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[76.102.12.149]) by smtp.gmail.com with ESMTPSA id 5a478bee46e88-2c16edde435sm4456638eec.25.2026.03.26.21.13.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 26 Mar 2026 21:13:36 -0700 (PDT) Date: Thu, 26 Mar 2026 21:13:36 -0700 From: Stanislav Fomichev To: Jason Xing Cc: davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org, netdev@vger.kernel.org Subject: Re: [PATCH net-next] net: add sysctl to toggle napi_consume_skb() alien skb defer Message-ID: References: <20260326144249.97213-1-kerneljasonxing@gmail.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20260326144249.97213-1-kerneljasonxing@gmail.com> On 03/26, Jason Xing wrote: > Commit e20dfbad8aab ("net: fix napi_consume_skb() with alien skbs") > defers freeing of alien SKBs (alloc_cpu != current cpu) via > skb_attempt_defer_free() on the TX completion path to reduce cross-NUMA > SLUB spinlock contention to improve multi-queue UDP workloads. > > However, this unconditionally impacts the napi_skb_cache fast recycle > path for single-flow / few-flow workloads (e.g. AF_XDP benchmarks[1]): > when the TX completion NAPI CPU differs from the SKB allocation CPU, > SKBs are deferred instead of being returned to the local napi_skb_cache, > forcing RX allocations back to the slow slab path. > > The existing net.core.skb_defer_max=0 could disable this, but it is a > global switch that also disables the defer mechanism in TCP/UDP/MPTCP > recvmsg paths, losing its positive SLUB locality benefits there. AF_XDP > can co-exist with other protocols. That's the reason why I gave up > reusing skb_defer_disable_key. Besides, if the defer path is disabled, > that means TCP/UDP/MPTCP in process path will trigger directly freeing > skb with enabling/disabling bottom half(in kfree_skb_napi_cache()) > which could affect others. So my thinking is not to touch this path. > > Add a dedicated sysctl net.core.napi_consume_skb_defer backed by a > static key to selectively control the alien skb defer feature. Let > users decide which is the best fit for their own requirements. For a non-zc path adding a userspace knob feels like too much. And there is zero documentation for users to decide which mode to use.