From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 852B840EBA5 for ; Wed, 15 Jul 2026 12:59:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784120397; cv=none; b=VzPnN2YnzsMahfUArSL+3Jk/4hX3x+XzG7eoulk7jKyKO9K4Lxmjn/0XdnSxrieRj3eQaMgaedvHXvAg41K+ZY/bCGn51l1+f1CaK4covdhxGJqhvlu6jUlbJnowpLncFCdC8X6KeU6IFXS4XjxagWKL2cbnQh4mpbTbbNvdNSw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784120397; c=relaxed/simple; bh=yiqRRnYui+nz9EzUT2sEFHq95yw9t5Bmf3PNN3mv5Ww=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=qi9ydC5vt0F+vRtIRzxGMHu3ii3d9ZSaUXmHX8jdCDCvnJhVjO6nw/WLRMgdf1eP/BCMbDXxq0QMBVYKhdqUTxUUDEeg757Z4VXblzeLDY44xM8bu+P2+47xUUoYtK8r2XVMPWHX45PvUcjoqNdM3ZT28v3Lx1Ph541GtBuBtEI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=TDojiWSB; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="TDojiWSB" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0A3AB1F000E9; Wed, 15 Jul 2026 12:59:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784120393; bh=U+/oYZie1wlRVt0EOqLK1nvaTbYvCLc0MdBf4vus2Mg=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=TDojiWSBNJJ6rI5kdkcQGawZH/Lu2ljtrTXZLi+k0l5LTVuEJ/9zJSYOEVSvjCdOm ED70I7IMb3mjoVFolWFueD2Snl+sDaZ+q4RCMAathPncNGWzmoU/GJYhhlU42jsjuU hMNwl9oImdcKVknG51AzxlN2yjIdwULEA8zLalknDJYo0XCCMSxkMYnIp26+JUFgff DsG+2pnLh+khDtBoIB2tD69ZHPyIpIAN6FSycNx2yFm4ZnuvwV6V4yqY2w9X0BZ6tV zvODD0cGQiAdt2/4KM11WWt6nGESNWdqddq2udmTrtZOxYqxmWLbk9xdQJxWLPfh5p tsdT20QzPNDLw== Message-ID: <70392f94-137a-4220-8ff5-64dd874cf5d9@kernel.org> Date: Wed, 15 Jul 2026 14:59:49 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH] mm/page_reporting: Add page_reporting_delay sysctl To: pratmal@google.com, Andrew Morton , Vlastimil Babka Cc: Greg Thelen , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260714171456.2350037-1-pratmal@google.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/14/26 19:14, pratmal@google.com wrote: > From: Pratyush Mallick > > Currently, the free page reporting daemon uses a hardcoded delay of > (2 HZ) between reporting intervals. While this is a reasonable > default, it lacks the flexibility to adapt to varying guest workloads. > > A low delay allows aggressive memory reclamation, returning unused > pages to the hypervisor as quickly as possible. However, during spiky > allocation/free churn, this immediate reporting can lead to a severe > performance penalty (nested page faults) as the guest re-allocates memory > that the hypervisor has just unmapped. In these scenarios, there is benefit > from increasing the delay to batch free pages over a longer window, > absorbing the churn without hypercall and re-fault overhead. > > This patch refactors the delay into a dynamically tunable sysctl, > /proc/sys/vm/page_reporting_delay, measured in milliseconds. The value > defaults to 2000ms to precisely match the original (2 HZ) behavior. > If the sysctl is modified across reporting windows, the sysctl handler > immediately issues a mod_delayed_work() to honor the new configuration > without waiting for the prior timeout to lapse. > > Signed-off-by: Pratyush Mallick > --- > > Sending this as an RFC to get thoughts on exposing this delay as a sysctl. > > Benchmark Results: > We kill a process allocated with 10GB memory within the VM and mesure the > time it takes to report all the memory to host as well the delay in > initiating the reporting. > > default(2s delay): https://drive.google.com/file/d/1Ouxm_raj4xPNthc_ryk0Mdbo-c_zXKiP/view?usp=sharing > Tuned to 0s delay: https://drive.google.com/file/d/1mk58LPiYgIF4Yk6kmX5JHplhrwIw0NbC/view?usp=sharing [...] Hi! > > #include "page_reporting.h" > @@ -47,15 +48,44 @@ MODULE_PARM_DESC(page_reporting_order, "Set page reporting order"); > */ > EXPORT_SYMBOL_GPL(page_reporting_order); > > -#define PAGE_REPORTING_DELAY (2 * HZ) > -static struct page_reporting_dev_info __rcu *pr_dev_info __read_mostly; > - Why are you moving that? > enum { > PAGE_REPORTING_IDLE = 0, > PAGE_REPORTING_REQUESTED, > PAGE_REPORTING_ACTIVE > }; > > +static unsigned int page_reporting_delay = 2000; Maybe "2 * MSEC_PER_SEC;" Would we want to call that page_reporting_delay_ms to make it clearer what we are dealing with? > +static struct page_reporting_dev_info __rcu *pr_dev_info __read_mostly; > + > +static int page_reporting_delay_sysctl(const struct ctl_table *table, int write, > + void *buffer, size_t *lenp, loff_t *ppos) We prefer two tabs here in MM land. > +{ > + int ret; > + struct page_reporting_dev_info *prdev; > + > + ret = proc_dointvec(table, write, buffer, lenp, ppos); > + if (ret < 0 || !write) > + return ret; Would we want to cap it at reasonable values? > + > + rcu_read_lock(); > + prdev = rcu_dereference(pr_dev_info); > + if (prdev && atomic_read(&prdev->state) == PAGE_REPORTING_REQUESTED) > + mod_delayed_work(system_wq, &prdev->work, msecs_to_jiffies(page_reporting_delay)); > + rcu_read_unlock(); Is that really required? Seems unnecessary given that we expect something in the range of a couple of seconds max. -- Cheers, David