Re: [PATCH v2] mm/khugepaged: add tracepoint to collapse_file()

[Gautam Menghani <gautammenghani201@gmail.com>]

On Tue, Oct 25, 2022 at 11:20:40AM -0700, Zach O'Keefe wrote:
> 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.
Yes I've sent a patch to refactor the mm_khugepaged_scan_file() tracepoint - 
lore.kernel.org/linux-mm/20221026044524.54793-1-gautammenghani201@gmail.com/T/#u