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SubjectRe: [PATCH] debug: More properly delay for secondary CPUs
On Fri, 14 Oct 2016 11:41:21 -0700 Douglas Anderson <dianders@chromium.org> wrote:

> We've got a delay loop waiting for secondary CPUs. That loop uses
> loops_per_jiffy. However, loops_per_jiffy doesn't actually mean how
> many tight loops make up a jiffy on all architectures. It is quite
> common to see things like this in the boot log:
> Calibrating delay loop (skipped), value calculated using timer
> frequency.. 48.00 BogoMIPS (lpj=24000)
>
> In my case I was seeing lots of cases where other CPUs timed out
> entering the debugging only to print their stack crawls shortly after
> the kdb> prompt was written.
>
> It appears that other code with similar loops (like __spin_lock_debug)
> adds an extra __delay(1) in there which makes it work better.
> Presumably the __delay(1) is very safe. At least on modern ARM/ARM64
> systems it will just do a CP15 instruction, which should be safe. On
> older ARM systems it will fall back to an actual delay loop, or perhaps
> another memory-mapped timer. On other platforms it must be safe too or
> it wouldn't be used in __spin_lock_debug.
>
> Note that we use __delay(100) instead of __delay(1) so we can get a
> little closer to a more accurate timeout on systems where __delay() is
> backed by a timer. It's better to have a more accurate timeout and the
> only penalty is that we might wait an extra 99 "loops" before we enter
> the debugger.
>
> --- a/kernel/debug/debug_core.c
> +++ b/kernel/debug/debug_core.c
> @@ -598,11 +598,11 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs,
> /*
> * Wait for the other CPUs to be notified and be waiting for us:
> */
> - time_left = loops_per_jiffy * HZ;
> + time_left = DIV_ROUND_UP(loops_per_jiffy * HZ, 100);
> while (kgdb_do_roundup && --time_left &&
> (atomic_read(&masters_in_kgdb) + atomic_read(&slaves_in_kgdb)) !=
> online_cpus)
> - cpu_relax();
> + __delay(100);
> if (!time_left)
> pr_crit("Timed out waiting for secondary CPUs.\n");
>

This is all rather vague, isn't it?

Can the code be redone using ndelay() or udelay()? That way we should
be able to get predictable, arch-independent, cpu-freq-independent
delay periods.

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