linux-kernel - Re: [PATCH tip 0/3] Improvements of scheduler related Tracepoints

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Date:   Fri, 15 Dec 2017 08:39:08 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexei Starovoitov <ast@...com>
Cc:     Teng Qin <qinteng@...com>, mingo@...hat.com, bgregg@...flix.com,
        daniel@...earbox.net, yhs@...com, linux-kernel@...r.kernel.org,
        Kernel-team@...com
Subject: Re: [PATCH tip 0/3] Improvements of scheduler related Tracepoints

On Thu, Dec 14, 2017 at 07:16:00PM -0800, Alexei Starovoitov wrote:
> On 12/14/17 12:49 PM, Peter Zijlstra wrote:
> > On Thu, Dec 14, 2017 at 12:20:41PM -0800, Teng Qin wrote:
> > > This set of commits attempts to improve three scheduler related
> > > Tracepoints: sched_switch, sched_process_fork, sched_process_exit.
> > > 
> > > Firstly, these commit add additional flag values, namely preempt,
> > > clone_flags and group_dead to these Tracepoints, to make information
> > > exposed via the Tracepoints more useful and complete.
> > > 
> > > Secondly, these commits exposes task_struct pointers in these
> > > Tracepoints. The task_struct pointers are arguments of the Tracepoints
> > > and currently only used to compute struct field values. But for BPF
> > > programs attached to these Tracepoints, we may want to read additional
> > > task information via the task_struct pointers. This is currently either
> > > impossible, or we have to make assumption of whether the Tracepoint is
> > > running from previous / parent or next / child, and use current pointer
> > > instead. Exposing the task_struct pointers explicitly makes such use
> > > case easier and more reliable.
> > > 
> > 
> > NAK
> 
> not sure what is the concern here.
> Is it first or second part of the above ?

Definitely the second, but also the first. You know I would have ripped
out all scheduler tracepoints if I could have. They're a pain in the
arse.

A lot of people want to add to the tracepoints, with the end result that
they'll end up a big bloated pile of useless crap. The first part is
just the pieces you want added.

As to the second, that's complete crap; that just makes everything
slower for bodies benefit. If you register a traceprobe you already get
access to these things.

I think your problem is that you use perf to get access to the
tracepoints, which them means you have to do disgusting things like
this.