linux-kernel - Re: [RFC PATCH 27/30] Code tagging based latency tracking

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Message-ID: <20220901215925.59ae5cb0@gandalf.local.home>
Date:   Thu, 1 Sep 2022 21:59:25 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Kent Overstreet <kent.overstreet@...ux.dev>
Cc:     Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org,
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Subject: Re: [RFC PATCH 27/30] Code tagging based latency tracking

On Thu, 1 Sep 2022 21:35:32 -0400
Kent Overstreet <kent.overstreet@...ux.dev> wrote:

> On Thu, Sep 01, 2022 at 08:23:11PM -0400, Steven Rostedt wrote:
> > If ftrace, perf, bpf can't do what you want, take a harder look to see if
> > you can modify them to do so.  
> 
> Maybe we can use this exchange to make both of our tools better. I like your
> histograms - the quantiles algorithm I've had for years is janky, I've been
> meaning to rip that out, I'd love to take a look at your code for that. And
> having an on/off switch is a good idea, I'll try to add that at some point.
> Maybe you got some ideas from my stuff too.
> 
> I'd love to get better tracepoints for measuring latency - what I added to
> init_wait() and finish_wait() was really only a starting point. Figuring out
> the right places to measure is where I'd like to be investing my time in this
> area, and there's no reason we couldn't both be making use of that.

Yes, this is exactly what I'm talking about. I'm not against your work, I
just want you to work more with everyone to come up with ideas that can
help everyone as a whole. That's how "open source communities" is suppose
to work ;-)

The histogram and synthetic events can use some more clean ups. There's a
lot of places that can be improved in that code. But I feel the ideas
behind that code is sound. It's just getting the implementation to be a bit
more efficient.

> 
> e.g. with kernel waitqueues, I looked at hooking prepare_to_wait() first but not
> all code uses that, init_wait() got me better coverage. But I've already seen
> that that misses things, too, there's more work to be done.

I picked prepare_to_wait() just because I was hacking up something quick
and thought that was "close enough" ;-)

-- Steve