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@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>