From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D8EB3225417 for ; Sat, 21 Mar 2026 01:21:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774056072; cv=none; b=GjoymksjoPvvIFW1gQTTRj7/IwKZDFiKasM00sRnI2oDEKoCA/fWqpkBP2nqMNha5fYB1bAN1uu1M+LlKMCCjmhkQ8nCzymGNCdujtaQh2AML6Gvk3nJP+rHIyoAImno5c5kh1tfV/surICrPNqALL9hWQ00nSpfuXncG1w1YJM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774056072; c=relaxed/simple; bh=WxqN67UcacsroUgxL2xNqkjk9VjkKorqv6BhRT1Upg8=; h=Date:To:From:Subject:Message-Id; b=RDNerQrtKdBclFtZ9NbuWGQ778xgxf3g59bFzhYz5CMDMI2G9Xb93KGzsVv6vMWMadVkhcPffTmvKklAphcpuEMVIjcIgne/+nkAJbHaa8iCq3+iqOfsqwZQ7K8c45nTULC2X+zRz8axQqD1nJb8sXJ4Aido04E8tNnoCfVfGjw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=LkKhwrs2; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="LkKhwrs2" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B02D3C4CEF7; Sat, 21 Mar 2026 01:21:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1774056072; bh=WxqN67UcacsroUgxL2xNqkjk9VjkKorqv6BhRT1Upg8=; h=Date:To:From:Subject:From; b=LkKhwrs216AyL4RmnAR+xYjXjcLXHVkbKAVRaoB4FgCG/lx7aMe1PU+WjtD3rF8Ju Hp/CM12Wy+RMTkfZamXmBVtJyqtEW8mThL39V+8JcbDySKOImzy3cn+VUVQWltzMIb pHE9wT9kkgseK6UKdFwKhT1NNGbIiKL3BK1Jtprc= Date: Fri, 20 Mar 2026 18:21:12 -0700 To: mm-commits@vger.kernel.org,richard@nod.at,marscheng@google.com,jserv@ccns.ncku.edu.tw,hch@infradead.org,eleanor15x@gmail.com,chengzhihao1@huawei.com,visitorckw@gmail.com,akpm@linux-foundation.org From: Andrew Morton Subject: + lib-list_sort-remove-dummy-cmp-calls-to-speed-up-merge_final.patch added to mm-nonmm-unstable branch Message-Id: <20260321012112.B02D3C4CEF7@smtp.kernel.org> Precedence: bulk X-Mailing-List: mm-commits@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: The patch titled Subject: lib/list_sort: Remove dummy cmp() calls to speed up merge_final() has been added to the -mm mm-nonmm-unstable branch. Its filename is lib-list_sort-remove-dummy-cmp-calls-to-speed-up-merge_final.patch This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/lib-list_sort-remove-dummy-cmp-calls-to-speed-up-merge_final.patch This patch will later appear in the mm-nonmm-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next via various branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there most days ------------------------------------------------------ From: Kuan-Wei Chiu Subject: lib/list_sort: Remove dummy cmp() calls to speed up merge_final() Date: Fri, 20 Mar 2026 18:09:38 +0000 Historically, list_sort() implemented a hack in merge_final(): if (unlikely(!++count)) cmp(priv, b, b); This was introduced 16 years ago in commit 835cc0c8477f ("lib: more scalable list_sort()") so that callers could periodically invoke cond_resched() within their comparison functions when merging highly unbalanced lists. An audit of the kernel tree reveals that fs/ubifs/ was the sole user of this mechanism. Recent discussions and inspections by Richard Weinberger confirm that UBIFS lists are strictly bounded in size (a few thousand elements at most), meaning it does not strictly rely on these dummy callbacks to prevent soft lockups. For the vast majority of list_sort() users (such as block layer IO schedulers and file systems), this hack results in completely wasted function calls. In the worst-case scenario (merging an already sorted list where 'a' is exhausted quickly), it results in approximately (N/2)/256 unnecessary cmp() invocations. Remove the dummy cmp(priv, b, b) fallback from merge_final(). This saves unnecessary function calls, avoids branching overhead in the tight loop, and slightly speeds up the final merge step for all generic list_sort() users. Link: https://lkml.kernel.org/r/20260320180938.1827148-3-visitorckw@gmail.com Signed-off-by: Kuan-Wei Chiu Cc: Ching-Chun (Jim) Huang Cc: Christoph Hellwig Cc: Mars Cheng Cc: Richard Weinberger Cc: Yu-Chun Lin Cc: Zhihao Cheng Signed-off-by: Andrew Morton --- lib/list_sort.c | 9 --------- 1 file changed, 9 deletions(-) --- a/lib/list_sort.c~lib-list_sort-remove-dummy-cmp-calls-to-speed-up-merge_final +++ a/lib/list_sort.c @@ -76,15 +76,6 @@ static void merge_final(void *priv, list /* Finish linking remainder of list b on to tail */ tail->next = b; do { - /* - * If the merge is highly unbalanced (e.g. the input is - * already sorted), this loop may run many iterations. - * Continue callbacks to the client even though no - * element comparison is needed, so the client's cmp() - * routine can invoke cond_resched() periodically. - */ - if (unlikely(!++count)) - cmp(priv, b, b); b->prev = tail; tail = b; b = b->next; _ Patches currently in -mm which might be from visitorckw@gmail.com are ubifs-remove-unnecessary-cond_resched-from-list_sort-compare.patch lib-list_sort-remove-dummy-cmp-calls-to-speed-up-merge_final.patch