(trimmed CC list a bit) On Tue, 29 Aug 2017 09:53:15 +0200 Florian Westphal <f...@strlen.de> wrote:
> Jesper Dangaard Brouer <bro...@redhat.com> wrote: > > On Mon, 28 Aug 2017 16:00:32 +0200 > > Florian Westphal <f...@strlen.de> wrote: > > > > > liujian (CE) <liujia...@huawei.com> wrote: > > > > Hi > > > > > > > > I checked our 3.10 kernel, we had backported all percpu_counter > > > > bug fix in lib/percpu_counter.c and include/linux/percpu_counter.h. > > > > And I check 4.13-rc6, also has the issue if NIC's rx cpu num big enough. > > > > > > > > > > > > the issue: > > > > > > > > Ip_defrag fail caused by frag_mem_limit reached > > > > > > > > 4M(frags.high_thresh). > > > > > > > > At this moment,sum_frag_mem_limit is about 10K. > > > > > > > > So should we change ipfrag high/low thresh to a reasonable value ? > > > > And if it is, is there a standard to change the value? > > > > > > Each cpu can have frag_percpu_counter_batch bytes rest doesn't know > > > about so with 64 cpus that is ~8 mbyte. > > > > > > possible solutions: > > > 1. reduce frag_percpu_counter_batch to 16k or so > > > 2. make both low and high thresh depend on NR_CPUS > > I take 2) back. Its wrong to do this, for large NR_CPU values it > would even overflow. Alternatively solution 3: Why do we want to maintain a (4MBytes) memory limit, across all CPUs? Couldn't we just allow each CPU to have a memory limit? > > To me it looks like we/I have been using the wrong API for comparing > > against percpu_counters. I guess we should have used > > __percpu_counter_compare(). > > Are you sure? For liujian use case (64 cores) it looks like we would > always fall through to percpu_counter_sum() so we eat spinlock_irqsave > cost for all compares. > > Before we entertain this we should consider reducing frag_percpu_counter_batch > to a smaller value. Yes, I agree, we really need to lower/reduce the frag_percpu_counter_batch. As you say, else the __percpu_counter_compare() call will be useless (around systems with >= 32 CPUs). I think the bug is in frag_mem_limit(). It just reads the global counter (fbc->count), without considering other CPUs can have upto 130K that haven't been subtracted yet (due to 3M low limit, become dangerous at >=24 CPUs). The __percpu_counter_compare() does the right thing, and takes into account the number of (online) CPUs and batch size, to account for this. If we choose 16K (16384), and use __percpu_counter_compare(), then we can scale to systems with 256 CPUs (4*1024*1024/16384=256), before this memory accounting becomes more expensive (than not using percpu_counters). But Liujian, reports he have a 384 CPU system, so he would still need to increase the lower+high threshold. $ grep -H . /proc/sys/net/ipv*/ip*frag_*_thresh /proc/sys/net/netfilter/nf_conntrack_frag6_*_thresh /proc/sys/net/ipv4/ipfrag_high_thresh:4194304 /proc/sys/net/ipv4/ipfrag_low_thresh:3145728 /proc/sys/net/ipv6/ip6frag_high_thresh:4194304 /proc/sys/net/ipv6/ip6frag_low_thresh:3145728 /proc/sys/net/netfilter/nf_conntrack_frag6_high_thresh:4194304 /proc/sys/net/netfilter/nf_conntrack_frag6_low_thresh:3145728 -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer
