linux-kernel - Re: [PATCH] add some drop_caches documentation and info messsge

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Date:	Thu, 25 Oct 2012 11:24:24 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>, linux-mm@...ck.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] add some drop_caches documentation and info messsge

On Wed, Oct 24, 2012 at 08:56:45PM -0400, KOSAKI Motohiro wrote:
> > That effectively means removing it from the kernel since distros ship
> > with those config options off.  We don't want to do that since there
> > _are_ valid, occasional uses like benchmarking that we want to be
> > consistent.
> 
> Agreed. we don't want to remove valid interface never.

Ok, duly noted.

But let's discuss this a bit further. So, for the benchmarking aspect,
you're either going to have to always require dmesg along with
benchmarking results or /proc/vmstat, depending on where the drop_caches
stats end up.

Is this how you envision it?

And then there are the VM bug cases, where you might not always get
full dmesg from a panicked system. In that case, you'd want the kernel
tainting thing too, so that it at least appears in the oops backtrace.

Although the tainting thing might not be enough - a user could
drop_caches at some point in time and the oops happening much later
could be unrelated but that can't be expressed in taint flags.

So you'd need some sort of a drop_caches counter, I'd guess. Or a last
drop_caches timestamp something.

Am I understanding the intent correctly?

Thanks.

-- 
Regards/Gruss,
    Boris.
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