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Date: Thu, 3 Feb 2022 09:34:43 -0800 From: Eric Dumazet <edumazet@...gle.com> To: Alexander H Duyck <alexander.duyck@...il.com> Cc: Eric Dumazet <eric.dumazet@...il.com>, "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, netdev <netdev@...r.kernel.org>, Coco Li <lixiaoyan@...gle.com> Subject: Re: [PATCH net-next 09/15] net: increase MAX_SKB_FRAGS On Thu, Feb 3, 2022 at 9:26 AM Alexander H Duyck <alexander.duyck@...il.com> wrote: > > On Wed, 2022-02-02 at 17:51 -0800, Eric Dumazet wrote: > > From: Eric Dumazet <edumazet@...gle.com> > > > > Currently, MAX_SKB_FRAGS value is 17. > > > > For standard tcp sendmsg() traffic, no big deal because tcp_sendmsg() > > attempts order-3 allocations, stuffing 32768 bytes per frag. > > > > But with zero copy, we use order-0 pages. > > > > For BIG TCP to show its full potential, we increase MAX_SKB_FRAGS > > to be able to fit 45 segments per skb. > > > > This is also needed for BIG TCP rx zerocopy, as zerocopy currently > > does not support skbs with frag list. > > > > We have used this MAX_SKB_FRAGS value for years at Google before > > we deployed 4K MTU, with no adverse effect. > > Back then, goal was to be able to receive full size (64KB) GRO > > packets without the frag_list overhead. > > > > Signed-off-by: Eric Dumazet <edumazet@...gle.com> > > So a big issue I see with this patch is the potential queueing issues > it may introduce on Tx queues. I suspect it will cause a number of > performance regressions and deadlocks as it will change the Tx queueing > behavior for many NICs. > > As I recall many of the Intel drivers are using MAX_SKB_FRAGS as one of > the ingredients for DESC_NEEDED in order to determine if the Tx queue > needs to stop. With this change the value for igb for instance is > jumping from 21 to 49, and the wake threshold is twice that, 98. As > such the minimum Tx descriptor threshold for the driver would need to > be updated beyond 80 otherwise it is likely to deadlock the first time > it has to pause. Are these limits hard coded in Intel drivers and firmware, or do you think this can be changed ? I could make MAX_SKB_FRAGS a config option, and default to 17, until all drivers have been fixed. Alternative is that I remove this patch from the series and we apply it to Google production kernels, as we did before.
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