[Skiboot] [PATCH] Run pollers in time_wait() when not booting

Andrew Jeffery andrew at aj.id.au
Tue Oct 30 22:15:54 AEDT 2018

On Tue, 30 Oct 2018, at 17:43, Stewart Smith wrote:
> This only bit us hard with hioma in one scenario.

Missing a 'p' in 'hiomap'

> Our OPAL API has been OPAL_POLL_EVENTS may be needed to make forward
> progress on ongoing operations, and the internal to skiboot API has been
> that time_wait() of a suitable time will run pollers (on at least one
> CPU) to help ensure forward progress can be made.
> In a perfect world, interrupts are used but they may a) be disabled, or
> b) the thing we're doing can't use interrupts because computers are
> generally terrible.
> Back in 3db397ea5892a (circa 2015), we changed skiboot so that we'd run
> pollers only on the boot CPU, and not if we held any locks. This was to
> reduce the chance of programming code that could deadlock, as well as to
> ensure that we didn't just thrash all the cachelines for running pollers
> all over a large system during boot, or hard spin on the same locks on
> all secondary CPUs.
> The problem arises if the OS we're booting makes an OPAL call early on,
> with interrupts disabled, that requires a poller to run to make forward
> progress. An example of this would be OPAL_WRITE_NVRAM early in Linux
> boot (where Linux sets up the partitions it wants) - something that
> occurs iff we've had to reformat NVRAM this boot (i.e. first boot or
> corrupted NVRAM).
> The hiomap implementation should arguably *not* rely on synchronous IPMI
> messages, but this is a future improvement (as was for mbox before it).
> The mbox-flash code solved this problem by spinning on check_timers().
> More generically though, the approach of running the pollers when no
> longer booting means we behave more in line with what the API is meant
> to be, rather than have this odd case of "time_wait() for a condition
> that could also be tripped by an interrupt works fine unless the OS is
> up and running but hasn't set interrupts up yet".

Nice description.

> Fixes: 529bdca0bc546a7ae3ecbd2c3134b7260072d8b0
> Fixes: 3db397ea5892a8b348cf412739996731884561b3
> Signed-off-by: Stewart Smith <stewart at linux.ibm.com>
> ---
>  core/timebase.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> diff --git a/core/timebase.c b/core/timebase.c
> index 24e72959f542..f9f1a55b8dbb 100644
> --- a/core/timebase.c
> +++ b/core/timebase.c
> @@ -57,7 +57,7 @@ void time_wait(unsigned long duration)
>  		return;
>  	}
> -	if (c != boot_cpu)
> +	if (c != boot_cpu && opal_booting())
>  		time_wait_nopoll(duration);
>  	else
>  		time_wait_poll(duration);
> -- 
> 2.17.2

Applying this to master and building gave me:

core/timebase.c: In function ‘time_wait’:
core/timebase.c:60:23: error: implicit declaration of function ‘opal_booting’ [-Werror=implicit-function-declaration]
  if (c != boot_cpu && opal_booting())

But it passes snowpatch according to patchwork :thinking_face:

I had to do this:

diff --git a/core/timebase.c b/core/timebase.c
index 24e72959f542..830ca53d2442 100644
--- a/core/timebase.c
+++ b/core/timebase.c
@@ -19,6 +19,7 @@
 #include <opal.h>
 #include <cpu.h>
 #include <chip.h>
+#include <debug_descriptor.h>
 unsigned long tb_hz = 512000000;
@@ -57,7 +58,7 @@ void time_wait(unsigned long duration)
-       if (c != boot_cpu)
+       if (c != boot_cpu && opal_booting())

Anyway, with the additional include the build looks like it rids ozrom2 of the stuck thread problem, which it was hitting consistently prior to applying it.

Thanks for chasing that down.


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