From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ming Lei Subject: Re: [PATCH 5/9] bpf: syscall: add percpu version of lookup/update elem Date: Wed, 13 Jan 2016 22:56:38 +0800 Message-ID: References: <1452527821-12276-1-git-send-email-tom.leiming@gmail.com> <1452527821-12276-6-git-send-email-tom.leiming@gmail.com> <20160111190248.GA26495@ast-mbp.thefacebook.com> <20160112054928.GA31180@ast-mbp.thefacebook.com> <20160112191051.GA67436@kafai-mba.local> <20160113022204.GA25270@kafai-mba.dhcp.thefacebook.com> <20160113053009.GC37858@ast-mbp.thefacebook.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Cc: Martin KaFai Lau , Linux Kernel Mailing List , Alexei Starovoitov , "David S. Miller" , Network Development , Daniel Borkmann To: Alexei Starovoitov Return-path: In-Reply-To: <20160113053009.GC37858@ast-mbp.thefacebook.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Wed, Jan 13, 2016 at 1:30 PM, Alexei Starovoitov wrote: > On Wed, Jan 13, 2016 at 11:17:23AM +0800, Ming Lei wrote: >> On Wed, Jan 13, 2016 at 10:22 AM, Martin KaFai Lau wrote: >> > On Wed, Jan 13, 2016 at 08:38:18AM +0800, Ming Lei wrote: >> >> > The userspace usually only aggregates value across all cpu every X seconds. >> >> >> >> That is just in your case, and Alexei worried the issue of data stale. >> > I believe we are talking about validity of a value. How to >> > make use of a less-stale but invalid data? >> >> About the 'invalidity' thing, it should be same between using >> smp_call(run in IPI irq handler) and simple memcpy(). >> >> When smp_call_function_single() is used to request to lookup element in >> the specific CPU, the value of the element may be in updating in that CPU >> and not completed yet in eBPF prog, then IPI comes and half updated >> data is still returned to syscall. > > hmm. I'm not following. bpf programs are executing with preempt disabled, > so smp_call_function_single suppose to execute when bpf is not running. Preempt disabled doesn't mean irq disabled, does it? So when bpf prog is running, the IPI irq for smp_call still may come on that CPU. Also in current non-percpu hash, the situation exists too between lookup elem syscall and updating value of element from bpf prog in SMP. -- Ming Lei