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@xxxxxxxxx> 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@xxxxxxxxxxxxxx/ > > 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
