linux-kernel - Re: [RFC PATCH 00/11] bpf, trace, dtrace: DTrace BPF program type implementation and sample use

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Message-ID: <20190524072726.GD2589@hirez.programming.kicks-ass.net>
Date:   Fri, 24 May 2019 09:27:26 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kris Van Hees <kris.van.hees@...cle.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        dtrace-devel@....oracle.com, linux-kernel@...r.kernel.org,
        rostedt@...dmis.org, mhiramat@...nel.org, acme@...nel.org,
        ast@...nel.org, daniel@...earbox.net
Subject: Re: [RFC PATCH 00/11] bpf, trace, dtrace: DTrace BPF program type
 implementation and sample use

On Wed, May 22, 2019 at 02:22:15PM -0400, Kris Van Hees wrote:

> > Let me further NAK it for adding all sorts of garbage to the code --
> > we're not going to do gaps and stay_in_page nonsense.
> 
> Could you give some guidance in terms of an alternative?  The ring buffer code
> provides both non-contiguous page allocation support and a vmalloc-based
> allocation, and the vmalloc version certainly would avoid the entire gap and
> page boundary stuff.  But since the allocator is chosen at build time based on
> the arch capabilities, there is no way to select a specific memory allocator.
> I'd be happy to use an alternative approach that allows direct writing into
> the ring buffer.

So why can't you do what the regular perf does? Use an output iterator
that knows about the page breaks? See perf_output_put() for example.

Anyway, I agree with Alexei and DaveM, get it working without/minimal
kernel changes first, and then we can talk about possible optimizations.