linux-kernel - Re: [PATCH 5/5] slab, documentation: add description of debugfs files for SLUB caches

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Message-ID: <1cc1eb98-fe46-4563-16d3-ac00b630ecd3@suse.cz>
Date:   Wed, 2 Mar 2022 17:31:53 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc:     David Rientjes <rientjes@...gle.com>,
        Christoph Lameter <cl@...ux.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Pekka Enberg <penberg@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        patches@...ts.linux.dev, linux-kernel@...r.kernel.org,
        Oliver Glitta <glittao@...il.com>,
        Faiyaz Mohammed <faiyazm@...eaurora.org>,
        Jonathan Corbet <corbet@....net>,
        Randy Dunlap <rdunlap@...radead.org>, linux-doc@...r.kernel.org
Subject: Re: [PATCH 5/5] slab, documentation: add description of debugfs files
 for SLUB caches

On 2/27/22 04:49, Hyeonggon Yoo wrote:
> I think it's not traces of "currently free objects"
> because index bit of free objects are set in obj_map bitmap?

Hm right, thanks.

> It's weird but it's traces of allocated objects that have been freed at
> least once (or <not available>)
> 
> I think we can fix the code or doc?

For now I'll fix the doc. Not clear to me myself what's the best usecase for
free_traces file. For alloc_traces it's clearly debugging memory leaks.
Freeing traces are most useful when a bug is detected and they are dumped in
dmesg. The debugfs file might be just for a rough idea where freeing usually
happens.

> Please tell me if I'm missing something :)
> 
>> +    Information in the output:
>> +    Number of objects, freeing function, minimal/average/maximal jiffies since free,
>> +    pid range of the freeing processes, cpu mask of freeing cpus, and stack trace.
>> +
>> +    Example:::
>> +
>> +    51 acpi_ut_update_ref_count+0x6a6/0x782 age=236886/237027/237772 pid=1 cpus=1
>> +	kfree+0x2db/0x420
>> +	acpi_ut_update_ref_count+0x6a6/0x782
>> +	acpi_ut_update_object_reference+0x1ad/0x234
>> +	acpi_ut_remove_reference+0x7d/0x84
>> +	acpi_rs_get_prt_method_data+0x97/0xd6
>> +	acpi_get_irq_routing_table+0x82/0xc4
>> +	acpi_pci_irq_find_prt_entry+0x8e/0x2e0
>> +	acpi_pci_irq_lookup+0x3a/0x1e0
>> +	acpi_pci_irq_enable+0x77/0x240
>> +	pcibios_enable_device+0x39/0x40
>> +	do_pci_enable_device.part.0+0x5d/0xe0
>> +	pci_enable_device_flags+0xfc/0x120
>> +	pci_enable_device+0x13/0x20
>> +	virtio_pci_probe+0x9e/0x170
>> +	local_pci_probe+0x48/0x80
>> +	pci_device_probe+0x105/0x1c0
>> +
> 
> Everything else looks nice!
> 
>>  Christoph Lameter, May 30, 2007
>>  Sergey Senozhatsky, October 23, 2015
>> -- 
>> 2.35.1
>> 
>> 
>