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

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Message-ID: <20210519193319.GC103930@fuller.cnet>
Date:   Wed, 19 May 2021 16:33:19 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Nicolas Saenz Julienne <nsaenzju@...hat.com>, rostedt@...dmis.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        mingo@...hat.com, corbet@....net
Subject: Re: [RFC] trace: Add option for polling ring buffers


Hi Willy,

On Wed, May 19, 2021 at 07:07:54PM +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.

+1 (yes please).

> > [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?  or are there situations where we only gather data in NMI
> context, and so would never succeed in waking up?  if so, maybe
> schedule the irq_work every 1000 failures to wake up.

We'd like to reduce overhead on the isolated (as in isolcpus=) CPUs as 
much as possible (but yes this option was suggested).