On Wed, 30 Aug 2017 13:58:20 +0200 Florian Westphal <f...@strlen.de> wrote:
> Jesper Dangaard Brouer <bro...@redhat.com> wrote: > > > 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? > > Consider ipv4, ipv6, nf ipv6 defrag, 6lowpan, and 8k cpus... This will > render any limit useless. With 8K CPUs I agree, that this might be a bad idea! > > > > 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. > > Right, I think we should at very least use __percpu_counter_compare > before denying a new frag queue allocation request. > > I'll create a patch. Oh, I've already started working on a patch, that I'm testing now. But if you want to take the assignment then I'm fine with that!. I just though that it was my responsibility to fix, given I introduced percpu_counter usage (back in 2013-01-28 / 6d7b857d541e). -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer
