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

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Date:	Wed, 9 Dec 2015 23:04:56 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Vlastimil Babka <vbabka@...e.cz>, 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 11:59:44AM +0900, Joonsoo Kim wrote:
> Ccing, Steven to ask trace-cmd problem.

Thanks, I should have been Cc'd for the rest as well.

> 
> On Fri, Dec 04, 2015 at 04:16:33PM +0100, Vlastimil Babka wrote:
> > In mm we use several kinds of flags bitfields that are sometimes printed for
> > debugging purposes, or exported to userspace via sysfs. To make them easier to
> > interpret independently on kernel version and config, we want to dump also the
> > symbolic flag names. So far this has been done with repeated calls to
> > pr_cont(), which is unreliable on SMP, and not usable for e.g. sysfs export.
> > 
> > To get a more reliable and universal solution, this patch extends printk()
> > format string for pointers to handle the page flags (%pgp), gfp_flags (%pgg)
> > and vma flags (%pgv). Existing users of dump_flag_names() are converted and
> > simplified.
> > 
> > It would be possible to pass flags by value instead of pointer, but the %p
> > format string for pointers already has extensions for various kernel
> > structures, so it's a good fit, and the extra indirection in a non-critical
> > path is negligible.
> 
> 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?
> Or it's better to pass flags by value?
> Or should I use something like show_gfp_flags()?

Yes this can be solved in perf and trace-cmd via the parse-events.c file. And
as soon as that happens, whatever method we decide upon becomes a userspace
ABI. So don't think you can change it later.

-- Steve

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