RE: sched/fair: scheduler not running high priority process on idle cpu

[David Laight <David.Laight@ACULAB.COM>]

From: Steven Rostedt
> Sent: 14 January 2020 17:48
...
> > The cost of ftrace function call entry/exit (about 200 clocks) makes it
> > rather unsuitable for any performance measurements unless only
> > a very few functions are traced - which rather requires you know
> > what the code is doing :-(
> >
> 
> Well, when I use function tracing, I start all of them, analyze the
> trace, then the functions I don't care about (usually spin locks and
> other utils), I add to the set_ftrace_notrace file,  which keeps them
> from being part of the trace. I keep doing this until I find a set of
> functions that doesn't hurt overhead as much and gives me enough
> information to know what is happening. It also helps to enable all or
> most events (at least scheduling events).

I've been using schedviz - but have had to 'fixup' wrapped traces so that
all the cpu traces start at the same time to get it to load them.
I managed to find what the worker thread was running - but only
because it ran for the entire time 'echo t >/proc/sysrq-trigger' took
to finish. Then I looked at the sources to find the code...

I'm surprised the 'normal case' for tracing function entry isn't done
in assembler without saving all the registers (etc).
For tsc stamps I think it should be possible saving just 3 registers
in under 32 instructions. Scaling to ns is a bit harder.
It's a shame the ns scaling isn't left to the reading code.

	David

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