linux-kernel - Re: [PATCH] slub: Fix misleading 'age' in verbose slub prints

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Date:   Wed, 7 Mar 2018 10:22:12 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Christopher Lameter <cl@...ux.com>
Cc:     Chintan Pandya <cpandya@...eaurora.org>, penberg@...nel.org,
        rientjes@...gle.com, iamjoonsoo.kim@....com,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] slub: Fix misleading 'age' in verbose slub prints

On Wed, Mar 07, 2018 at 12:13:56PM -0600, Christopher Lameter wrote:
> On Wed, 7 Mar 2018, Chintan Pandya wrote:
> > In this case, object got freed later but 'age' shows
> > otherwise. This could be because, while printing
> > this info, we print allocation traces first and
> > free traces thereafter. In between, if we get schedule
> > out, (jiffies - t->when) could become meaningless.
> 
> Ok then get the jiffies earlier?
> 
> > So, simply print when the object was allocated/freed.
> 
> The tick value may not related to anything in the logs that is why the
> "age" is there. How do I know how long ago the allocation was if I look at
> the log and only see long and large number of ticks since bootup?

I missed that the first read-through too.  The trick is that there are two printks:

[ 6044.170804] INFO: Allocated in binder_transaction+0x4b0/0x2448 age=731 cpu=3 pid=5314
...
[ 6044.216696] INFO: Freed in binder_free_transaction+0x2c/0x58 age=735 cpu=6 pid=2079

If you print the raw value, then you can do the subtraction yourself;
if you've subtracted it from jiffies each time, you've at least introduced
jitter, and possibly enough jitter to confuse and mislead.