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 5B5E9519DE3; Fri, 16 Oct 2026 16:25:22 +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=1792167925; cv=none; b=Ko7dG0cWm+OeFUDc3Z3IEOmxhn/PZFNJrRb9PJH+hypXmJfpjN3KUnCD0R0FWvInABPuMoHOxF57Xe4UBo/uIj7dBKsTYKcqtQ1+KSFEgpTaZbPYgv5c/iJKH2q0qjCwTRmj2lAToV/L9lixMst8QP1v/nc2jK2Dn95a5LbTWiw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792167925; c=relaxed/simple; bh=kviyD256rRK49BqVnAS7bdkwL+XgsFYGmHgBq0Pbx8E=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=f6SGczvMcnufvVOEK6S4MyrpTE45uVAWvyAMYpMCYKrSrE3OMa+diyybRW6MVP7xvE0NTkp1bpBE/GzNC7vxxpn9PBZXVwQkEmy4SP8fScnkr0U71M77WEVqXGsHCxtqwDPEoMpT2sLYipON64hdD8rEKE6/BuK/9SCSmdL/PkQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=DDnQLX4u; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="DDnQLX4u" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 258D21F000C4; Fri, 16 Oct 2026 16:25:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1792167922; bh=TWT4roPjmISJxTKvGQzQWU1LR7xLXo2Q7pvoSudceyg=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=DDnQLX4uJmcDHyArI08r+DiFRSvDYIcfJTA5a0dqMtZcD+xdbLb/9FxW1MXvjWPp9 BF80p/3Y2fiKJlnsTIEMDNGn0BLGHrm9SaoNMZ9H2oVJPvq5OulDE0YynulNAxE57o qemrVo0M+buj3OKz389eclN6eGLG6kepRL1D14fM= Date: Fri, 16 Oct 2026 18:25:26 +0200 From: Greg Kroah-Hartman To: Peng Wang Cc: Anna-Maria Behnsen , Frederic Weisbecker , Ingo Molnar , Thomas Gleixner , "Rafael J . Wysocki" , Danilo Krummrich , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Peter Zijlstra , Waiman Long , Tejun Heo , He Rongguang , linux-kernel@vger.kernel.org, linux-api@vger.kernel.org, driver-core@lists.linux.dev, linux-trace-kernel@vger.kernel.org, cgroups@vger.kernel.org Subject: Re: [RFC PATCH 2/2] cpu: Add sysfs control of full dynticks policy Message-ID: <2026101646-speller-gutter-c184@gregkh> References: Precedence: bulk X-Mailing-List: driver-core@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Thu, Oct 15, 2026 at 08:43:09PM +0800, Peng Wang wrote: > Expose nohz_full_allowed under /sys/devices/system/cpu/ as the subset of > nohz_full CPUs permitted to suppress their non-idle scheduler tick. > > Writes replace the entire list; reads report policy, not current tick > state. Reject invalid input before updating policy. Document asynchronous > revocation, offline CPU configuration and persistence across hotplug. > The existing nohz_full attribute continues to report membership. > > Co-developed-by: He Rongguang > Signed-off-by: He Rongguang > Signed-off-by: Peng Wang > --- > .../ABI/testing/sysfs-devices-system-cpu | 40 ++++++++++++++++ > drivers/base/cpu.c | 46 +++++++++++++++++++ > 2 files changed, 86 insertions(+) > > diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu > index 82d10d556cc89..edf3408fd84c8 100644 > --- a/Documentation/ABI/testing/sysfs-devices-system-cpu > +++ b/Documentation/ABI/testing/sysfs-devices-system-cpu > @@ -767,6 +767,46 @@ Description: Preferred MTE tag checking mode > > See also: Documentation/arch/arm64/memory-tagging-extension.rst > > +What: /sys/devices/system/cpu/nohz_full_allowed > +Date: October 2026 > +Contact: Linux kernel mailing list > +Description: > + (RW) CPUs permitted to stop the scheduler tick while non-idle. > + Available with CONFIG_NO_HZ_FULL. The default is the complete > + nohz_full CPU list. Readback reports permission, not the current > + tick state: all the usual tick dependencies still apply. > + > + A write replaces the entire list, using CPU-list syntax (for > + example, "1-3,5"). An empty line revokes permission from all > + nohz_full CPUs. Invalid input is rejected without changing > + policy. A parsed list containing CPUs outside nohz_full fails > + with EINVAL. CPU numbers beyond the mask range and numeric > + overflows are rejected by the CPU-list parser with ERANGE and > + EOVERFLOW, respectively. Input must be shorter than PAGE_SIZE > + bytes, otherwise E2BIG is returned. Embedded NULs and newlines > + other than a trailing newline are rejected with EINVAL. Valid > + writes without an active nohz_full configuration fail with > + EOPNOTSUPP. > + > + Reads fail with EOVERFLOW if the formatted output reaches > + PAGE_SIZE - 1 bytes, rather than returning a potentially > + truncated CPU list. > + > + Revoking permission adds a per-CPU tick dependency. Online CPUs > + are notified through the existing asynchronous kick mechanism > + to restart a stopped non-idle tick. A successful write means > + the policy is updated and any needed notification is queued; > + it does not wait for tick restart. Changes to multiple CPUs > + are not simultaneous. Granting permission only removes this > + dependency and does not force the tick to stop. > + > + Offline CPUs may be configured without sending notifications. > + Policy persists across CPU offline/online. Idle tick suppression > + is unchanged; the policy applies when a CPU leaves idle. > + > + This interface does not change nohz_full membership, context > + tracking, RCU offloading, or housekeeping configuration. > + > What: /sys/devices/system/cpu/nohz_full > Date: Apr 2015 > Contact: Linux kernel mailing list > diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c > index 69e52fed42415..299ce61cef895 100644 > --- a/drivers/base/cpu.c > +++ b/drivers/base/cpu.c > @@ -324,6 +324,51 @@ static ssize_t nohz_full_show(struct device *dev, > return sysfs_emit(buf, "\n"); > } > static DEVICE_ATTR_RO(nohz_full); > + > +static ssize_t nohz_full_allowed_show(struct device *dev, > + struct device_attribute *attr, char *buf) > +{ > + cpumask_var_t mask; > + ssize_t ret; > + > + if (!alloc_cpumask_var(&mask, GFP_KERNEL)) > + return -ENOMEM; > + > + tick_nohz_full_get_allowed(mask); > + ret = sysfs_emit(buf, "%*pbl\n", cpumask_pr_args(mask)); > + > + free_cpumask_var(mask); > + > + /* Never present a truncated CPU list as the complete policy. */ > + return ret == PAGE_SIZE - 1 ? -EOVERFLOW : ret; Why not? What will userspace do then if it gets -EOVERFLOW as it can not get the full list? > +} > + > +static ssize_t nohz_full_allowed_store(struct device *dev, > + struct device_attribute *attr, > + const char *buf, size_t count) > +{ > + cpumask_var_t mask; > + int ret; > + > + /* kernfs may truncate an oversized write to PAGE_SIZE. */ > + if (count >= PAGE_SIZE) > + return -E2BIG; How are you getting a larger than PAGE_SIZE value here? > + if (memchr(buf, '\0', count) || > + (count > 1 && memchr(buf, '\n', count - 1))) > + return -EINVAL; What are you testing for here, sorry, I can't easily figure it out. thanks, greg k-h