linux-kernel - Re: [PATCH v2] mm/khugepaged: add tracepoint to collapse

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Message-ID: <Y1duhy5kbhLoMqWO@biggie>
Date:   Tue, 25 Oct 2022 10:35:11 +0530
From:   Gautam Menghani <gautammenghani201@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     rostedt@...dmis.org, mhiramat@...nel.org, zokeefe@...gle.com,
        shy828301@...il.com, vbabka@...e.cz, david@...hat.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v2] mm/khugepaged: add tracepoint to collapse_file()

On Mon, Oct 24, 2022 at 01:17:06PM -0700, Andrew Morton wrote:
> On Mon, 24 Oct 2022 23:05:58 +0530 Gautam Menghani <gautammenghani201@...il.com> wrote:
> 
> > In the file mm/khugepaged.c, a TODO in the function collapse_file() asks
> > to add tracepoints. Add the tracepoint named "mm_khugepaged_collapse_file".
> 
> This isn't a very satisfying explanation for changing the kernel.  Maybe
> the comment is stale are this tracepoint is unneeded.
> 
> Please explain afresh how this addition benefits kernel users?
> 
The function collapse_file() is called by the function hpage_collapse_scan_file(). 
Without a tracepoint in collapse_file(), we won't know if it was called or not and as a result,
we also won't know if it returned successfully or not. Also, as Zach mentioned earlier [1]:

there are a few scan result codes that overlap between hpage_collapse_scan_file() and those
possibly returned in collapse_file() such that, if we only have the one tracepoint in 
hpage_collapse_scan_file(), it could be ambiguous what callsite the error path stemmed from.

[1]:https://lore.kernel.org/lkml/CAAa6QmSKtj6T2dW1tkg5_HVj2+rXj5inOLdEzr0MkJzQxxcPXQ@mail.gmail.com/

Please do let me know if a v3 is needed.

Thanks,
Gautam