Re: [PATCH] Fix a race in pid generation that causes pids to be reused immediately.

Andrew Morton
2010-06-15T19:37:51Z

On Tue, 15 Jun 2010 16:35:52 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> > I suspect sched_clock.c might be generating fair amounts of code which
> > UP builds don't need.
>
> Only sched_clock_remote() and its caller, something like the below, not
> much code..
>
> UP machines can still have utterly sucky TSC, although the
> inter-cpu-drift thing isn't much of an issue ;-)
>
> ---
> kernel/sched_clock.c | 4 ++++
> 1 files changed, 4 insertions(+), 0 deletions(-)
>
> diff --git a/kernel/sched_clock.c b/kernel/sched_clock.c
> index 52f1a14..7ff5b56 100644
> --- a/kernel/sched_clock.c
> +++ b/kernel/sched_clock.c
> @@ -170,6 +170,7 @@ again:
> return clock;
> }
>
> +#ifdef CONFIG_SMP
> static u64 sched_clock_remote(struct sched_clock_data *scd)
> {
> struct sched_clock_data *my_scd = this_scd();
> @@ -205,6 +206,7 @@ again:
>
> return val;
> }
> +#endif
>
> /*
> * Similar to cpu_clock(), but requires local IRQs to be disabled.
> @@ -226,9 +228,11 @@ u64 sched_clock_cpu(int cpu)
>
> scd = cpu_sdc(cpu);
>
> +#ifdef CONFIG_SMP
> if (cpu != smp_processor_id())
> clock = sched_clock_remote(scd);
> else
> +#endif
> clock = sched_clock_local(scd);
>
> return clock;

hm, OK, I was actually looking at sched_clock_local() at the time. Can
clocks go backwards on UP hardware? What breaks if we do #define
sched_clock_local sched_clock?

I've mentioned this before, but sched_clock.c is really opaque - it
would be a formidable task for anyone to get in there and work on the
code if they hadn't already been working on it for years.