linux-kernel - Re: [RFC] trace: Add option for polling ring buffers

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Date:   Thu, 20 May 2021 10:57:21 +0200
From:   Nicolas Saenz Julienne <nsaenzju@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     rostedt@...dmis.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, mingo@...hat.com, corbet@....net,
        mtosatti@...hat.com
Subject: Re: [RFC] trace: Add option for polling ring buffers

Hi Matthew, thanks for your comments.

On Wed, 2021-05-19 at 19:07 +0100, Matthew Wilcox wrote:
> On Wed, May 19, 2021 at 07:57:55PM +0200, Nicolas Saenz Julienne wrote:
> > To minimize trace's effect on isolated CPUs. That is, CPUs were only a
> > handful or a single, process are allowed to run. Introduce a new trace
> > option: 'poll-rb'.
> 
> maybe this should take a parameter in ms (us?) saying how frequently
> to poll?  it seems like a reasonable assumption that somebody running in
> this kind of RT environment would be able to judge how often their
> monitoring task needs to collect data.

I'll look into it.

> > [1] The IPI, in this case, an irq_work, is needed since trace might run
> > in NMI context. Which is not suitable for wake-ups.
> 
> could we also consider a try-wakeup which would not succeed if in NMI
> context?

Yes, in a similar vein, my original idea was to defer the wakeup process into a
non-isolated CPU using irq_work_on(). But that irq_work flavor is not NMI safe
(nor any other IPI mechanisms targeting other CPUs).

> or are there situations where we only gather data in NMI
> context, and so would never succeed in waking up?

Yes, that's a use-case. For ex. 'trace-cmd record -e nmi'.

> if so, maybe schedule the irq_work every 1000 failures to wake up.

You'd be generating latency spikes nonetheless. Which might eventually break
the isolated application latency requirements.

As Marcelo said, the least code we run on the isolated CPU the better.

-- 
Nicolás Sáenz