* Byungchul Park <byungchul.park@xxxxxxx> wrote:
> Johan Hovold reported a performance regression by crossrelease like:
>
> > Boot time (from "Linux version" to login prompt) had in fact doubled
> > since 4.13 where it took 17 seconds (with my current config) compared to
> > the 35 seconds I now see with 4.14-rc4.
> >
> > I quick bisect pointed to lockdep and specifically the following commit:
> >
> > 28a903f63ec0 ("locking/lockdep: Handle non(or multi)-acquisition
> > of a crosslock")
> >
> > which I've verified is the commit which doubled the boot time (compared
> > to 28a903f63ec0^) (added by lockdep crossrelease series [1]).
>
> Currently crossrelease performs unwind on every acquisition. But, that
> overloads systems too much. So this patch makes unwind optional and set
> it to N as default. Instead, it records only acquire_ip normally. Of
> course, unwind is sometimes required for full analysis. In that case, we
> can set CROSSRELEASE_STACK_TRACE to Y and use it.
>
> In my qemu ubuntu machin (x86_64, 4 cores, 512M), the regression was
> fixed like, measuring timestamp of "Freeing unused kernel memory":
>
> 1. No lockdep enabled
> Average : 1.543353 secs
>
> 2. Lockdep enabled
> Average : 1.570806 secs
>
> 3. Lockdep enabled + crossrelease enabled
> Average : 1.870317 secs
>
> 4. Lockdep enabled + crossrelease enabled + this patch applied
> Average : 1.574143 secs
Ok, that looks really nice, recovers almost all of the lost performance, right?
Could you please run perf stat --null --repeat type of stats of a boot test (for
example running init=/bin/true should boot up Qemu and make it exit), so that we
can see how stable the numbers are and what the real slowdown is?
> +config CROSSRELEASE_STACK_TRACE
> + bool "Record more than one entity of stack trace in crossrelease"
> + depends on LOCKDEP_CROSSRELEASE
> + default n
> + help
> + Crossrelease feature needs to record stack traces for all
> + acquisitions for later use. And only acquire_ip is normally
> + recorded because the unwind operation is too expensive. However,
> + sometimes more than acquire_ip are required for full analysis.
> + In the case that we need to record more than one entity of
> + stack trace using unwind, this feature would be useful, with
> + taking more overhead.
> +
> + If unsure, say N.
Fixed the text for you:
> + The lockdep "cross-release" feature needs to record stack traces
> + (of calling functions) for all acquisitions, for eventual later use
> + during analysis.
> + By default only a single caller is recorded, because the unwind
> + operation can be very expensive with deeper stack chains.
> + However, sometimes deeper traces are required for full analysis.
> + This option turns on the saving of the full stack trace entries.
> +
> + If unsure, say N.
BTW., have you attempted limiting the depth of the stack traces? I suspect more
than 2-4 are rarely required to disambiguate the calling context.
Thanks,
Ingo
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