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rv: Add nrp and sssw per-task monitors
Add 2 per-task monitors as part of the sched model: * nrp: need-resched preempts Monitor to ensure preemption requires need resched. * sssw: set state sleep and wakeup Monitor to ensure sched_set_state to sleepable leads to sleeping and sleeping tasks require wakeup. Cc: Ingo Molnar <[email protected]> Cc: Jonathan Corbet <[email protected]> Cc: Masami Hiramatsu <[email protected]> Cc: Tomas Glozar <[email protected]> Cc: Juri Lelli <[email protected]> Cc: Clark Williams <[email protected]> Cc: John Kacur <[email protected]> Cc: Peter Zijlstra <[email protected]> Link: https://lore.kernel.org/[email protected] Signed-off-by: Gabriele Monaco <[email protected]> Acked-by: Nam Cao <[email protected]> Tested-by: Nam Cao <[email protected]> Signed-off-by: Steven Rostedt (Google) <[email protected]>
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‎Documentation/trace/rv/monitor_sched.rst‎

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@@ -174,6 +174,173 @@ running one, no real task switch occurs but interrupts are disabled nonetheless:
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| | irq_entry
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+---------------+ irq_enable
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Monitor nrp
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-----------
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The need resched preempts (nrp) monitor ensures preemption requires
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``need_resched``. Only kernel preemption is considered, since preemption
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while returning to userspace, for this monitor, is indistinguishable from
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``sched_switch_yield`` (described in the sssw monitor).
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A kernel preemption is whenever ``__schedule`` is called with the preemption
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flag set to true (e.g. from preempt_enable or exiting from interrupts). This
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type of preemption occurs after the need for ``rescheduling`` has been set.
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This is not valid for the *lazy* variant of the flag, which causes only
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userspace preemption.
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A ``schedule_entry_preempt`` may involve a task switch or not, in the latter
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case, a task goes through the scheduler from a preemption context but it is
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picked as the next task to run. Since the scheduler runs, this clears the need
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to reschedule. The ``any_thread_running`` state does not imply the monitored
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task is not running as this monitor does not track the outcome of scheduling.
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In theory, a preemption can only occur after the ``need_resched`` flag is set. In
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practice, however, it is possible to see a preemption where the flag is not
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set. This can happen in one specific condition::
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need_resched
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preempt_schedule()
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preempt_schedule_irq()
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__schedule()
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!need_resched
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__schedule()
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In the situation above, standard preemption starts (e.g. from preempt_enable
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when the flag is set), an interrupt occurs before scheduling and, on its exit
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path, it schedules, which clears the ``need_resched`` flag.
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When the preempted task runs again, the standard preemption started earlier
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resumes, although the flag is no longer set. The monitor considers this a
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``nested_preemption``, this allows another preemption without re-setting the
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flag. This condition relaxes the monitor constraints and may catch false
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negatives (i.e. no real ``nested_preemptions``) but makes the monitor more
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robust and able to validate other scenarios.
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For simplicity, the monitor starts in ``preempt_irq``, although no interrupt
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occurred, as the situation above is hard to pinpoint::
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schedule_entry
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irq_entry #===========================================#
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+-------------------------- H H
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| H H
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+-------------------------> H any_thread_running H
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H H
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+-------------------------> H H
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| #===========================================#
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| schedule_entry | ^
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| schedule_entry_preempt | sched_need_resched | schedule_entry
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| | schedule_entry_preempt
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| v |
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| +----------------------+ |
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| +--- | | |
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| sched_need_resched | | rescheduling | -+
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| +--> | |
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| +----------------------+
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| | irq_entry
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| v
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| +----------------------+
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| | | ---+
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| ---> | | | sched_need_resched
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| | preempt_irq | | irq_entry
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| | | <--+
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| | | <--+
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| +----------------------+ |
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| | schedule_entry | sched_need_resched
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| | schedule_entry_preempt |
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| v |
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| +-----------------------+ |
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+-------------------------- | nested_preempt | --+
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+-----------------------+
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^ irq_entry |
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+-------------------+
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Due to how the ``need_resched`` flag on the preemption count works on arm64,
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this monitor is unstable on that architecture, as it often records preemption
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when the flag is not set, even in presence of the workaround above.
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For the time being, the monitor is disabled by default on arm64.
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Monitor sssw
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------------
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The set state sleep and wakeup (sssw) monitor ensures ``set_state`` to
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sleepable leads to sleeping and sleeping tasks require wakeup. It includes the
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following types of switch:
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* ``switch_suspend``:
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a task puts itself to sleep, this can happen only after explicitly setting
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the task to ``sleepable``. After a task is suspended, it needs to be woken up
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(``waking`` state) before being switched in again.
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Setting the task's state to ``sleepable`` can be reverted before switching if it
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is woken up or set to ``runnable``.
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* ``switch_blocking``:
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a special case of a ``switch_suspend`` where the task is waiting on a
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sleeping RT lock (``PREEMPT_RT`` only), it is common to see wakeup and set
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state events racing with each other and this leads the model to perceive this
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type of switch when the task is not set to sleepable. This is a limitation of
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the model in SMP system and workarounds may slow down the system.
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* ``switch_preempt``:
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a task switch as a result of kernel preemption (``schedule_entry_preempt`` in
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the nrp model).
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* ``switch_yield``:
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a task explicitly calls the scheduler or is preempted while returning to
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userspace. It can happen after a ``yield`` system call, from the idle task or
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if the ``need_resched`` flag is set. By definition, a task cannot yield while
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``sleepable`` as that would be a suspension. A special case of a yield occurs
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when a task in ``TASK_INTERRUPTIBLE`` calls the scheduler while a signal is
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pending. The task doesn't go through the usual blocking/waking and is set
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back to runnable, the resulting switch (if there) looks like a yield to the
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``signal_wakeup`` state and is followed by the signal delivery. From this
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state, the monitor expects a signal even if it sees a wakeup event, although
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not necessary, to rule out false negatives.
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This monitor doesn't include a running state, ``sleepable`` and ``runnable``
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are only referring to the task's desired state, which could be scheduled out
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(e.g. due to preemption). However, it does include the event
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``sched_switch_in`` to represent when a task is allowed to become running. This
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can be triggered also by preemption, but cannot occur after the task got to
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``sleeping`` before a ``wakeup`` occurs::
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+--------------------------------------------------------------------------+
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| |
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| |
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| switch_suspend | |
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| switch_blocking | |
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v v |
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+----------+ #==========================# set_state_runnable |
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| | H H wakeup |
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| | H H switch_in |
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| | H H switch_yield |
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| sleeping | H H switch_preempt |
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| | H H signal_deliver |
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| | switch_ H H ------+ |
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| | _blocking H runnable H | |
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| | <----------- H H <-----+ |
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+----------+ H H |
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| wakeup H H |
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+---------------------> H H |
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H H |
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+---------> H H |
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| #==========================# |
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| | ^ |
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| | | set_state_runnable |
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| | | wakeup |
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| set_state_sleepable | +------------------------+
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| v | |
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| +--------------------------+ set_state_sleepable
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| | | switch_in
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| | | switch_preempt
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signal_deliver | sleepable | signal_deliver
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| | | ------+
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| | | |
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| | | <-----+
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| +--------------------------+
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| | ^
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| switch_yield | set_state_sleepable
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| v |
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| +---------------+ |
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+---------- | signal_wakeup | -+
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+---------------+
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^ | switch_in
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| | switch_preempt
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| | switch_yield
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+-----------+ wakeup
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References
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----------
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‎kernel/trace/rv/Kconfig‎

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@@ -55,6 +55,8 @@ source "kernel/trace/rv/monitors/snroc/Kconfig"
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source "kernel/trace/rv/monitors/scpd/Kconfig"
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source "kernel/trace/rv/monitors/snep/Kconfig"
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source "kernel/trace/rv/monitors/sts/Kconfig"
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source "kernel/trace/rv/monitors/nrp/Kconfig"
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source "kernel/trace/rv/monitors/sssw/Kconfig"
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# Add new sched monitors here
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source "kernel/trace/rv/monitors/rtapp/Kconfig"

‎kernel/trace/rv/Makefile‎

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@@ -14,6 +14,8 @@ obj-$(CONFIG_RV_MON_RTAPP) += monitors/rtapp/rtapp.o
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obj-$(CONFIG_RV_MON_PAGEFAULT) += monitors/pagefault/pagefault.o
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obj-$(CONFIG_RV_MON_SLEEP) += monitors/sleep/sleep.o
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obj-$(CONFIG_RV_MON_STS) += monitors/sts/sts.o
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obj-$(CONFIG_RV_MON_NRP) += monitors/nrp/nrp.o
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obj-$(CONFIG_RV_MON_SSSW) += monitors/sssw/sssw.o
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# Add new monitors here
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obj-$(CONFIG_RV_REACTORS) += rv_reactors.o
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obj-$(CONFIG_RV_REACT_PRINTK) += reactor_printk.o

‎kernel/trace/rv/monitors/nrp/Kconfig‎

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# SPDX-License-Identifier: GPL-2.0-only
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#
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config RV_MON_NRP
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depends on RV
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depends on RV_MON_SCHED
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default y if !ARM64
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select DA_MON_EVENTS_ID
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bool "nrp monitor"
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help
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Monitor to ensure preemption requires need resched.
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This monitor is part of the sched monitors collection.
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This monitor is unstable on arm64, say N unless you are testing it.
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For further information, see:
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Documentation/trace/rv/monitor_sched.rst

‎kernel/trace/rv/monitors/nrp/nrp.c‎

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// SPDX-License-Identifier: GPL-2.0
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#include <linux/ftrace.h>
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#include <linux/tracepoint.h>
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#include <linux/kernel.h>
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#include <linux/module.h>
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#include <linux/init.h>
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#include <linux/rv.h>
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#include <rv/instrumentation.h>
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#include <rv/da_monitor.h>
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#define MODULE_NAME "nrp"
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#include <trace/events/irq.h>
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#include <trace/events/sched.h>
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#include <rv_trace.h>
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#include <monitors/sched/sched.h>
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#include "nrp.h"
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static struct rv_monitor rv_nrp;
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DECLARE_DA_MON_PER_TASK(nrp, unsigned char);
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#ifdef CONFIG_X86_LOCAL_APIC
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#include <asm/trace/irq_vectors.h>
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static void handle_vector_irq_entry(void *data, int vector)
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{
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da_handle_event_nrp(current, irq_entry_nrp);
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}
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static void attach_vector_irq(void)
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{
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rv_attach_trace_probe("nrp", local_timer_entry, handle_vector_irq_entry);
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if (IS_ENABLED(CONFIG_IRQ_WORK))
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rv_attach_trace_probe("nrp", irq_work_entry, handle_vector_irq_entry);
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if (IS_ENABLED(CONFIG_SMP)) {
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rv_attach_trace_probe("nrp", reschedule_entry, handle_vector_irq_entry);
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rv_attach_trace_probe("nrp", call_function_entry, handle_vector_irq_entry);
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rv_attach_trace_probe("nrp", call_function_single_entry, handle_vector_irq_entry);
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}
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}
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static void detach_vector_irq(void)
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{
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rv_detach_trace_probe("nrp", local_timer_entry, handle_vector_irq_entry);
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if (IS_ENABLED(CONFIG_IRQ_WORK))
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rv_detach_trace_probe("nrp", irq_work_entry, handle_vector_irq_entry);
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if (IS_ENABLED(CONFIG_SMP)) {
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rv_detach_trace_probe("nrp", reschedule_entry, handle_vector_irq_entry);
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rv_detach_trace_probe("nrp", call_function_entry, handle_vector_irq_entry);
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rv_detach_trace_probe("nrp", call_function_single_entry, handle_vector_irq_entry);
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}
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}
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#else
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/* We assume irq_entry tracepoints are sufficient on other architectures */
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static void attach_vector_irq(void) { }
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static void detach_vector_irq(void) { }
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#endif
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static void handle_irq_entry(void *data, int irq, struct irqaction *action)
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{
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da_handle_event_nrp(current, irq_entry_nrp);
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}
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static void handle_sched_need_resched(void *data, struct task_struct *tsk,
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int cpu, int tif)
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{
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/*
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* Although need_resched leads to both the rescheduling and preempt_irq
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* states, it is safer to start the monitor always in preempt_irq,
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* which may not mirror the system state but makes the monitor simpler,
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*/
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if (tif == TIF_NEED_RESCHED)
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da_handle_start_event_nrp(tsk, sched_need_resched_nrp);
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}
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static void handle_schedule_entry(void *data, bool preempt)
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{
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if (preempt)
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da_handle_event_nrp(current, schedule_entry_preempt_nrp);
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else
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da_handle_event_nrp(current, schedule_entry_nrp);
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}
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static int enable_nrp(void)
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{
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int retval;
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retval = da_monitor_init_nrp();
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if (retval)
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return retval;
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rv_attach_trace_probe("nrp", irq_handler_entry, handle_irq_entry);
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rv_attach_trace_probe("nrp", sched_set_need_resched_tp, handle_sched_need_resched);
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rv_attach_trace_probe("nrp", sched_entry_tp, handle_schedule_entry);
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attach_vector_irq();
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return 0;
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}
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static void disable_nrp(void)
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{
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rv_nrp.enabled = 0;
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rv_detach_trace_probe("nrp", irq_handler_entry, handle_irq_entry);
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rv_detach_trace_probe("nrp", sched_set_need_resched_tp, handle_sched_need_resched);
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rv_detach_trace_probe("nrp", sched_entry_tp, handle_schedule_entry);
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detach_vector_irq();
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da_monitor_destroy_nrp();
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}
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static struct rv_monitor rv_nrp = {
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.name = "nrp",
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.description = "need resched preempts.",
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.enable = enable_nrp,
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.disable = disable_nrp,
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.reset = da_monitor_reset_all_nrp,
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.enabled = 0,
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};
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static int __init register_nrp(void)
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{
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return rv_register_monitor(&rv_nrp, &rv_sched);
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}
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static void __exit unregister_nrp(void)
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{
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rv_unregister_monitor(&rv_nrp);
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}
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module_init(register_nrp);
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module_exit(unregister_nrp);
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MODULE_LICENSE("GPL");
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MODULE_AUTHOR("Gabriele Monaco <[email protected]>");
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MODULE_DESCRIPTION("nrp: need resched preempts.");

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