Commit 723b1133 authored by Nicholas Piggin's avatar Nicholas Piggin Committed by Michael Ellerman

powerpc/watchdog: improve watchdog comments

The overview comments in the powerpc watchdog are out of date after
several iterations and changes of the code. Bring them up to date.
Signed-off-by: default avatarNicholas Piggin <npiggin@gmail.com>
Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
parent 8183d99f
......@@ -26,15 +26,45 @@
#include <asm/paca.h>
/*
* The watchdog has a simple timer that runs on each CPU, once per timer
* period. This is the heartbeat.
* The powerpc watchdog ensures that each CPU is able to service timers.
* The watchdog sets up a simple timer on each CPU to run once per timer
* period, and updates a per-cpu timestamp and a "pending" cpumask. This is
* the heartbeat.
*
* Then there are checks to see if the heartbeat has not triggered on a CPU
* for the panic timeout period. Currently the watchdog only supports an
* SMP check, so the heartbeat only turns on when we have 2 or more CPUs.
* Then there are two systems to check that the heartbeat is still running.
* The local soft-NMI, and the SMP checker.
*
* This is not an NMI watchdog, but Linux uses that name for a generic
* watchdog in some cases, so NMI gets used in some places.
* The soft-NMI checker can detect lockups on the local CPU. When interrupts
* are disabled with local_irq_disable(), platforms that use soft-masking
* can leave hardware interrupts enabled and handle them with a masked
* interrupt handler. The masked handler can send the timer interrupt to the
* watchdog's soft_nmi_interrupt(), which appears to Linux as an NMI
* interrupt, and can be used to detect CPUs stuck with IRQs disabled.
*
* The soft-NMI checker will compare the heartbeat timestamp for this CPU
* with the current time, and take action if the difference exceeds the
* watchdog threshold.
*
* The limitation of the soft-NMI watchdog is that it does not work when
* interrupts are hard disabled or otherwise not being serviced. This is
* solved by also having a SMP watchdog where all CPUs check all other
* CPUs heartbeat.
*
* The SMP checker can detect lockups on other CPUs. A gobal "pending"
* cpumask is kept, containing all CPUs which enable the watchdog. Each
* CPU clears their pending bit in their heartbeat timer. When the bitmask
* becomes empty, the last CPU to clear its pending bit updates a global
* timestamp and refills the pending bitmask.
*
* In the heartbeat timer, if any CPU notices that the global timestamp has
* not been updated for a period exceeding the watchdog threshold, then it
* means the CPU(s) with their bit still set in the pending mask have had
* their heartbeat stop, and action is taken.
*
* Some platforms implement true NMI IPIs, which can by used by the SMP
* watchdog to detect an unresponsive CPU and pull it out of its stuck
* state with the NMI IPI, to get crash/debug data from it. This way the
* SMP watchdog can detect hardware interrupts off lockups.
*/
static cpumask_t wd_cpus_enabled __read_mostly;
......@@ -47,19 +77,7 @@ static u64 wd_timer_period_ms __read_mostly; /* interval between heartbeat */
static DEFINE_PER_CPU(struct timer_list, wd_timer);
static DEFINE_PER_CPU(u64, wd_timer_tb);
/*
* These are for the SMP checker. CPUs clear their pending bit in their
* heartbeat. If the bitmask becomes empty, the time is noted and the
* bitmask is refilled.
*
* All CPUs clear their bit in the pending mask every timer period.
* Once all have cleared, the time is noted and the bits are reset.
* If the time since all clear was greater than the panic timeout,
* we can panic with the list of stuck CPUs.
*
* This will work best with NMI IPIs for crash code so the stuck CPUs
* can be pulled out to get their backtraces.
*/
/* SMP checker bits */
static unsigned long __wd_smp_lock;
static cpumask_t wd_smp_cpus_pending;
static cpumask_t wd_smp_cpus_stuck;
......
Markdown is supported
0%
or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment