Re: [PATCH] BPF: Disable on PREEMPT_RT

[Clark Williams <williams@redhat.com>]

On Fri, 18 Oct 2019 10:38:06 +0200 (CEST)
Thomas Gleixner <tglx@linutronix.de> wrote:

> On Thu, 17 Oct 2019, David Miller wrote:
> 
> > From: Clark Williams <williams@redhat.com>
> > Date: Thu, 17 Oct 2019 21:49:17 -0500
> >   
> > > BPF programs cannot loop and are limited to 4096 instructions.  
> > 
> > The limit was increased to 1 million not too long ago.  
> 
> Assuming a instruction/cycle ratio of 1.0 and a CPU frequency of 2GHz,
> that's 500us of preempt disabled time. Out of bounds by at least one order
> of magntiude for a lot of RT scenarios.
> 

So if I do my arithmetic right, 4096 instructions would be around 2us. In
many cases that would just be noise, but there are some customer cases on
the horizon where 2us would be a significant fraction of their max latency.

Hmmm. A quick grep through the Kconfigs didn't show me a BPF config that
was set to a large numeric value (e.g. 1000000). Is the instruction limit
hard coded into the verifier/VM logic and if so, could we lift it out and
set a smaller limit for PREEMPT_RT? Not a silver bullet, but might be a
start, since it sounds like BPF code presumes it has the cpu for the duration
of a program run. 

Clark

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