From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793979549; b=n9HWBJGWpsaqB15007Gw/mzZVR+9G0ilbjHeE7XHgs6cjVtJ4tKlBu47/ZH+OMIjgIgw aX+8PXsuqDme1vIMZCvC2Lr4WSYfw4duPb28hXgB8w+e6brDOlxnrnky8IFoo7k9crwz1 19yY16sJzsmBLXXKsV5IjFbhTeJYfoWQJk2MioCy60TO1j/fFtFceBClCfGA55NrlOayR +8IZvfC7EYLlqDClXXvoNC4uHxpKtBVrlprqDS0dTjfX9Wf/LM3hnyefYPcrrWqq2whu+ iJB2arOIa2UHq+qSufuOtfpm2j9DENwBH8bOCh6qA30wGTt8d5upocfZxnjd/PFECew== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793979549; h=Received:DKIM-Signature:Date:From:To:Cc:Subject:Message-ID: References:MIME-Version:Content-Type:Content-Disposition:In-Reply-To; bh=343eayTvifkkMlrwy/xdVN6F48t4446WEUYad6Mst4M=; b=R2oxt4VDloB2NA5b3sjc+luv4YVyJ+ximeq3+G6AzftmoFKw+cDDdTfzr5TATC8Mg+jO K5bcfoyEdkIi30G6y0ROFLIU9RQ3HEuV+/57hZk4IMivSKxIQNFlGtGDMkQPlArekfeRN jeBS5acegi0cBCSswkVHHmhHoppztv9ImzCql0T6XjWQuzroUDyVXfXK/grjjD2+XbLD7 WTcJKFZBu4XHxPN+HDW3ftBdd3H2Nm0eUuW39OxnrDaJdYCI2IMwupZdy+TznJ/NSgfLV kfWi68HOMQNO+sguLadGhTy9XWS4bygIkzXKDR9wUQngZcjewG+q0Ov1qOlRgwEBlDA== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=RhNUG8v3; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id DE969606A329 for ; Fri, 6 Nov 2026 15:39:08 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=RhNUG8v3 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6A10E1F008A5; Fri, 6 Nov 2026 15:39:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793979548; bh=343eayTvifkkMlrwy/xdVN6F48t4446WEUYad6Mst4M=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=RhNUG8v3jLxEuKfkjzjh2O3hzkT/GXHoHwjFXad+jlqrHMmoroWCSEGOJ0whXTbw2 xSujptrJUOFXO6gYslpAoW99b5HL12/fyBJuJOuhsieu3ICH/oxVleEoAIkh7RcG6U SIwdwG+dP6IuacVdo+ipywY41G0+dKBsxkxgOtACZ7bUEtTcEo4MTSd+nd3GadPzyU bAeFw2ZOH3g8/lu99jX2UxMQ9VF/cEMNnaOpzFIGQfKnsGek3+2LNwGgETaY9PfRiY DwG+UpF2w89nS7IKxvChsEE0SCP5nqnZ8YrJWZEf4Pf3/iugNsee2VmRdh2bUVHvqT DatnXZ2gE5+og== Date: Fri, 6 Nov 2026 15:39:02 +0000 From: "Lorenzo Stoakes (ARM)" To: Vernon Yang Cc: David Hildenbrand , Zi Yan , Baolin Wang , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 3/3] mm/khugepaged: release contended mmap read lock after scanning enough VMAs Message-ID: References: <20261105-khugepaged-latency-fix-v2-0-4384bbfe4606@kernel.org> <20261105-khugepaged-latency-fix-v2-3-4384bbfe4606@kernel.org> <798cd192-9ec2-4f37-9cba-44db275a269c@gmail.com> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Fri, Nov 06, 2026 at 03:24:20PM +0000, Lorenzo Stoakes (ARM) wrote: > On Fri, Nov 06, 2026 at 09:26:01PM +0800, Vernon Yang wrote: > > On Fri, Nov 06, 2026 at 12:07:53PM +0000, Lorenzo Stoakes (ARM) wrote: > > > On reflection, I think this would be better implemented as something like: > > > > > > static void collapse_scan_mm_slot(unsigned int progress_max, ...) > > > { > > > unsigned int long lock_start = jiffies; > > > ... > > > for_each_vma(vmi, vma) { > > > ... > > > if (time_after(jiffies, lock_start) && mmap_lock_is_contended(mm)) { > > > mmap_read_unlock(mm); > > > cond_resched(); > > > mmap_read_lock(mm); > > > lock_start = jiffies; > > > vma_iter_set(&vmi, mm_slot->address); > > > continue; > > > } > > > } > > > } > > > > > > Which avoids magic numbers, and means if we hold the lock for 1-10ms (depending > > > on HZ), which is a long time in the kernel to hold a lock, AND the lock is > > > contended, we drop/reacquire. > > > > Looks good to me. Is the test result of this solution the same as the > > current one? > > Thanks! I will update the commit to reflect the changes. Hmm, actually the poor granularity of jiffies makes things measurably worse. I think I have to just use a fixed constant still, and explain how it was derived. -- Cheers, Lorenzo