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Subject[PATCH RFC net-next v2 6/7] net: skbuff: avoid accessing page_pool if !napi_safe when returning page
Date
Currently, pp->p.napi is always read, but the actual variable it gets
assigned to is read-only when @napi_safe is true. For the !napi_safe
cases, which yet is still a pack, it's an unneeded operation.
Moreover, it can lead to premature or even redundant page_pool
cacheline access. For example, when page_pool_is_last_frag() returns
false (with the recent frag improvements).
Thus, read it only when @napi_safe is true. This also allows moving
@napi inside the condition block itself. Constify it while we are
here, because why not.

Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
---
net/core/skbuff.c | 12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 4de280c454e4..fc1470aab5cf 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -881,9 +881,8 @@ static void skb_clone_fraglist(struct sk_buff *skb)

bool page_pool_return_skb_page(struct page *page, bool napi_safe)
{
- struct napi_struct *napi;
+ bool allow_direct = false;
struct page_pool *pp;
- bool allow_direct;

page = compound_head(page);

@@ -903,9 +902,12 @@ bool page_pool_return_skb_page(struct page *page, bool napi_safe)
* in the same context as the consumer would run, so there's
* no possible race.
*/
- napi = READ_ONCE(pp->p.napi);
- allow_direct = napi_safe && napi &&
- READ_ONCE(napi->list_owner) == smp_processor_id();
+ if (napi_safe) {
+ const struct napi_struct *napi = READ_ONCE(pp->p.napi);
+
+ allow_direct = napi &&
+ READ_ONCE(napi->list_owner) == smp_processor_id();
+ }

/* Driver set this to memory recycling info. Reset it on recycle.
* This will *not* work for NIC using a split-page memory model.
--
2.41.0
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