From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pl1-f202.google.com (mail-pl1-f202.google.com [209.85.214.202]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 96AA82F1FDE for ; Tue, 14 Jul 2026 17:15:13 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.214.202 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784049315; cv=none; b=eh32FU8IjA5z58biNNiWwAV2nswdKV/36lpsMMeSAfSeK0jvCjpdNc7EIGGn37kEWegZoJlCTTxsKMnOceqOTKskpzmY/mIl5ta7X2a40lh5My5+J5RUH/UUuIuayalwP02i2XIhFj9+poQ74kfPPIXQVXveC9OqQMthd5oG4ms= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784049315; c=relaxed/simple; bh=0O+fCYtAeEL6m2xQKGY4DDGMngZVKDto9UMiyhbPAOQ=; h=Date:Mime-Version:Message-ID:Subject:From:To:Cc:Content-Type; b=ZFsdTykdvIkOdfqiW+CJGHQOzfvDqFNNKvvxjMgnI3WeVK+GDxM7tKyHYlEb/biwpClDEm9DG3UPmlRbiBLL5FdiPp/VUYct6zKb6BPs10/ZNuIrS5pFobzAOdPjzuhoCW6j6lIzvvoezJU1RVMM2dhrKpjp+J6OtCYWKK2dnRc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--pratmal.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=vnJMFVzt; arc=none smtp.client-ip=209.85.214.202 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--pratmal.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="vnJMFVzt" Received: by mail-pl1-f202.google.com with SMTP id d9443c01a7336-2ca5d2474c7so30211715ad.2 for ; Tue, 14 Jul 2026 10:15:13 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20251104; t=1784049313; x=1784654113; darn=vger.kernel.org; h=content-type:cc:to:from:subject:message-id:mime-version:date:from :to:cc:subject:date:message-id:reply-to:content-type; bh=HQl/YhNIycgwR9ahJpRcbgcx4L53MFsyko1Sj3ozvDU=; b=vnJMFVztkcbrg5sZe5sweWpoOidwxrUE3yrZhtSXTLYFbyFj5WOpt6uxZa5L6M6hZu de6KetEiAehlp0FC3ODxVXhLXDYwEIM6I1LbEvX7nltBmdllSq9hhn4tsLq47NloG3TJ diohmLW+SS5fxla7omKb+mHpZtua2XKvciEAJ3OcPB7qWobt4YYVs2ik9MyWSstNhlbx 3ptWPoD8C3tLANAbWCBPWBidkPJf7Eu7IgLrSV0gwJORqU8WUfmD9h9hYFOT710icsED kngzBWSnETX4oqGut35ZjNENfJY4wAas8qh//DYlrgoNHl5gyMNBRRP4edf4mcuaeYwK VilA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1784049313; x=1784654113; h=content-type:cc:to:from:subject:message-id:mime-version:date :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to :content-type; bh=HQl/YhNIycgwR9ahJpRcbgcx4L53MFsyko1Sj3ozvDU=; b=TVDhf9I7RtySq+EnutfM4D9ittiZB0zxnqZ+Juen0Zb2zVSMdb9OIcX8tkPY4d9z2T x7Ve1Ek9zCm2UqtuQ+WDDeBq/whns714d7MmKCnqg0+aFP1CXzKX2k1LtLXsxwc/vbCR 2Cno7iPWKoW3xl/zjVFukNYHaoz5j709cOYPC+SOSU2/s7uE802RvDNsULYeKGtHW9ME kUwcEiR8YtjPjWin2H75lUo2P8mpCPHYK2dOIejyqUF8XFYPkFFHPH6tKzsJjFoVsEtX s/koAd6j1pb80omA7k5xvfOTFhH8uZUAcqZt9PAc/GMvJDvTwY4G+3YDbCjhy546MXTN w9Xg== X-Forwarded-Encrypted: i=1; AHgh+Rpr6WlXYvLWRh2wHUpmdaRUwogvkxet5nMExlItoA8IlpLv6lfBLh6di6RqZTAY6GpIr6lhabiAA2jrocE=@vger.kernel.org X-Gm-Message-State: AOJu0Yx/6Y/hPBTpFjn2aELosYMjZ7EeCKYoNvAWIophSJy3WIti1W/1 IuTo1sfDdbzDy+QFCLREvr5tfF48JXkCyBAxWNIeLUDWfc8CMc6ynwtSORD8ZmsBHZ1Lpuln4kG CK4bKKTQfIg== X-Received: from dled1-n2.prod.google.com ([2002:a05:701b:42c1:20b0:13b:9e9c:c86a]) (user=pratmal job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6a21:3394:b0:3a8:21e0:1ffa with SMTP id adf61e73a8af0-3c110160cabmr18302925637.27.1784049311355; Tue, 14 Jul 2026 10:15:11 -0700 (PDT) Date: Tue, 14 Jul 2026 17:14:55 +0000 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 X-Mailer: git-send-email 2.55.0.141.g00534a21ce-goog Message-ID: <20260714171456.2350037-1-pratmal@google.com> Subject: [RFC PATCH] mm/page_reporting: Add page_reporting_delay sysctl From: pratmal@google.com To: Andrew Morton , Vlastimil Babka Cc: David Hildenbrand , Greg Thelen , Pratyush Mallick , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan , linux-mm@kvack.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" 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 The data shows (sysctl.vm.page_reporting_delay=0) that within ~2 seconds, the guest reports ~10 GiB and the reporting is initiated as soon as 100ms. With the default 2s delay, it takes around ~7 sec to report the same and initiation delay is around 2 sec. mm/page_reporting.c | 47 ++++++++++++++++++++++++++++++++++++++++----- 1 file changed, 42 insertions(+), 5 deletions(-) diff --git a/mm/page_reporting.c b/mm/page_reporting.c index f0042d5743af..6fde542cbe09 100644 --- a/mm/page_reporting.c +++ b/mm/page_reporting.c @@ -6,6 +6,7 @@ #include #include #include +#include #include #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; - enum { PAGE_REPORTING_IDLE = 0, PAGE_REPORTING_REQUESTED, PAGE_REPORTING_ACTIVE }; +static unsigned int page_reporting_delay = 2000; +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) +{ + int ret; + struct page_reporting_dev_info *prdev; + + ret = proc_dointvec(table, write, buffer, lenp, ppos); + if (ret < 0 || !write) + return ret; + + 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(); + + return 0; +} + +static struct ctl_table page_reporting_sysctls[] = { + { + .procname = "page_reporting_delay", + .data = &page_reporting_delay, + .maxlen = sizeof(unsigned int), + .mode = 0644, + .proc_handler = page_reporting_delay_sysctl, + }, +}; + /* request page reporting */ static void __page_reporting_request(struct page_reporting_dev_info *prdev) @@ -80,7 +110,7 @@ __page_reporting_request(struct page_reporting_dev_info *prdev) * now we are limiting this to running no more than once every * couple of seconds. */ - schedule_delayed_work(&prdev->work, PAGE_REPORTING_DELAY); + schedule_delayed_work(&prdev->work, msecs_to_jiffies(page_reporting_delay)); } /* notify prdev of free page reporting request */ @@ -343,7 +373,7 @@ static void page_reporting_process(struct work_struct *work) */ state = atomic_cmpxchg(&prdev->state, state, PAGE_REPORTING_IDLE); if (state == PAGE_REPORTING_REQUESTED) - schedule_delayed_work(&prdev->work, PAGE_REPORTING_DELAY); + schedule_delayed_work(&prdev->work, msecs_to_jiffies(page_reporting_delay)); } static DEFINE_MUTEX(page_reporting_mutex); @@ -415,3 +445,10 @@ void page_reporting_unregister(struct page_reporting_dev_info *prdev) mutex_unlock(&page_reporting_mutex); } EXPORT_SYMBOL_GPL(page_reporting_unregister); + +static int __init page_reporting_sysctl_init(void) +{ + register_sysctl_init("vm", page_reporting_sysctls); + return 0; +} +late_initcall(page_reporting_sysctl_init); -- 2.55.0.141.g00534a21ce-goog