On Thu, Oct 15, 2026 at 12:07:14PM -0400, Waiman Long wrote: > > On 10/15/26 8:43 AM, 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 <[email protected]> > > Signed-off-by: He Rongguang <[email protected]> > > Signed-off-by: Peng Wang <[email protected]> > > --- > > .../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 > > I do have a problem with the name of this sysfs file. nohz_full is more than > just the tick. The naming may introduce the wrong impression that all > nohz_full behavior can be controlled. May be a name like "nohz_tick_allowed" > which is much more specific will be better.
Hi Waiman, Agreed, nohz_tick_allowed describes the scope more accurately. If the control remains in sysfs, I'll use that name in the next version. Thanks, Peng > > Regards, > Longman > > > +Date: October 2026 > > +Contact: Linux kernel mailing list <[email protected]> > > +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 <[email protected]> > > 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; > > +} > > + > > +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; > > + if (memchr(buf, '\0', count) || > > + (count > 1 && memchr(buf, '\n', count - 1))) > > + return -EINVAL; > > + > > + if (!alloc_cpumask_var(&mask, GFP_KERNEL)) > > + return -ENOMEM; > > + > > + ret = cpulist_parse(buf, mask); > > + if (!ret) > > + ret = tick_nohz_full_set_allowed(mask); > > + > > + free_cpumask_var(mask); > > + > > + return ret ? ret : count; > > +} > > +static DEVICE_ATTR_RW(nohz_full_allowed); > > #endif > > #ifdef CONFIG_CRASH_HOTPLUG > > @@ -525,6 +570,7 @@ static struct attribute *cpu_root_attrs[] = { > > &dev_attr_housekeeping.attr, > > #ifdef CONFIG_NO_HZ_FULL > > &dev_attr_nohz_full.attr, > > + &dev_attr_nohz_full_allowed.attr, > > #endif > > #ifdef CONFIG_CRASH_HOTPLUG > > &dev_attr_crash_hotplug.attr, >
