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

[Borislav Petkov <bp@alien8.de>]

On Thu, Oct 25, 2012 at 04:57:11AM -0700, Dave Hansen wrote:
> On 10/25/2012 02:24 AM, Borislav Petkov wrote:
> > 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.
> 
> Here's the problem: Joe Kernel Developer gets a bug report, usually
> something like "the kernel is slow", or "the kernel is eating up all my
> memory".  We then start going and digging in to the problem with the
> usual tools.  We almost *ALWAYS* get dmesg, and it's reasonably common,
> but less likely, that we get things like vmstat along with such a bug
> report.
> 
> Joe Kernel Developer digs in the statistics or the dmesg and tries to
> figure out what happened.  I've run in to a couple of cases in practice
> (and I assume Michal has too) where the bug reporter was using
> drop_caches _heavily_ and did not realize the implications.  It was
> quite hard to track down exactly how the page cache and dentries/inodes
> were getting purged.
> 
> There are rarely oopses involved in these scenarios.
> 
> The primary goal of this patch is to make debugging those scenarios
> easier so that we can quickly realize that drop_caches is the reason our
> caches went away, not some anomalous VM activity.  A secondary goal is
> to tell the user: "Hey, maybe this isn't something you want to be doing
> all the time."

Ok, understood. So you will be requiring dmesg, ok, then it makes sense.

This way you're also getting timestamps of when exactly and how many
times drop_caches was used. For that, though, you'll need to add the
timestamp explicitly to the printk because CONFIG_PRINTK_TIME is not
always enabled.

Thanks.

-- 
Regards/Gruss,
Boris.

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>