linux-kernel - Re: [PATCH] mm: simplify node/zone name printing

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Message-ID: <20161219102759.GM393@pathway.suse.cz>
Date:   Mon, 19 Dec 2016 11:27:59 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        Johannes Weiner <hannes@...xchg.org>,
        Joonsoo Kim <js1304@...il.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH] mm: simplify node/zone name printing

On Mon 2016-12-19 09:12:10, Michal Hocko wrote:
> On Mon 19-12-16 16:32:28, Sergey Senozhatsky wrote:
> [...]
> > as far as I can tell, now for_each_populated_zone() iterations are
> > split by non-CONT printk() from show_zone_node(), which previously
> > has been   printk(KERN_CONT "%s: ", zone->name), so pr_cont(\n)
> > between iterations was important, but now that non-CONT printk()
> > should do the trick. it's _a bit_ hacky, though.
> 
> Do you consider that more hacky than the original? At least for me,
> starting with KERN_CONT and relying on an explicit \n sounds more error
> prone than leaving the last pr_cont without \n and relying on the
> implicit flushing.

The missing '\n' will cause the string will not be flushed
until another printk happens. It is not a problem here because
other printk follows. But it might take a while in general.

There was a commit[1] that flushed the cont lines when the log
buffer was read via /dev/kmsg or syslog. Also there was a patch[2]
that flushed cont lines using a timer. But the commit caused problems
and was reverted[3]. Also the patch needs more testing. So, it might
take a while until flushing partial cont lines is "guaranteed".

I would personally prefer to keep that pr_cont("\n") call when
it was already there. It makes it clear that the line is over.
It does not rely on any further printk's or fallbacks.

[1] https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=bfd8d3f23b51018388be0411ccbc2d56277fe294
[2]
https://lkml.kernel.org/r/CA+55aFwKYnrMJr_vSE+GfDGszeUGyd=CPUD15-zZ8yWQW61GBA@mail.gmail.com
[3] https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=f5c9f9c72395c3291c2e35c905dedae2b98475a4

Best Regards,
Petr