netdev - Re: [PATCH v4 bpf-next 1/9] bpf: introduce bpf_spin

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Message-ID: <20190125234403.iisj5woztm4afwgh@ast-mbp.dhcp.thefacebook.com>
Date:   Fri, 25 Jan 2019 15:44:05 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     Jann Horn <jannh@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexei Starovoitov <ast@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        jakub.kicinski@...ronome.com,
        Network Development <netdev@...r.kernel.org>,
        kernel-team@...com, Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH v4 bpf-next 1/9] bpf: introduce bpf_spin_lock

On Fri, Jan 25, 2019 at 02:51:12PM -0800, Paul E. McKenney wrote:
> > >
> > > So no more than (say) 100 milliseconds?
> > 
> > Depends on RLIMIT_MEMLOCK and on how hard userspace is trying to make
> > things slow, I guess - if userspace manages to create a hashtable,
> > with a few dozen megabytes in size, with worst-case assignment of
> > elements to buckets (everything in a single bucket), every lookup call
> > on that bucket becomes a linked list traversal through a list that
> > must be stored in main memory because it's too big for the CPU caches.
> > I don't know into how much time that translates.
> 
> So perhaps you have a candidate BPF program for the RCU CPU stall warning
> challenge, then.  ;-)

I'd like to see one that can defeat jhash + random seed.