linux-kernel - Re: [PATCH] debug: More properly delay for secondary CPUs

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Date:   Tue, 18 Oct 2016 10:48:27 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Douglas Anderson <dianders@...omium.org>,
        Jason Wessel <jason.wessel@...driver.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        briannorris@...omium.org, kgdb-bugreport@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] debug: More properly delay for secondary CPUs

On 14/10/16 19:41, Douglas Anderson 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.

It would probably be better to switch this code fully over to udelay(10) 
instead (and forget about loops_per_jiffy entirely). Even udelay(10) is 
still plenty fast enough not to be human detectable when bringing up the 
debug prompt.

Note udelay() is already used internally to kgdb so there should be 
little risk introducing it here.


Daniel.


>
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> ---
>  kernel/debug/debug_core.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c
> index 0874e2edd275..454150d98dbc 100644
> --- 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");
>
>