Use "bool napi_safe" rather than bare "int budget",
> the further we get from NAPI the more confusing the budget
> argument may seem (particularly whether 0 or MAX is the
> correct value to pass in when not in NAPI).
>
> Reviewed-by: Tariq Toukan <tariqt@...dia.com>
> Signed-off-by: Jakub Kicinski <kuba@...nel.org>
> ---
> v2:
> - clarify the budget 0 and fix the name of the argument in
> the commit message
> v1:
> - feed the cache in __kfree_skb_defer(), it's in NAPI
> - s/in_normal_napi/napi_safe/
> ---
> net/core/skbuff.c | 38 ++++++++++++++++++++------------------
> 1 file changed, 20 insertions(+), 18 deletions(-)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 050a875d09c5..b2092166f7e2 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -839,7 +839,7 @@ static void skb_clone_fraglist(struct sk_buff *skb)
> skb_get(list);
> }
>
> -static bool skb_pp_recycle(struct sk_buff *skb, void *data)
> +static bool skb_pp_recycle(struct sk_buff *skb, void *data, bool napi_safe)
> {
> if (!IS_ENABLED(CONFIG_PAGE_POOL) || !skb->pp_recycle)
> return false;
> @@ -856,12 +856,12 @@ static void skb_kfree_head(void *head, unsigned int end_offset)
> kfree(head);
> }
>
> -static void skb_free_head(struct sk_buff *skb)
> +static void skb_free_head(struct sk_buff *skb, bool napi_safe)
> {
> unsigned char *head = skb->head;
>
> if (skb->head_frag) {
> - if (skb_pp_recycle(skb, head))
> + if (skb_pp_recycle(skb, head, napi_safe))
> return;
> skb_free_frag(head);
> } else {
> @@ -869,7 +869,8 @@ static void skb_free_head(struct sk_buff *skb)
> }
> }
>
> -static void skb_release_data(struct sk_buff *skb, enum skb_drop_reason reason)
> +static void skb_release_data(struct sk_buff *skb, enum skb_drop_reason reason,
> + bool napi_safe)
> {
> struct skb_shared_info *shinfo = skb_shinfo(skb);
> int i;
> @@ -894,7 +895,7 @@ static void skb_release_data(struct sk_buff *skb, enum skb_drop_reason reason)
> if (shinfo->frag_list)
> kfree_skb_list_reason(shinfo->frag_list, reason);
>
> - skb_free_head(skb);
> + skb_free_head(skb, napi_safe);
> exit:
> /* When we clone an SKB we copy the reycling bit. The pp_recycle
> * bit is only set on the head though, so in order to avoid races
> @@ -955,11 +956,12 @@ void skb_release_head_state(struct sk_buff *skb)
> }
>
> /* Free everything but the sk_buff shell. */
> -static void skb_release_all(struct sk_buff *skb, enum skb_drop_reason reason)
> +static void skb_release_all(struct sk_buff *skb, enum skb_drop_reason reason,
> + bool napi_safe)
> {
> skb_release_head_state(skb);
> if (likely(skb->head))
> - skb_release_data(skb, reason);
> + skb_release_data(skb, reason, napi_safe);
> }
>
> /**
> @@ -973,7 +975,7 @@ static void skb_release_all(struct sk_buff *skb, enum skb_drop_reason reason)
>
> void __kfree_skb(struct sk_buff *skb)
> {
> - skb_release_all(skb, SKB_DROP_REASON_NOT_SPECIFIED);
> + skb_release_all(skb, SKB_DROP_REASON_NOT_SPECIFIED, false);
> kfree_skbmem(skb);
> }
> EXPORT_SYMBOL(__kfree_skb);
> @@ -1027,7 +1029,7 @@ static void kfree_skb_add_bulk(struct sk_buff *skb,
> return;
> }
>
> - skb_release_all(skb, reason);
> + skb_release_all(skb, reason, false);
> sa->skb_array[sa->skb_count++] = skb;
>
> if (unlikely(sa->skb_count == KFREE_SKB_BULK_SIZE)) {
> @@ -1201,7 +1203,7 @@ EXPORT_SYMBOL(consume_skb);
> void __consume_stateless_skb(struct sk_buff *skb)
> {
> trace_consume_skb(skb, __builtin_return_address(0));
> - skb_release_data(skb, SKB_CONSUMED);
> + skb_release_data(skb, SKB_CONSUMED, false);
> kfree_skbmem(skb);
> }
>
> @@ -1226,7 +1228,7 @@ static void napi_skb_cache_put(struct sk_buff *skb)
>
> void __kfree_skb_defer(struct sk_buff *skb)
> {
> - skb_release_all(skb, SKB_DROP_REASON_NOT_SPECIFIED);
> + skb_release_all(skb, SKB_DROP_REASON_NOT_SPECIFIED, true);
> napi_skb_cache_put(skb);
> }
>
> @@ -1264,7 +1266,7 @@ void napi_consume_skb(struct sk_buff *skb, int budget)
> return;
> }
>
> - skb_release_all(skb, SKB_CONSUMED);
> + skb_release_all(skb, SKB_CONSUMED, !!budget);
If it is not normal NAPI context, dev_consume_skb_any() is called and
return at the begin, we may just call skb_release_all() with napi_safe
being true here.
> napi_skb_cache_put(skb);
> }
> EXPORT_SYMBOL(napi_consume_skb);
> @@ -1395,7 +1397,7 @@ EXPORT_SYMBOL_GPL(alloc_skb_for_msg);
> */
> struct sk_buff *skb_morph(struct sk_buff *dst, struct sk_buff *src)
> {
> - skb_release_all(dst, SKB_CONSUMED);
> + skb_release_all(dst, SKB_CONSUMED, false);
> return __skb_clone(dst, src);
> }
> EXPORT_SYMBOL_GPL(skb_morph);
> @@ -2018,9 +2020,9 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
> if (skb_has_frag_list(skb))
> skb_clone_fraglist(skb);
>
> - skb_release_data(skb, SKB_CONSUMED);
> + skb_release_data(skb, SKB_CONSUMED, false);
> } else {
> - skb_free_head(skb);
> + skb_free_head(skb, false);
> }
> off = (data + nhead) - skb->head;
>
> @@ -6389,12 +6391,12 @@ static int pskb_carve_inside_header(struct sk_buff *skb, const u32 off,
> skb_frag_ref(skb, i);
> if (skb_has_frag_list(skb))
> skb_clone_fraglist(skb);
> - skb_release_data(skb, SKB_CONSUMED);
> + skb_release_data(skb, SKB_CONSUMED, false);
> } else {
> /* we can reuse existing recount- all we did was
> * relocate values
> */
> - skb_free_head(skb);
> + skb_free_head(skb, false);
> }
>
> skb->head = data;
> @@ -6529,7 +6531,7 @@ static int pskb_carve_inside_nonlinear(struct sk_buff *skb, const u32 off,
> skb_kfree_head(data, size);
> return -ENOMEM;
> }
> - skb_release_data(skb, SKB_CONSUMED);
> + skb_release_data(skb, SKB_CONSUMED, false);
>
> skb->head = data;
> skb->head_frag = 0;
>
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