Re: [PATCH v9 2/3] watchdog: add watchdog_cpumask sysctl to assist nohz

Don Zickus
2015-04-29T13:10:43Z

On Tue, Apr 28, 2015 at 02:07:59PM -0400, Chris Metcalf wrote:
> On 04/21/2015 08:32 AM, Ulrich Obergfell wrote:
> >Chris,
> >
> >in v9, smpboot_update_cpumask_percpu_thread() allocates 'tmp' mask dynamically.
> >This allocation can fail and thus the function can now return an error. However,
> >this error is being ignored by proc_watchdog_cpumask().
>
> Yes, I did that intentionally, because it seemed like a pretty
> extreme corner
> case (not enough memory to allocate one cpumask), and a relatively
> unproblematic outcome (we don't actually modify the running set of watchdog
> threads the way the /proc knob requested).
>
> The problem with your proposal (to save the old cpumask and put it back on
> failure) is that we will almost certainly not be able to do that
> either if we can't
> successfully run smpboot_update_cpumask_percpu_thread(), since that's
> exactly the allocation that we're presuming is going to fail internally.
>
> I went down this rathole and decided it wasn't worth worrying about.
> Let me know if you think we need to beat on it some more :-)

Hi Chris,

It seems we should at least output something to the user that their request
failed. I think it would be more frustrating to the user if they changed
the mask and the system did not respond the way they intended, not knowing
it did not successfully configure.

I understand and agree with the difficulty of handling this corner case, but
we can still trap and report it in hopes some day we get smarter. :-)

Cheers,
Don

>
> >
> >+int proc_watchdog_cpumask(struct ctl_table *table, int write,
> >+ void __user *buffer, size_t *lenp, loff_t *ppos)
> >+{
> >+ int err;
> >+
> >+ mutex_lock(&watchdog_proc_mutex);
> >+ err = proc_do_large_bitmap(table, write, buffer, lenp, ppos);
> >+ if (!err && write) {
> >+ /* Remove impossible cpus to keep sysctl output cleaner. */
> >+ cpumask_and(watchdog_cpumask, watchdog_cpumask,
> >+ cpu_possible_mask);
> >+
> >+ if (watchdog_enabled && watchdog_thresh)
> >+ smpboot_update_cpumask_percpu_thread(&watchdog_threads,
> >+ watchdog_cpumask);
> >+ }
> >+ mutex_unlock(&watchdog_proc_mutex);
> >+ return err;
> >+}
> >
> >You may want to consider handling the error, for example something like:
> >
> > save watchdog_cpumask because proc_do_large_bitmap() is going to change it
> > ...
> > err = smpboot_update_cpumask_percpu_thread()
> > if (err)
> > restore saved watchdog_cpumask
> > ...
> > return err so that the user becomes aware of the failure
> >
> >
> >Regards,
> >
> >Uli
> >
> >
> >----- Original Message -----
> >From: "Chris Metcalf" <cmetcalf@ezchip.com>
> >To: "Frederic Weisbecker" <fweisbec@gmail.com>, "Don Zickus" <dzickus@redhat.com>, "Ingo Molnar" <mingo@kernel.org>, "Andrew Morton" <akpm@linux-foundation.org>, "Andrew Jones" <drjones@redhat.com>, "chai wen" <chaiw.fnst@cn.fujitsu.com>, "Ulrich Obergfell" <uobergfe@redhat.com>, "Fabian Frederick" <fabf@skynet.be>, "Aaron Tomlin" <atomlin@redhat.com>, "Ben Zhang" <benzh@chromium.org>, "Christoph Lameter" <cl@linux.com>, "Gilad Ben-Yossef" <gilad@benyossef.com>, "Steven Rostedt" <rostedt@goodmis.org>, linux-kernel@vger.kernel.org, "Jonathan Corbet" <corbet@lwn.net>, linux-doc@vger.kernel.org, "Thomas Gleixner" <tglx@linutronix.de>, "Peter Zijlstra" <peterz@infradead.org>
> >Cc: "Chris Metcalf" <cmetcalf@ezchip.com>
> >Sent: Friday, April 17, 2015 8:37:17 PM
> >Subject: [PATCH v9 2/3] watchdog: add watchdog_cpumask sysctl to assist nohz
> >
> >Change the default behavior of watchdog so it only runs on the
> >housekeeping cores when nohz_full is enabled at build and boot time.
> >Allow modifying the set of cores the watchdog is currently running
> >on with a new kernel.watchdog_cpumask sysctl.
> >
> >If we allowed the watchdog to run on nohz_full cores, the timer
> >interrupts and scheduler work would prevent the desired tickless
> >operation on those cores. But if we disable the watchdog globally,
> >then the housekeeping cores can't benefit from the watchdog
> >functionality. So we allow disabling it only on some cores.
> >See Documentation/lockup-watchdogs.txt for more information.
> >
> >Acked-by: Don Zickus <dzickus@redhat.com>
> >Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>
> >---
> >v9: use new, new semantics of smpboot_update_cpumask_percpu_thread() [Frederic]
> > add and use for_each_watchdog_cpu() [Uli]
> > check alloc_cpumask_var for failure [Chai Wen]
> >
> >v8: use new semantics of smpboot_update_cpumask_percpu_thread() [Frederic]
> > improve documentation in "Documentation/" and in changelog [akpm]
> >
> >v7: use cpumask field instead of valid_cpu() callback
> >
> >v6: use alloc_cpumask_var() [Sasha Levin]
> > switch from watchdog_exclude to watchdog_cpumask [Frederic]
> > simplify the smp_hotplug_thread API to watchdog [Frederic]
> > add Don's Acked-by
> >
> > Documentation/lockup-watchdogs.txt | 18 +++++++++++
> > Documentation/sysctl/kernel.txt | 15 +++++++++
> > include/linux/nmi.h | 3 ++
> > kernel/sysctl.c | 7 +++++
> > kernel/watchdog.c | 63 +++++++++++++++++++++++++++++++++++---
> > 5 files changed, 101 insertions(+), 5 deletions(-)
> >
> >diff --git a/Documentation/lockup-watchdogs.txt b/Documentation/lockup-watchdogs.txt
> >index ab0baa692c13..22dd6af2e4bd 100644
> >--- a/Documentation/lockup-watchdogs.txt
> >+++ b/Documentation/lockup-watchdogs.txt
> >@@ -61,3 +61,21 @@ As explained above, a kernel knob is provided that allows
> > administrators to configure the period of the hrtimer and the perf
> > event. The right value for a particular environment is a trade-off
> > between fast response to lockups and detection overhead.
> >+
> >+By default, the watchdog runs on all online cores. However, on a
> >+kernel configured with NO_HZ_FULL, by default the watchdog runs only
> >+on the housekeeping cores, not the cores specified in the "nohz_full"
> >+boot argument. If we allowed the watchdog to run by default on
> >+the "nohz_full" cores, we would have to run timer ticks to activate
> >+the scheduler, which would prevent the "nohz_full" functionality
> >+from protecting the user code on those cores from the kernel.
> >+Of course, disabling it by default on the nohz_full cores means that
> >+when those cores do enter the kernel, by default we will not be
> >+able to detect if they lock up. However, allowing the watchdog
> >+to continue to run on the housekeeping (non-tickless) cores means
> >+that we will continue to detect lockups properly on those cores.
> >+
> >+In either case, the set of cores excluded from running the watchdog
> >+may be adjusted via the kernel.watchdog_cpumask sysctl. For
> >+nohz_full cores, this may be useful for debugging a case where the
> >+kernel seems to be hanging on the nohz_full cores.
> >diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
> >index c831001c45f1..f1697858d71c 100644
> >--- a/Documentation/sysctl/kernel.txt
> >+++ b/Documentation/sysctl/kernel.txt
> >@@ -923,6 +923,21 @@ and nmi_watchdog.
> > ==============================================================
> >+watchdog_cpumask:
> >+
> >+This value can be used to control on which cpus the watchdog may run.
> >+The default cpumask is all possible cores, but if NO_HZ_FULL is
> >+enabled in the kernel config, and cores are specified with the
> >+nohz_full= boot argument, those cores are excluded by default.
> >+Offline cores can be included in this mask, and if the core is later
> >+brought online, the watchdog will be started based on the mask value.
> >+
> >+Typically this value would only be touched in the nohz_full case
> >+to re-enable cores that by default were not running the watchdog,
> >+if a kernel lockup was suspected on those cores.
> >+
> >+==============================================================
> >+
> > watchdog_thresh:
> > This value can be used to control the frequency of hrtimer and NMI
> >diff --git a/include/linux/nmi.h b/include/linux/nmi.h
> >index 3d46fb4708e0..f94da0e65dea 100644
> >--- a/include/linux/nmi.h
> >+++ b/include/linux/nmi.h
> >@@ -67,6 +67,7 @@ extern int nmi_watchdog_enabled;
> > extern int soft_watchdog_enabled;
> > extern int watchdog_user_enabled;
> > extern int watchdog_thresh;
> >+extern unsigned long *watchdog_cpumask_bits;
> > extern int sysctl_softlockup_all_cpu_backtrace;
> > struct ctl_table;
> > extern int proc_watchdog(struct ctl_table *, int ,
> >@@ -77,6 +78,8 @@ extern int proc_soft_watchdog(struct ctl_table *, int ,
> > void __user *, size_t *, loff_t *);
> > extern int proc_watchdog_thresh(struct ctl_table *, int ,
> > void __user *, size_t *, loff_t *);
> >+extern int proc_watchdog_cpumask(struct ctl_table *, int,
> >+ void __user *, size_t *, loff_t *);
> > #endif
> > #ifdef CONFIG_HAVE_ACPI_APEI_NMI
> >diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> >index 2082b1a88fb9..699571a74e3b 100644
> >--- a/kernel/sysctl.c
> >+++ b/kernel/sysctl.c
> >@@ -881,6 +881,13 @@ static struct ctl_table kern_table[] = {
> > .extra2 = &one,
> > },
> > {
> >+ .procname = "watchdog_cpumask",
> >+ .data = &watchdog_cpumask_bits,
> >+ .maxlen = NR_CPUS,
> >+ .mode = 0644,
> >+ .proc_handler = proc_watchdog_cpumask,
> >+ },
> >+ {
> > .procname = "softlockup_panic",
> > .data = &softlockup_panic,
> > .maxlen = sizeof(int),
> >diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> >index 2316f50b07a4..8875717b6616 100644
> >--- a/kernel/watchdog.c
> >+++ b/kernel/watchdog.c
> >@@ -19,6 +19,7 @@
> > #include <linux/sysctl.h>
> > #include <linux/smpboot.h>
> > #include <linux/sched/rt.h>
> >+#include <linux/tick.h>
> > #include <asm/irq_regs.h>
> > #include <linux/kvm_para.h>
> >@@ -56,6 +57,12 @@ int __read_mostly sysctl_softlockup_all_cpu_backtrace;
> > #else
> > #define sysctl_softlockup_all_cpu_backtrace 0
> > #endif
> >+static cpumask_var_t watchdog_cpumask;
> >+unsigned long *watchdog_cpumask_bits;
> >+
> >+/* Helper for online, unparked cpus. */
> >+#define for_each_watchdog_cpu(cpu) \
> >+ for_each_cpu_and((cpu), cpu_online_mask, watchdog_cpumask)
> > static int __read_mostly watchdog_running;
> > static u64 __read_mostly sample_period;
> >@@ -205,7 +212,7 @@ void touch_all_softlockup_watchdogs(void)
> > * do we care if a 0 races with a timestamp?
> > * all it means is the softlock check starts one cycle later
> > */
> >- for_each_online_cpu(cpu)
> >+ for_each_watchdog_cpu(cpu)
> > per_cpu(watchdog_touch_ts, cpu) = 0;
> > }
> >@@ -612,7 +619,7 @@ void watchdog_nmi_enable_all(void)
> > return;
> > get_online_cpus();
> >- for_each_online_cpu(cpu)
> >+ for_each_watchdog_cpu(cpu)
> > watchdog_nmi_enable(cpu);
> > put_online_cpus();
> > }
> >@@ -625,7 +632,7 @@ void watchdog_nmi_disable_all(void)
> > return;
> > get_online_cpus();
> >- for_each_online_cpu(cpu)
> >+ for_each_watchdog_cpu(cpu)
> > watchdog_nmi_disable(cpu);
> > put_online_cpus();
> > }
> >@@ -684,7 +691,7 @@ static void update_watchdog_all_cpus(void)
> > int cpu;
> > get_online_cpus();
> >- for_each_online_cpu(cpu)
> >+ for_each_watchdog_cpu(cpu)
> > update_watchdog(cpu);
> > put_online_cpus();
> > }
> >@@ -697,8 +704,12 @@ static int watchdog_enable_all_cpus(void)
> > err = smpboot_register_percpu_thread(&watchdog_threads);
> > if (err)
> > pr_err("Failed to create watchdog threads, disabled\n");
> >- else
> >+ else {
> >+ if (smpboot_update_cpumask_percpu_thread(
> >+ &watchdog_threads, watchdog_cpumask))
> >+ pr_err("Failed to set cpumask for watchdog threads\n");
> > watchdog_running = 1;
> >+ }
> > } else {
> > /*
> > * Enable/disable the lockup detectors or
> >@@ -869,12 +880,54 @@ out:
> > mutex_unlock(&watchdog_proc_mutex);
> > return err;
> > }
> >+
> >+/*
> >+ * The cpumask is the mask of possible cpus that the watchdog can run
> >+ * on, not the mask of cpus it is actually running on. This allows the
> >+ * user to specify a mask that will include cpus that have not yet
> >+ * been brought online, if desired.
> >+ */
> >+int proc_watchdog_cpumask(struct ctl_table *table, int write,
> >+ void __user *buffer, size_t *lenp, loff_t *ppos)
> >+{
> >+ int err;
> >+
> >+ mutex_lock(&watchdog_proc_mutex);
> >+ err = proc_do_large_bitmap(table, write, buffer, lenp, ppos);
> >+ if (!err && write) {
> >+ /* Remove impossible cpus to keep sysctl output cleaner. */
> >+ cpumask_and(watchdog_cpumask, watchdog_cpumask,
> >+ cpu_possible_mask);
> >+
> >+ if (watchdog_enabled && watchdog_thresh)
> >+ smpboot_update_cpumask_percpu_thread(&watchdog_threads,
> >+ watchdog_cpumask);
> >+ }
> >+ mutex_unlock(&watchdog_proc_mutex);
> >+ return err;
> >+}
> >+
> > #endif /* CONFIG_SYSCTL */
> > void __init lockup_detector_init(void)
> > {
> > set_sample_period();
> >+ if (!alloc_cpumask_var(&watchdog_cpumask, GFP_KERNEL)) {
> >+ pr_err("Failed to allocate cpumask for watchdog");
> >+ return;
> >+ }
> >+ watchdog_cpumask_bits = cpumask_bits(watchdog_cpumask);
> >+
> >+#ifdef CONFIG_NO_HZ_FULL
> >+ if (!cpumask_empty(tick_nohz_full_mask))
> >+ pr_info("Disabling watchdog on nohz_full cores by default\n");
> >+ cpumask_andnot(watchdog_cpumask, cpu_possible_mask,
> >+ tick_nohz_full_mask);
> >+#else
> >+ cpumask_copy(watchdog_cpumask, cpu_possible_mask);
> >+#endif
> >+
> > if (watchdog_enabled)
> > watchdog_enable_all_cpus();
> > }
>
> --
> Chris Metcalf, EZChip Semiconductor
> http://www.ezchip.com
>