From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Wang Subject: Re: [RFC PATCH v2] ptr_ring: linked list fallback Date: Tue, 27 Feb 2018 10:29:26 +0800 Message-ID: <0316016a-717b-9d3f-5aef-dccaf34d0fae@redhat.com> References: <1519607771-20613-1-git-send-email-mst@redhat.com> <01aff5eb-a92f-2170-05f7-664220985070@redhat.com> <20180226223252-mutt-send-email-mst@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Cc: linux-kernel@vger.kernel.org, John Fastabend , netdev@vger.kernel.org, David Miller To: "Michael S. Tsirkin" Return-path: In-Reply-To: <20180226223252-mutt-send-email-mst@kernel.org> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On 2018年02月27日 04:34, Michael S. Tsirkin wrote: > On Mon, Feb 26, 2018 at 11:15:42AM +0800, Jason Wang wrote: >> On 2018年02月26日 09:17, Michael S. Tsirkin wrote: >>> So pointer rings work fine, but they have a problem: make them too small >>> and not enough entries fit. Make them too large and you start flushing >>> your cache and running out of memory. >>> >>> This is a new idea of mine: a ring backed by a linked list. Once you run >>> out of ring entries, instead of a drop you fall back on a list with a >>> common lock. >>> >>> Should work well for the case where the ring is typically sized >>> correctly, but will help address the fact that some user try to set e.g. >>> tx queue length to 1000000. >>> >>> In other words, the idea is that if a user sets a really huge TX queue >>> length, we allocate a ptr_ring which is smaller, and use the backup >>> linked list when necessary to provide the requested TX queue length >>> legitimately. >>> >>> My hope this will move us closer to direction where e.g. fw codel can >>> use ptr rings without locking at all. The API is still very rough, and >>> I really need to take a hard look at lock nesting. >>> >>> Compiled only, sending for early feedback/flames. >>> >>> Signed-off-by: Michael S. Tsirkin >>> --- >>> >>> changes from v1: >>> - added clarifications by DaveM in the commit log >>> - build fixes >>> >>> include/linux/ptr_ring.h | 64 +++++++++++++++++++++++++++++++++++++++++++++--- >>> 1 file changed, 61 insertions(+), 3 deletions(-) >>> >>> diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h >>> index d72b2e7..8aa8882 100644 >>> --- a/include/linux/ptr_ring.h >>> +++ b/include/linux/ptr_ring.h >>> @@ -31,11 +31,18 @@ >>> #include >>> #endif >>> +/* entries must start with the following structure */ >>> +struct plist { >>> + struct plist *next; >>> + struct plist *last; /* only valid in the 1st entry */ >>> +}; >> So I wonder whether or not it's better to do this in e.g skb_array >> implementation. Then it can use its own prev/next field. > XDP uses ptr ring directly, doesn't it? > Well I believe the main user for this is qdisc, which use skb array. And we can not use what implemented in this patch directly for sk_buff without some changes on the data structure. For XDP, we need to embed plist in struct xdp_buff too, so it looks to me that the better approach is to have separated function for ptr ring and skb array. Thanks