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Shutemov" Subject: Re: [PATCH v2] mm/khugepaged: add tracepoint to collapse_file() Message-ID: References: <20221024173559.332324-1-gautammenghani201@gmail.com> <20221024131706.3d58bd92c332684386c7df13@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: ARC-Authentication-Results: i=1; imf11.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b="b5y/M9Jw"; spf=pass (imf11.hostedemail.com: domain of gautammenghani201@gmail.com designates 209.85.215.180 as permitted sender) smtp.mailfrom=gautammenghani201@gmail.com; dmarc=pass (policy=none) header.from=gmail.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1666759989; a=rsa-sha256; cv=none; b=t1Scddn+Q8hqZISftKwYbNzF8nzPCF1vEOZEohmjVnAJHHHpkZNpeRf2/qlT9ANlZLO6Tg SfoHwrDe2HaL6NxEBr8gFVMD92GWTXTSMo/mjIHXkBWaikHNUnkJqpvcs6F3DZT2MkJ78y DzHS1dJsl7+R8EAIj9JibdPKCeKAQnQ= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1666759989; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=WUatcekvUHl52e6kKZ24DbmCCp6yIPudmgeORti4bbc=; b=JyG7ssVO6btu0YkuF0t2JQIYz8vkCeJ6DzeRxK4e+Ev9Ylr5TlBMnQsuN5mJnpHUn6Hyn/ LThylxkiEno34OWC0USh2Ja7gf7VllXsbU8EXZeuTZzPmjzSHZdn3F6elsXDqZt8JqSyAL SJfhPb8nPX/9qmaTmkAI1ZoIvm/8Yek= X-Rspamd-Queue-Id: 418AC40006 Authentication-Results: imf11.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b="b5y/M9Jw"; spf=pass (imf11.hostedemail.com: domain of gautammenghani201@gmail.com designates 209.85.215.180 as permitted sender) smtp.mailfrom=gautammenghani201@gmail.com; dmarc=pass (policy=none) header.from=gmail.com X-Rspamd-Server: rspam02 X-Rspam-User: X-Stat-Signature: 55xk96m6qxcd8br7cj8rau9r7o5s5ohs X-HE-Tag: 1666759989-53678 X-Bogosity: Ham, tests=bogofilter, spamicity=0.008698, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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 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