On 25/10/2024 13.16, Toke Høiland-Jørgensen wrote:
Yunsheng Lin <linyunsheng@xxxxxxxxxx> writes:On 2024/10/24 22:40, Toke Høiland-Jørgensen wrote: ...I really really dislike this approach! Nacked-by: Jesper Dangaard Brouer <hawk@xxxxxxxxxx> Having to keep an array to record all the pages including the ones which are handed over to network stack, goes against the very principle behind page_pool. We added members to struct page, such that pages could be "outstanding".Before and after this patch both support "outstanding", the difference is how many "outstanding" pages do they support. The question seems to be do we really need unlimited inflight page for page_pool to work as mentioned in [1]? 1. https://lore.kernel.org/all/5d9ea7bd-67bb-4a9d-a120-c8f290c31a47@xxxxxxxxxx/Well, yes? Imposing an arbitrary limit on the number of in-flight packets (especially such a low one as in this series) is a complete non-starter. Servers have hundreds of gigs of memory these days, and if someone wants to use that for storing in-flight packets, the kernel definitely shouldn't impose some (hard-coded!) limit on that.
I agree this limit is a non-starter.
You and Jesper seems to be mentioning a possible fact that there might be 'hundreds of gigs of memory' needed for inflight pages, it would be nice to provide more info or reasoning above why 'hundreds of gigs of memory' is needed here so that we don't do a over-designed thing to support recording unlimited in-flight pages if the driver unbound stalling turns out impossible and the inflight pages do need to be recorded.I don't have a concrete example of a use that will blow the limit you are setting (but maybe Jesper does), I am simply objecting to the arbitrary imposing of any limit at all. It smells a lot of "640k ought to be enough for anyone".
As I wrote before. In *production* I'm seeing TCP memory reach 24 GiB (on machines with 384GiB memory). I have attached a grafana screenshot to prove what I'm saying. As my co-worker Mike Freemon, have explain to me (and more details in blogposts[1]). It is no coincident that graph have a strange "sealing" close to 24 GiB (on machines with 384GiB total memory). This is because TCP network stack goes into a memory "under pressure" state when 6.25% of total memory is used by TCP-stack. (Detail: The system will stay in that mode until allocated TCP memory falls below 4.68% of total memory).[1] https://blog.cloudflare.com/unbounded-memory-usage-by-tcp-for-receive-buffers-and-how-we-fixed-it/
I guess it is common sense to start with easy one until someone complains with some testcase and detailed reasoning if we need to go the hard way as you and Jesper are also prefering waiting over having to record the inflight pages.AFAIU Jakub's comment on his RFC patch for waiting, he was suggesting exactly this: Add the wait, and see if the cases where it can stall turn out to be problems in practice.
+1 I like Jakub's approach. --Jesper
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