linux-kernel - Re: [PATCH v2 1/3] mm, printk: introduce new format string for flags

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Date:	Mon, 14 Dec 2015 12:03:17 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Steven Rostedt <rostedt@...dmis.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan@...nel.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...e.cz>,
	Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: Re: [PATCH v2 1/3] mm, printk: introduce new format string for flags

On Thu, Dec 10, 2015 at 11:03:34AM +0100, Vlastimil Babka wrote:
> On 12/10/2015 05:04 AM, Steven Rostedt wrote:
> >On Thu, Dec 10, 2015 at 11:59:44AM +0900, Joonsoo Kim wrote:
> >>Ccing, Steven to ask trace-cmd problem.
> >>
> >>I'd like to use %pgp in tracepoint output. It works well when I do
> >>'cat /sys/kernel/debug/tracing/trace' but not works well when I do
> >>'./trace-cmd report'. It prints following error log.
> >>
> >>   [page_ref:page_ref_unfreeze] bad op token &
> >>   [page_ref:page_ref_set] bad op token &
> >>   [page_ref:page_ref_mod_unless] bad op token &
> >>   [page_ref:page_ref_mod_and_test] bad op token &
> >>   [page_ref:page_ref_mod_and_return] bad op token &
> >>   [page_ref:page_ref_mod] bad op token &
> >>   [page_ref:page_ref_freeze] bad op token &
> >>
> >>Following is the format I used.
> >>
> >>TP_printk("pfn=0x%lx flags=%pgp count=%d mapcount=%d mapping=%p mt=%d val=%d ret=%d",
> >>                 __entry->pfn, &__entry->flags, __entry->count,
> >>                 __entry->mapcount, __entry->mapping, __entry->mt,
> >>                 __entry->val, __entry->ret)
> >>
> >>Could it be solved by 'trace-cmd' itself?
> 
> You mean that trace-cmd/parse-events.c would interpret the raw value
> of flags by itself? That would mean the flags became fixed ABI, not
> a good idea...
> 
> >>Or it's better to pass flags by value?
> 
> If it's value (as opposed to a pointer in %pgp), that doesn't change
> much wrt. having to intepret them?
> 
> >>Or should I use something like show_gfp_flags()?
> 
> Sounds like least pain to me, at least for now. We just need to have
> the translation tables available as #define with __print_flags() in
> some trace/events header, like the existing trace/events/gfpflags.h
> for gfp flags. These tables can still be reused within mm/debug.c or
> printk code without copy/paste, like I did in "[PATCH v2 6/9] mm,
> debug: introduce dump_gfpflag_names() for symbolic printing of
> gfp_flags" [1]. Maybe it's not the most elegant solution, but works
> without changing parse-events.c using the existing format export.
> 
> So if you agree, I can do this in the next spin.
> 

Okay. I'm okay with this approach.

Thanks.
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