From: Zach O'Keefe <zokeefe@google.com>
To: Gautam Menghani <gautammenghani201@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
rostedt@goodmis.org, mhiramat@kernel.org, shy828301@gmail.com,
vbabka@suse.cz, david@redhat.com, linux-mm@kvack.org,
linux-kernel@vger.kernel.org,
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH v2] mm/khugepaged: add tracepoint to collapse_file()
Date: Tue, 25 Oct 2022 11:20:40 -0700 [thread overview]
Message-ID: <Y1go+BSdpGELmIZS@google.com> (raw)
In-Reply-To: <Y1duhy5kbhLoMqWO@biggie>
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@gmail.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
next prev parent reply other threads:[~2022-10-25 18:20 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-24 17:35 [PATCH v2] mm/khugepaged: add tracepoint to collapse_file() Gautam Menghani
2022-10-24 20:17 ` Andrew Morton
2022-10-25 5:05 ` Gautam Menghani
2022-10-25 18:20 ` Zach O'Keefe [this message]
2022-10-26 4:53 ` Gautam Menghani
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