linux-kernel - Re: Profiling execution time for __alloc_pages

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Message-ID: <03693c3f-e95e-d0a7-d7de-f2cda92ba6ea@redhat.com>
Date:   Mon, 10 May 2021 17:01:55 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Shivank Garg <shivankgarg98@...il.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, gregkh@...uxfoundation.org,
        sergey.senozhatsky@...il.com, pmladek@...e.com
Subject: Re: Profiling execution time for __alloc_pages_nodemask

On 10.05.21 16:57, Shivank Garg wrote:
> Hi Everyone!
> 
> I want to profile the time taken to execute the __alloc_pages_nodemask
> for different linux configurations/parameters.
> To measure the execution time, I use the ktime_get() apis. I get the
> ktime_get() on the top, and I want to do ktime_sub(ktime_get(),ktime)
> and record it in a tracepoint.

Don't tracepoints already record a timestamp? It's bee a while since I 
worked with tracepoints ...

> However, the patch on implementation prevents the kernel from booting
> up. I debugged the bug to find out that the issue recurs on adding
> ktime_get() inside the __alloc_pages_nodemask path. So, that the
> kernel fails to boot up ( and show the blank screen without any logs )
> I'm using the Linux kernel 5.6.13 (5821a5593fa9f28eb6fcc95c35d00454d9bb8624)

Are you maybe calling ktime_get() too early during boot, where some 
subsystems are just about to be brought up?

> 
> Is it an expected behavior? or a BUG? Has anyone else faced the same issue?
> 
> Can you please suggest, what would be a good way to measure execution
> time for page allocation (if not ktime_get)

I could have sworn ordinary tracepoints would already give you a 
timestamp ...

-- 
Thanks,

David / dhildenb