[Skiboot] [PATCH] FSP/CONSOLE: Workaround for unresponsive ipmi daemon

[Stewart Smith]

Vasant Hegde <hegdevasant at linux.vnet.ibm.com> writes:

> We use TCE mapped area to write data to console. Console header
> (fsp_serbuf_hdr) is modified by both FSP and OPAL (OPAL updates
> next_in pointer in fsp_serbuf_hdr and FSP updates next_out pointer).
>
> Kernel makes opal_console_write() OPAL call to write data to console.
> OPAL write data to TCE mapped area and sends MBOX command to FSP.
> If our console becomes full and we have data to write to console,
> we keep on waiting until FSP reads data.
>
> In some corner cases, where FSP is active but not responding to
> console MBOX message (due to buggy IPMI) and we have heavy console
> write happening from kernel, then eventually our console buffer
> becomes full. At this point OPAL starts sending OPAL_BUSY_EVENT to
> kernel. Kernel will keep on retrying. This is creating kernel soft
> lockups. In some extreme case when every CPU is trying to write to
> console, user will not be able to ssh and thinks system is hang.
>
> If we reset FSP or restart IPMI daemon on FSP, system recovers and
> everything becomes normal.
>
> This patch adds workaround to above issue by returning OPAL_HARDWARE
> when cosole is full. Side effect of this patch is, we may endup dropping
> latest console data. But better to drop console data than system hang.
>
> Alternative approach is to drop old data from console buffer, make space
> for new data. But in normal condition only FSP can update 'next_out'
> pointer and if we touch that pointer, it may introduce some other
> race conditions. Hence we decided to just new console write request.
>
> Signed-off-by: Vasant Hegde <hegdevasant at linux.vnet.ibm.com>
> ---
>  hw/fsp/fsp-console.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/hw/fsp/fsp-console.c b/hw/fsp/fsp-console.c
> index fd67b20..a7b0281 100644
> --- a/hw/fsp/fsp-console.c
> +++ b/hw/fsp/fsp-console.c
> @@ -610,7 +610,7 @@ static int64_t fsp_console_write(int64_t term_number, int64_t *length,
>  	*length = written;
>  	unlock(&fsp_con_lock);
>
> -	return written ? OPAL_SUCCESS : OPAL_BUSY_EVENT;
> +	return written ? OPAL_SUCCESS : OPAL_HARDWARE;
>  }
>
>  static int64_t fsp_console_write_buffer_space(int64_t term_number,

I've been thinking about this problem a bit... and I'm not quite
convinced that this is the best solution. This would have us start
dropping console output fairly soon after the FSP slows down or stops
responding for a bit.....

What about an approach where we start returting OPAL_HARDWARE if we haven't
seen any progress from the FSP in, say, a second or something? (and
probably also log an error log and/or do a HIR?)

That way, if the FSP is just heavily loaded and being a bit slow, it
should all be okay, and not potentially loose important console
information (e.g. panic(), bug, oops etc).

thoughts?
-- 
Stewart Smith
OPAL Architect, IBM.