Merge tag 'trace-v7.3-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-tracegrafted

Pull tracing fixes from Steven Rostedt:

 - Don't destroy user event fields when removal fails

   User event fields are destroyed before the event is removed from
   visibility. But that can fail leaving the still visible event with no
   fields. Move the destroying of the fields to after the event is
   successfully removed from visibility.

 - Initialize function graph state is fork before calling
   copy_exec_state()

   For non-CLONE_VM forks, copy_exec_state() allocates a new
   task_exec_state. If that allocation fails, ftrace_graph_exit_task()
   will free the tasks ret_stack pointer. Since that pointer is still
   using the parent's ret_stack, it mistakenly frees the parent's
   pointer too.

   Call ftrace_graph_init() on the task first which will NULL out the
   new tasks's ret_stack and if the copy fails, it will not free
   anything.

 - Remove FGRAPH_MAX_INDEX

   The macro FGRAPH_MAX_INDEX was added but never used. Remove it.

 - Save ent_size in function graph printing of nested functions

   The function graph tracer needs to look at the next event to see if
   the next event is the return of the current function entry. If it is,
   it prints a single line:

	ktime_get();

   Otherwise it prints it like a nested function:

	tick_nohz_irq_exit() {
	    ktime_get();
	    kcpustat_irq_exit();
	}

   In order to look at the next event, it must save the current event so
   that it has the information to print from it. It saves the event in
   the iterator descriptor called "ent". What it doesn't save is the
   ent_size of the event which is now used to know if the function graph
   arguments are to be printed. The peek doesn't save the size so the
   size used happens to be that of the size of the last event that was
   seen.

   Save the entry event size in the iterator descriptor so that the
   correct size is used.

 - Fix several errors with freeing data in the histogram code

   The histogram code had a lot of leaked or or incorrect accounting
   when failures happen. Correct them.

 - Fix histogram regression of .percent and .graph modifiers

   Up until 6.3 histogram values could have "percent" or "graph"
   modifiers that changed how they were printed. But a change that added
   restricting histograms values from being strings, stack traces and
   other modifiers inadvertently prevented them from using the percent
   and graph modifiers, which were legal use cases for values.

   Put back the percent and graph modifiers.

 - Fix various typos in the comments

 - Set the trace_clock before initializing a histogram with clock
   argument

   The histogram API allows the user to specific which trace clock to
   use via a "clock=" string. The histogram is set up first before the
   clock is checked. If the passed in clock is not valid, it exits
   without fully fixing up the histogram leaving it on the list and a
   use-after-free can trigger.

   Update the clock argument first and if it fails then exit gracefully
   before the histogram trigger is placed on any lists.

 - Restore :mod: trailer after parsing in ftrace_set_clr_event

   The function ftrace_set_clr_event() modifies the parse string and
   needs to put it back to what was passed in. It searches for ":mod:"
   via a strsep() but fails to put back the first ':' in the string.

   Add back the ':' in the passed in string.

 - Take trace_array reference when opening a tracer options file

   The options files are dynamically created and some tracers add their
   own options. When a tracer adds their own list of options, the
   trace_array holding them has an array to hold the list of options for
   each tracer. This array increases in size via a krealloc(), and the
   new entry gets a newly allocated array to hold the options of the new
   tracer being added.

   The element in each entry of the tracer's option array holds a
   pointer back to the trace_array, a pointer to the tracer it is
   associated to, a pointer to the flags of the option.

   The issue is that these arrays are freed when the trace_array is
   freed when its instance it represents is removed from the instances
   directory. There's a race that an open of one of these options files
   can happen when the instance is being removed.

   Add a new helper function to be called by the open function of the
   options file to iterate all existing trace_arrays under a lock and
   find the one that has the given option element in one of it's tracer
   arrays. If found, then update the associated trace_array's reference
   counter to keep it from being freed. If not found, have the open call
   return -ENODEV.

 - Disable interrupts when acquiring the lock in rb_wake_up_waiters()

   The function rb_wake_up_waiters() assumes it will be called in
   interrupt context and does not disable irqs when taking
   cpu_buffer->reader_lock, which can be called in hard interrupt
   context. The issue is in PREEMPT_RT, this function is called in
   thread context leaving this lock open to a deadlock.

   Take the lock with interrupts disabled.

 - Use rcu_assign_pointer() for tmp_ops filter hash

   The tmp_ops used in update_ftrace_direct_mod() assigns its
   filter_hash field directly, but that field is annotated as __rcu and
   sparse complains. Assign it with rcu_assign_pointer()

 - Fix use-after-free in enable_trigger_private_data_free()

   The trace_event_call is accessed through the event_trigger_data's
   trace_event_file pointer to put the trace_event_call on freeing. The
   issue is that the trace_event_file data may have been freed already
   causing a use-after-free. Add a field to the event_trigger_data that
   points directly to the trace_event_call so that it can decrement its
   reference directly without needing to go through the
   trace_event_file.

 - Fix accounting of buffer data remote headers

   trace_buffer_desc_size() and trace_remote_alloc_buffer() undercount
   the number of pages is needed for the asked for size as it doesn't
   take into account the meta data on each page. Add a helper function
   to do the calculation properly and use that in these functions.

 - Catch nr_page_va overflow in ring_buffer_desc sizing

   The number of pages per remote ring buffer is capped by
   ring_buffer_desc::nr_page_va (32 bits). A buffer_size large enough to
   overflow that field would silently allocate a descriptor smaller than
   what was asked for.

 - Do not resize the subbuf order if any per_cpu buffer is disabled

   The mmapping of ring buffers disables resizing the subbuffers, but it
   is done per-cpu whereas the subbuf size change is done for all the
   per_cpu buffers under the buffer->mutex. It could change the size of
   some while the mapping is happening on others. Have the resize of the
   subbuf order check all the per_cpu buffers under the lock to see if
   any of them is disabled before starting and causing an inconsistency
   between buffers that are being mapped.

* tag 'trace-v7.3-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace: (25 commits)
  ring-buffer: Check resize_disabled before publishing the new subbuf order
  tracing/remotes: Catch nr_page_va overflow in ring_buffer_desc sizing
  tracing/remotes: Account for ring buffer page header in size calculation
  tracing: Don't dereference trace_event_file in deferred trigger free
  ftrace: Use rcu_assign_pointer() for tmp_ops filter hash
  ring-buffer: Acquire the lock with irqsave in rb_wake_up_waiters()
  tracing: Take trace_array reference when opening a tracer options file
  tracing: Fix ring_buffer_read_page_size() kernel-doc
  tracing: Restore :mod: trailer after parsing in ftrace_set_clr_event()
  tracing: Fix memory corruption from a "STACKTRACE" histogram key
  tracing: Fix memory corruption from the histogram stacktrace modifier
  tracing: Undo the registration when enabling the histogram trigger fails
  tracing: Take the reference before publishing the named histogram trigger
  tracing: Set the trace clock before registering the histogram trigger
  tracing: Fix typo "preceeded" in comment
  tracing: Fix typo "availabe" in comment
  tracing: Let histogram values keep the percent and graph modifiers
  tracing: Keep the entry count when the histogram stats allocation fails
  tracing: Free histogram the field rejected for a bad modifier
  tracing: Free histogram the var ref when its initialization fails
  ...

1 files changed, 357 insertions, 0 deletions

diff --git a/Documentation/admin-guide/cpu-isolation.rst b/Documentation/admin-guide/cpu-isolation.rst
new file mode 100644
index 000000000..8c65d03fd
--- /dev/null
+++ b/Documentation/admin-guide/cpu-isolation.rst
@@ -0,0 +1,357 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=============
+CPU Isolation
+=============
+
+Introduction
+============
+
+"CPU Isolation" means leaving a CPU exclusive to a given workload
+without any undesired code interference from the kernel.
+
+Those interferences, commonly pointed out as "noise", can be triggered
+by asynchronous events (interrupts, timers, scheduler preemption by
+workqueues and kthreads, ...) or synchronous events (syscalls and page
+faults).
+
+Such noise usually goes unnoticed. After all, synchronous events are a
+component of the requested kernel service. And asynchronous events are
+either sufficiently well-distributed by the scheduler when executed
+as tasks or reasonably fast when executed as interrupt. The timer
+interrupt can even execute 1024 times per seconds without a significant
+and measurable impact most of the time.
+
+However some rare and extreme workloads can be quite sensitive to
+those kinds of noise. This is the case, for example, with high
+bandwidth network processing that can't afford losing a single packet
+or very low latency network processing. Typically those use cases
+involve DPDK, bypassing the kernel networking stack and performing
+direct access to the networking device from userspace.
+
+In order to run a CPU without or with limited kernel noise, the
+related housekeeping work needs to be either shut down, migrated or
+offloaded.
+
+Housekeeping
+============
+
+In the CPU isolation terminology, housekeeping is the work, often
+asynchronous, that the kernel needs to process in order to maintain
+all its services. It matches the noises and disturbances enumerated
+above except when at least one CPU is isolated. Then housekeeping may
+make use of further coping mechanisms if CPU-tied work must be
+offloaded.
+
+Housekeeping CPUs are the non-isolated CPUs where the kernel noise
+is moved away from isolated CPUs.
+
+The isolation can be implemented in several ways depending on the
+nature of the noise:
+
+- Unbound work, where "unbound" means not tied to any CPU, can be
+  simply migrated away from isolated CPUs to housekeeping CPUs.
+  This is the case of unbound workqueues, kthreads and timers.
+
+- Bound work, where "bound" means tied to a specific CPU, usually
+  can't be moved away as-is by nature. Either:
+
+	- The work must switch to a locked implementation. E.g.:
+	  This is the case of RCU with CONFIG_RCU_NOCB_CPU.
+
+	- The related feature must be shut down and considered
+	  incompatible with isolated CPUs. E.g.: Lockup watchdog,
+	  unreliable clocksources, etc...
+
+	- An elaborate and heavyweight coping mechanism stands as a
+	  replacement. E.g.: the timer tick is shut down on nohz_full
+	  CPUs but with the constraint of running a single task on
+	  them. A significant cost penalty is added on kernel entry/exit
+	  and a residual 1Hz scheduler tick is offloaded to housekeeping
+	  CPUs.
+
+In any case, housekeeping work has to be handled, which is why there
+must be at least one housekeeping CPU in the system, preferably more
+if the machine runs a lot of CPUs. For example one per node on NUMA
+systems.
+
+Also CPU isolation often means a tradeoff between noise-free isolated
+CPUs and added overhead on housekeeping CPUs, sometimes even on
+isolated CPUs entering the kernel.
+
+Isolation features
+==================
+
+Different levels of isolation can be configured in the kernel, each of
+which has its own drawbacks and tradeoffs.
+
+Scheduler domain isolation
+--------------------------
+
+This feature isolates a CPU from the scheduler topology. As a result,
+the target isn't part of the load balancing. Tasks won't migrate
+either from or to it unless affined explicitly.
+
+As a side effect the CPU is also isolated from unbound workqueues and
+unbound kthreads.
+
+Requirements
+~~~~~~~~~~~~
+
+- CONFIG_CPUSETS=y for the cpusets-based interface
+
+Tradeoffs
+~~~~~~~~~
+
+By nature, the system load is overall less distributed since some CPUs
+are extracted from the global load balancing.
+
+Interfaces
+~~~~~~~~~~
+
+- Documentation/admin-guide/cgroup-v2.rst cpuset isolated partitions are recommended
+  because they are tunable at runtime.
+
+- The 'isolcpus=' kernel boot parameter with the 'domain' flag is a
+  less flexible alternative that doesn't allow for runtime
+  reconfiguration.
+
+IRQs isolation
+--------------
+
+Isolate the IRQs whenever possible, so that they don't fire on the
+target CPUs.
+
+Interfaces
+~~~~~~~~~~
+
+- The file /proc/irq/\*/smp_affinity as explained in detail in
+  Documentation/core-api/irq/irq-affinity.rst page.
+
+- The "irqaffinity=" kernel boot parameter for a default setting.
+
+- The "managed_irq" flag in the "isolcpus=" kernel boot parameter
+  tries a best effort affinity override for managed IRQs.
+
+Full Dynticks (aka nohz_full)
+-----------------------------
+
+Full dynticks extends the dynticks idle mode, which stops the tick when
+the CPU is idle, to CPUs running a single task in userspace. That is,
+the timer tick is stopped if the environment allows it.
+
+Global timer callbacks are also isolated from the nohz_full CPUs.
+
+Requirements
+~~~~~~~~~~~~
+
+- CONFIG_NO_HZ_FULL=y
+
+Constraints
+~~~~~~~~~~~
+
+- The isolated CPUs must run a single task only. Multitask requires
+  the tick to maintain preemption. This is usually fine since the
+  workload usually can't stand the latency of random context switches.
+
+- No call to the kernel from isolated CPUs, at the risk of triggering
+  random noise.
+
+- No use of POSIX CPU timers on isolated CPUs.
+
+- Architecture must have a stable and reliable clocksource (no
+  unreliable TSC that requires the watchdog).
+
+
+Tradeoffs
+~~~~~~~~~
+
+In terms of cost, this is the most invasive isolation feature. It is
+assumed to be used when the workload spends most of its time in
+userspace and doesn't rely on the kernel except for preparatory
+work because:
+
+- RCU adds more overhead due to the locked, offloaded and threaded
+  callbacks processing (the same that would be obtained with "rcu_nocbs"
+  boot parameter).
+
+- Kernel entry/exit through syscalls, exceptions and IRQs are more
+  costly due to fully ordered RmW operations that maintain userspace
+  as RCU extended quiescent state. Also the CPU time is accounted on
+  kernel boundaries instead of periodically from the tick.
+
+- Housekeeping CPUs must run a 1Hz residual remote scheduler tick
+  on behalf of the isolated CPUs.
+
+Checklist
+=========
+
+You have set up each of the above isolation features but you still
+observe jitters that trash your workload? Make sure to check a few
+elements before proceeding.
+
+Some of these checklist items are similar to those of real-time
+workloads:
+
+- Use mlock() to prevent your pages from being swapped away. Page
+  faults are usually not compatible with jitter sensitive workloads.
+
+- Avoid SMT to prevent your hardware thread from being "preempted"
+  by another one.
+
+- CPU frequency changes may induce subtle sorts of jitter in a
+  workload. Cpufreq should be used and tuned with caution.
+
+- Deep C-states may result in latency issues upon wake-up. If this
+  happens to be a problem, C-states can be limited via kernel boot
+  parameters such as processor.max_cstate or intel_idle.max_cstate.
+  More finegrained tunings are described in
+  Documentation/admin-guide/pm/cpuidle.rst page
+
+- Your system may be subject to firmware-originating interrupts - x86 has
+  System Management Interrupts (SMIs) for example. Check your system BIOS
+  to disable such interference, and with some luck your vendor will have
+  a BIOS tuning guidance for low-latency operations.
+
+
+Full isolation example
+======================
+
+In this example, the system has 8 CPUs and the 8th is to be fully
+isolated. Since CPUs start from 0, the 8th CPU is CPU 7.
+
+Kernel parameters
+-----------------
+
+Set the following kernel boot parameters to disable SMT and setup tick
+and IRQ isolation:
+
+- Full dynticks: nohz_full=7
+
+- IRQs isolation: irqaffinity=0-6
+
+- Managed IRQs isolation: isolcpus=managed_irq,7
+
+- Prevent SMT: nosmt
+
+The full command line is then:
+
+  nohz_full=7 irqaffinity=0-6 isolcpus=managed_irq,7 nosmt
+
+CPUSET configuration (cgroup v2)
+--------------------------------
+
+Assuming cgroup v2 is mounted to /sys/fs/cgroup, the following script
+isolates CPU 7 from scheduler domains.
+
+::
+
+  cd /sys/fs/cgroup
+  # Activate the cpuset subsystem
+  echo +cpuset > cgroup.subtree_control
+  # Create partition to be isolated
+  mkdir test
+  cd test
+  echo +cpuset > cgroup.subtree_control
+  # Isolate CPU 7
+  echo 7 > cpuset.cpus
+  echo "isolated" > cpuset.cpus.partition
+
+The userspace workload
+----------------------
+
+Fake a pure userspace workload, the program below runs a dummy
+userspace loop on the isolated CPU 7.
+
+::
+
+  #include <stdio.h>
+  #include <fcntl.h>
+  #include <unistd.h>
+  #include <errno.h>
+  int main(void)
+  {
+      // Move the current task to the isolated cpuset (bind to CPU 7)
+      int fd = open("/sys/fs/cgroup/test/cgroup.procs", O_WRONLY);
+      if (fd < 0) {
+          perror("Can't open cpuset file...\n");
+          return 0;
+      }
+
+      write(fd, "0\n", 2);
+      close(fd);
+
+      // Run an endless dummy loop until the launcher kills us
+      while (1)
+      ;
+
+      return 0;
+  }
+
+Build it and save for later step:
+
+::
+
+  # gcc user_loop.c -o user_loop
+
+The launcher
+------------
+
+The below launcher runs the above program for 10 seconds and traces
+the noise resulting from preempting tasks and IRQs.
+
+::
+
+  TRACING=/sys/kernel/tracing/
+  # Make sure tracing is off for now
+  echo 0 > $TRACING/tracing_on
+  # Flush previous traces
+  echo > $TRACING/trace
+  # Record disturbance from other tasks
+  echo 1 > $TRACING/events/sched/sched_switch/enable
+  # Record disturbance from interrupts
+  echo 1 > $TRACING/events/irq_vectors/enable
+  # Now we can start tracing
+  echo 1 > $TRACING/tracing_on
+  # Run the dummy user_loop for 10 seconds on CPU 7
+  ./user_loop &
+  USER_LOOP_PID=$!
+  sleep 10
+  kill $USER_LOOP_PID
+  # Disable tracing and save traces from CPU 7 in a file
+  echo 0 > $TRACING/tracing_on
+  cat $TRACING/per_cpu/cpu7/trace > trace.7
+
+If no specific problem arose, the output of trace.7 should look like
+the following:
+
+::
+
+  <idle>-0 [007] d..2. 1980.976624: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=user_loop next_pid=1553 next_prio=120
+  user_loop-1553 [007] d.h.. 1990.946593: reschedule_entry: vector=253
+  user_loop-1553 [007] d.h.. 1990.946593: reschedule_exit: vector=253
+
+That is, no specific noise triggered between the first trace and the
+second during 10 seconds when user_loop was running.
+
+Debugging
+=========
+
+Of course things are never so easy, especially on this matter.
+Chances are that actual noise will be observed in the aforementioned
+trace.7 file.
+
+The best way to investigate further is to enable finer grained
+tracepoints such as those of subsystems producing asynchronous
+events: workqueue, timer, irq_vector, etc... It also can be
+interesting to enable the tick_stop event to diagnose why the tick is
+retained when that happens.
+
+Some tools may also be useful for higher level analysis:
+
+- Documentation/tools/rtla/rtla.rst provides a suite of tools to analyze
+  latency and noise in the system. For example Documentation/tools/rtla/rtla-osnoise.rst
+  runs a kernel tracer that analyzes and output a summary of the noises.
+
+- dynticks-testing does something similar to rtla-osnoise but in userspace. It is available
+  at git://git.kernel.org/pub/scm/linux/kernel/git/frederic/dynticks-testing.git