On 11/4/20 4:51 AM, Eric Dumazet wrote: > > > On 11/4/20 1:36 PM, Matthew Wilcox wrote: >> On Wed, Nov 04, 2020 at 09:50:30AM +0100, Eric Dumazet wrote: >>> On 11/4/20 2:16 AM, Rama Nichanamatlu wrote: >>>>> Thanks for providing the numbers. Do you think that dropping (up to) >>>>> 7 packets is acceptable? >>>> >>>> net.ipv4.tcp_syn_retries = 6 >>>> >>>> tcp clients wouldn't even get that far leading to connect establish issues. >>> >>> This does not really matter. If host was under memory pressure, >>> dropping a few packets is really not an issue. >>> >>> Please do not add expensive checks in fast path, just to "not drop a packet" >>> even if the world is collapsing. >> >> Right, that was my first patch -- to only recheck if we're about to >> reuse the page. Do you think that's acceptable, or is that still too >> close to the fast path? > > I think it is totally acceptable. > > The same strategy is used in NIC drivers, before recycling a page. > > If page_is_pfmemalloc() returns true, they simply release the 'problematic'page > and attempt a new allocation. > > ( git grep -n page_is_pfmemalloc -- drivers/net/ethernet/ ) While the drivers may implement their own page_frag_cache to manage skb->frags ... ... the skb->data is usually allocated via __netdev_alloc_skb() or napi_alloc_skb(), which end up to the global this_cpu_ptr(&netdev_alloc_cache) or this_cpu_ptr(&napi_alloc_cache.page). > > >> >>> Also consider that NIC typically have thousands of pre-allocated page/frags >>> for their RX ring buffers, they might all have pfmemalloc set, so we are speaking >>> of thousands of packet drops before the RX-ring can be refilled with normal (non pfmemalloc) page/frags. >>> >>> If we want to solve this issue more generically, we would have to try >>> to copy data into a non pfmemalloc frag instead of dropping skb that >>> had frags allocated minutes ago under memory pressure. >> >> I don't think we need to copy anything. We need to figure out if the >> system is still under memory pressure, and if not, we can clear the >> pfmemalloc bit on the frag, as in my second patch. The 'least change' >> way of doing that is to try to allocate a page, but the VM could export >> a symbol that says "we're not under memory pressure any more". >> >> Did you want to move checking that into the networking layer, or do you >> want to keep it in the pagefrag allocator? > > I think your proposal is fine, thanks ! Hi Matthew, are you going to send out the patch to avoid pfmemalloc recycle? Thank you very much! Dongli Zhang
