Re: [PATCH v2 0/4] Dump off-cpu samples directly

[Namhyung Kim]

Hello,

On Wed, May 15, 2024 at 9:56 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
>
> On Wed, May 15, 2024 at 9:24 PM Howard Chu <howardchu95@xxxxxxxxx> wrote:
> >
> > Hello,
> >
> > Here is a little update on --off-cpu.
> >
> > > > It would be nice to start landing this work so I'm wondering what the
> > > > minimal way to do that is. It seems putting behavior behind a flag is
> > > > a first step.
> >
> > The flag to determine output threshold of off-cpu has been implemented.
> > If the accumulated off-cpu time exceeds this threshold, output the sample
> > directly; otherwise, save it later for off_cpu_write.
> >
> > But adding an extra pass to handle off-cpu samples introduces performance
> > issues, here's the processing rate of --off-cpu sampling(with the
> > extra pass to extract raw
> > sample data) and without. The --off-cpu-threshold is in nanoseconds.
> >
> > +-----------------------------------------------------+---------------------------------------+----------------------+
> > | comm                                                | type
> >                        | process rate         |
> > +-----------------------------------------------------+---------------------------------------+----------------------+
> > | -F 4999 -a                                          | regular
> > samples (w/o extra pass)      | 13128.675 samples/ms |
> > +-----------------------------------------------------+---------------------------------------+----------------------+
> > | -F 1 -a --off-cpu --off-cpu-threshold 100           | offcpu samples
> > (extra pass)           |  2843.247 samples/ms |
> > +-----------------------------------------------------+---------------------------------------+----------------------+
> > | -F 4999 -a --off-cpu --off-cpu-threshold 100        | offcpu &
> > regular samples (extra pass) |  3910.686 samples/ms |
> > +-----------------------------------------------------+---------------------------------------+----------------------+
> > | -F 4999 -a --off-cpu --off-cpu-threshold 1000000000 | few offcpu &
> > regular (extra pass)     |  4661.229 samples/ms |
> > +-----------------------------------------------------+---------------------------------------+----------------------+

What does the process rate mean?  Is the sample for the
off-cpu event or other (cpu-cycles)?  Is it from a single CPU
or system-wide or per-task?

> >
> > It's not ideal. I will find a way to reduce overhead. For example
> > process them samples
> > at save time as Ian mentioned.
> >
> > > > To turn the bpf-output samples into off-cpu events there is a pass
> > > > added to the saving. I wonder if that can be more generic, like a save
> > > > time perf inject.
> >
> > And I will find a default value for such a threshold based on performance
> > and common use cases.
> >
> > > Sounds good.  We might add an option to specify the threshold to
> > > determine whether to dump the data or to save it for later.  But ideally
> > > it should be able to find a good default.
> >
> > These will be done before the GSoC kick-off on May 27.
>
> This all sounds good. 100ns seems like quite a low threshold and 1s
> extremely high, shame such a high threshold is marginal for the
> context switch performance change. I wonder 100 microseconds may be a
> more sensible threshold. It's 100 times larger than the cost of 1
> context switch but considerably less than a frame redraw at 60FPS (16
> milliseconds).

I don't know what's the sensible default.  But 1 msec could be
another candidate for the similar reason. :)

Thanks,
Namhyung