From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-124.freemail.mail.aliyun.com (out30-124.freemail.mail.aliyun.com [115.124.30.124]) (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 288D457267E; Fri, 16 Oct 2026 17:27:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792171637; cv=none; b=ugpXa6xmc5E7Nk8CCAuVVbuTVZ+c3BV3+E47hc0hv+xt+XTWAA/kBtLycLYCzzxIWtUpRAtpeFnkLl0eIi5L0hB0lkyJggwUAo0lEDfPu51NdO2GjXajZQVebs/h4wKu0xFK5kC3jRDN1plDBaDQvq9v33lGo7hWtMak4mYHmEc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792171637; c=relaxed/simple; bh=STyj/lYVqM8gAurdqYy6Pjs7GEOpRb+Xus0p1OR+ffU=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=XRaI6nrWV28srHevAL3C0cBozdRIkZ3I/nAvWHBFg3zaD03rK8NGgRfEkmDnA8sEjc/ohpaJOzUoHNEfO7/Yf6IGLCY7xIFvdhnlo1Mf080P3KOcJmReTtTZ9MkjWcoRbPYUIsXaT0TD+2LtKvGwK3uyoJcVmo3qaFFaAOYUZ0k= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com; spf=pass smtp.mailfrom=linux.alibaba.com; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b=vTk+hQhE; arc=none smtp.client-ip=115.124.30.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b="vTk+hQhE" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1792171630; h=Date:From:To:Subject:Message-ID:MIME-Version:Content-Type; bh=1juLGbyehF1Q7piOkFmo3ZV7cET5DQh/zeO/hMNZHdU=; b=vTk+hQhEe3thvP15S5N+dOnPLikFnr1gqSW5J4K6tFO9hekQ4dm9bQOHBtrhoWP7NEbXZFDXvI+l045KmwFSy2OBMAqHxn8xvYu5CyM+VONJAuWlayqYorBj4xvjMtmJqeiylVnFgSxDiO6dc1ixoTg1ALAAt6u4UHq1Ozt3Sbw= X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R681e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=maildocker-contentspam033045098064;MF=peng_wang@linux.alibaba.com;NM=1;PH=DS;RN=19;SR=0;TI=SMTPD_---0XCvPRM5_1792171628; Received: from localhost(mailfrom:peng_wang@linux.alibaba.com fp:SMTPD_---0XCvPRM5_1792171628 cluster:ay36) by smtp.aliyun-inc.com; Sat, 17 Oct 2026 01:27:09 +0800 Date: Sat, 17 Oct 2026 01:27:07 +0800 From: Peng Wang To: Greg Kroah-Hartman 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: Reply-To: Peng Wang References: <2026101646-speller-gutter-c184@gregkh> Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <2026101646-speller-gutter-c184@gregkh> On Fri, Oct 16, 2026 at 06:25:26PM +0200, Greg Kroah-Hartman wrote: > 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? Hi Greg, There is no way to retrieve the full list through this attribute. The intent was to avoid returning a truncated list that userspace could write back, unintentionally revoking permission for the omitted CPUs. Returning an error does not solve that interface limitation. > > > +} > > + > > +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? I cannot; kernfs caps count at PAGE_SIZE. The check is intended for count == PAGE_SIZE, to avoid applying a potentially truncated prefix as a complete list. It also rejects an exactly page-sized input. > > > + 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. The checks reject user-supplied NULs and non-final newlines. For example, cpulist_parse() accepts "1\n2\n" as CPU 1 only, while store() would report the full input as consumed. The NUL appended by kernfs is at buf[count], outside the checked range. I propose replacing the global attribute with /sys/devices/system/cpu/cpuN/nohz_tick_allowed, available only on nohz_full CPUs. Writing 0 would prevent non-idle tick suppression, using the existing asynchronous kick when needed. Writing 1 would allow suppression, subject to the existing tick dependencies. Readback would report policy, not the current tick state. Each write would affect only that CPU, without changing nohz_full membership or other isolation settings. This would give up the single-write whole-list replacement. Would that be a better fit? Thanks, Peng > > thanks, > > greg k-h