stap shows kfree() is 5 times more than kallocs, how could be?

stap shows kfree() is 5 times more than kallocs, how could be?

[Lev Olshvang]

Hello all,

I am debugging kernel module and use  SystemTap to monitor requested and freed memory.

I see that SystemTap statistics shows that kfree() is called 5 times more than kalloc. 
It happens not only on my module, it happens on VirtualBox vboxsf driver as well.

See hits count below  kmalloc hits: 21561
                                  kfree      hits: 102175


----- probe hit report:
kernel.function("__kmalloc at mm/slub.c:3706"), (./kmalloc_argus.stp:10:1), hits: 21561, cycles: 21228min/52226avg/334688max, from: kernel.function("__kmalloc at mm/slub.c:3706") from: kernel.function("__kmalloc"), index: 0
kernel.function("__kmalloc at mm/slub.c:3706").return, (./kmalloc_argus.stp:25:1), hits: 21561, cycles: 18843min/51228avg/406816max, from: kernel.function("__kmalloc at mm/slub.c:3706").return from: kernel.function("__kmalloc").return, index: 1
kernel.function("kfree at mm/slub.c:3843"), (./kmalloc_argus.stp:38:1), hits: 102175, cycles: 400min/1557avg/499754max, from: kernel.function("kfree at mm/slub.c:3843") from: kernel.function("kfree"), index: 2


Please comment.

Lev

stap shows kfree() is 5 times more than kallocs, how could be?

[Bernd Petrovitsch]

Hi all!

On 15/10/18 14:17, Lev Olshvang wrote:
[...]
> I am debugging kernel module and use  SystemTap to monitor requested and freed memory.
> 
> I see that SystemTap statistics shows that kfree() is called 5 times more than kalloc. 
> It happens not only on my module, it happens on VirtualBox vboxsf driver as well.
> 
> See hits count below  kmalloc hits: 21561
>                                   kfree      hits: 102175

"kfree(NULL)" is a legal NO-OP (and basically all other similar
functions are expected to gracefully accept a NULL pointer).

MfG,
	Bernd
-- 
"I dislike type abstraction if it has no real reason. And saving
on typing is not a good reason - if your typing speed is the main
issue when you're coding, you're doing something seriously wrong."
    - Linus Torvalds
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