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 8C30F37F0F1 for ; Mon, 12 Jan 2026 17:47:09 +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=1768240029; cv=none; b=fkgplpfyKJ1gr56MBe7trCLeqAufxXW0Ud3W4RfCJPDL3gHmmGyiMF1ieDHNdC95YTRAS5K5arJ/0l2ws6u3xCsfdtZ+Gs+ZyL79yl+T1i2URM8MVtAXQCP4JF31W8vIlSlcY3HjFSyJHso27Skd77vfeXjafdsjns5gE62/pzA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768240029; c=relaxed/simple; bh=2dTDkiVRgcHJftm9NWhRy0KEikPB7jlsnjfXZvmzkhc=; h=Date:From:To:Cc:Subject:Message-Id:In-Reply-To:References: Mime-Version:Content-Type; b=BpcY3gtdHQNTC8JxDZnKnSvMN4Oa2WQi1QPcGrwQZkXJjGM6HVgnK/kK8ygsZFm6xJTdfteyc/J4qjjm0cUN9sM/rS3GLhKNm8JbdgqtjLXGd5/pWl7LVFUfQ3910HUUa1hBHtfa+okvXH4+VxAyQVvRBDPRMhoJm7RVF/gr9Yg= 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=LpZXpoyc; 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="LpZXpoyc" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 097BAC16AAE; Mon, 12 Jan 2026 17:47:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1768240029; bh=2dTDkiVRgcHJftm9NWhRy0KEikPB7jlsnjfXZvmzkhc=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=LpZXpoycG9xeQz4UqKy9nE4QofwTeJAeRpTkkEfcOPwezXFimm7Axu0rIjHBOi0uY qb+P6PBEkSCiJHYvlovFkaXAVENCNIZh6zyuRe5RgLZSWNQ/Kj6FRVAN2oB6i6YQ7D Dc25g6UF8eE6GxZ3rjIVBmOgsL4bgOEONPpvcIK4= Date: Mon, 12 Jan 2026 09:47:08 -0800 From: Andrew Morton To: Cc: , , , , , , Subject: Re: [PATCH 2/2] ksm: Optimize rmap_walk_ksm by passing a suitable =?ISO-8859-1?Q?address=A0=A0range?= Message-Id: <20260112094708.e965f00cb36678f41b840cc2@linux-foundation.org> In-Reply-To: <20260112220143497dgs9w3S7sfdTUNRbflDtb@zte.com.cn> References: <20260112215315996jocrkFSqeYfhABkZxqs4T@zte.com.cn> <20260112220143497dgs9w3S7sfdTUNRbflDtb@zte.com.cn> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Mon, 12 Jan 2026 22:01:43 +0800 (CST) wrote: > From: xu xin > > Problem > ======= > When available memory is extremely tight, causing KSM pages to be swapped > out, or when there is significant memory fragmentation and THP triggers > memory compaction, the system will invoke the rmap_walk_ksm function to > perform reverse mapping. However, we observed that this function becomes > particularly time-consuming when a large number of VMAs (e.g., 20,000) > share the same anon_vma. Through debug trace analysis, we found that most > of the latency occurs within anon_vma_interval_tree_foreach, leading to an > excessively long hold time on the anon_vma lock (even reaching 500ms or > more), which in turn causes upper-layer applications (waiting for the > anon_vma lock) to be blocked for extended periods. > > Root Reaon > ========== > Further investigation revealed that 99.9% of iterations inside the > anon_vma_interval_tree_foreach loop are skipped due to the first check > "if (addr < vma->vm_start || addr >= vma->vm_end)), indicating that a large > number of loop iterations are ineffective. This inefficiency arises because > the pgoff_start and pgoff_end parameters passed to > anon_vma_interval_tree_foreach span the entire address space from 0 to > ULONG_MAX, resulting in very poor loop efficiency. > > Solution > ======== > In fact, we can significantly improve performance by passing a more precise > range based on the given addr. Since the original pages merged by KSM > correspond to anonymous VMAs, the page offset can be calculated as > pgoff = address >> PAGE_SHIFT. Therefore, we can optimize the call by > defining: > > pgoff_start = rmap_item->address >> PAGE_SHIFT; > pgoff_end = pgoff_start + folio_nr_pages(folio) - 1; > > Performance > =========== > In our real embedded Linux environment, the measured metrcis were as follows: > > 1) Time_ms: Max time for holding anon_vma lock in a single rmap_walk_ksm. > 2) Nr_iteration_total: The max times of iterations in a loop of anon_vma_interval_tree_foreach > 3) Skip_addr_out_of_range: The max times of skipping due to the first check (vma->vm_start > and vma->vm_end) in a loop of anon_vma_interval_tree_foreach. > 4) Skip_mm_mismatch: The max times of skipping due to the second check (rmap_item->mm == vma->vm_mm) > in a loop of anon_vma_interval_tree_foreach. > > The result is as follows: > > Time_ms Nr_iteration_total Skip_addr_out_of_range Skip_mm_mismatch > Before patched: 228.65 22169 22168 0 > After pacthed: 0.396 3 0 2 Wow. This was not the best code we've ever delivered. It's really old code - over a decade? Your workload seems a reasonable one and I wonder why it took so long to find this. > --- a/mm/ksm.c > +++ b/mm/ksm.c > @@ -3172,6 +3172,7 @@ void rmap_walk_ksm(struct folio *folio, struct rmap_walk_control *rwc) > struct anon_vma_chain *vmac; > struct vm_area_struct *vma; > unsigned long addr; > + pgoff_t pgoff_start, pgoff_end; > > cond_resched(); > if (!anon_vma_trylock_read(anon_vma)) { > @@ -3185,8 +3186,11 @@ void rmap_walk_ksm(struct folio *folio, struct rmap_walk_control *rwc) > /* Ignore the stable/unstable/sqnr flags */ > addr = rmap_item->address & PAGE_MASK; > > + pgoff_start = rmap_item->address >> PAGE_SHIFT; > + pgoff_end = pgoff_start + folio_nr_pages(folio) - 1; > + > anon_vma_interval_tree_foreach(vmac, &anon_vma->rb_root, > - 0, ULONG_MAX) { > + pgoff_start, pgoff_end) { > > cond_resched(); > vma = vmac->vma; Thanks, I'll queue this for testing - hopefully somehugh will find time to check the change.