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

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Date:   Tue, 25 Oct 2022 11:20:40 -0700
From:   Zach O'Keefe <zokeefe@...gle.com>
To:     Gautam Menghani <gautammenghani201@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, rostedt@...dmis.org,
        mhiramat@...nel.org, 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 Oct 25 10:35, Gautam Menghani wrote:
> 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

Thanks Guatam,

The ambiguous codes in particular are:

SCAN_PTE_MAPPED_HUGEPAGE
SCAN_PAGE_COMPOUND
SCAN_PAGE_COUNT

; properties of a page that are checked before/after the page is
locked/isolated.  This personally hasn't been an issue for me, as someone who's
spent considerable time in these codepaths over the last little while.  There
are certainly other codes (like SCAN_FAIL) which are ambiguous within functions
(and likewise haven't been an issue) so this reason alone isn't particularly
motivating.

However, some of the extra information (is_shmem, index), at times, would have
been useful if was already available -- but it was never enough of a pain to
force my hand to put this tracepoint in.

If the tracepoint is staying, then 2 nits:

CHECK: Alignment should match open parenthesis
#74: FILE: mm/khugepaged.c:2064:
+       trace_mm_khugepaged_collapse_file(mm, hpage, index, is_shmem,
+               addr, file, nr, result);

and, apply Steven's advice to trace_mm_khugepaged_scan_file() for consistency.

Thanks,
Zach