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

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Message-ID: <20220902013532.6n5cyf3oofntljho@moria.home.lan>
Date:   Thu, 1 Sep 2022 21:35:32 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Steven Rostedt <rostedt@...dmis.org>
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, 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.

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.

random thought: might try adding a warning in schedule() any time it's called
and codetag_time_stats_start() hasn't been called, that'll be a starting
point...