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

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Message-ID: <da1be252-403f-6725-a1b8-223222f7f946@codeaurora.org>
Date:   Thu, 8 Mar 2018 12:21:19 +0530
From:   Chintan Pandya <cpandya@...eaurora.org>
To:     Matthew Wilcox <willy@...radead.org>,
        Christopher Lameter <cl@...ux.com>
Cc:     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 3/7/2018 11:52 PM, Matthew Wilcox wrote:
> 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.
> 
This is exactly what I was thinking. But looking up 'age' is easy 
compared to 'when' and this seems required as from Christopher's
reply. So, will raise new patch cleaning commit message a bit.

Chintan
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