Merge tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-tracegrafted

Pull tracing fixes from Steven Rostedt:

 - Fix error output of boot instance creation failure

   Currently if a boot instance creation fails, instead of printing out
   the name of the instance that failed, it prints "(null)". That is
   because it prints "cur_str" that had already been processed by
   strsep(). Print the saved name instead.

   While at it, print the error code of the failure.

 - Fix use-after-free for same named historgrams

   Histograms can be named so that they can be used in multiple events.
   But if the named histogram has a variable attached, the second event
   that uses the named histogram which duplicates it and needs to free
   the original after duplication leaves the old variable in place and
   still visible. If another histogram uses than variable, it will use
   the stale one which will try to reference the freed duplicate
   histogram and crash the kernel.

   Free the duplicate variables along with the duplicated histogram
   data.

 - Check return value of kthread_run() in event self test

   The events self tests uses a kthread for testing but does not check
   if it succeeded in creating a kthread. If the kthread creation were
   to fail, the code will still try to call kthread_stop() on the error
   returned.

 - Fix race between reading trace_pipe and updating subbuffer size

   If a user is reading the trace_pipe file at the same time they update
   the ring buffer sub-buffer size, can cause the trace_pipe read to
   read stale data. Add trace_access_lock() around updating the ring
   buffer sub-buffer size.

 - Fix eventfs_inode on failure path in creation of the events directory

   In the creation of the "events" directory, if after allocating the
   eventfs_inode a failure is detected, it calls cleanup_ei() which
   calls free_ei(). The free_ei() will test if eventfs_inode being freed
   has no children. It is a bug if it does. But on the failure case of
   the creation of the "events" directory, the children lists have not
   yet been initialized and the free will trigger a warning because
   list_empty() on an uninitialized list returns false.

   Move the initialization into init_ei() where it makes more sense and
   makes sure that a created eventfs_inode has its lists initialized
   upon creation.

 - Check return value of kthread_run() in ftrace direct sample code

   The sample code that shows how to use the ftrace direct calls does
   not test the return of kthread_run() to see if it succeeds. Return a
   failure if the kthread_run() doesn't succeed.

 - Clear user events state on fork in case of alloc failure

   On fork, the child gets a pointer to the parent's user events state.
   It makes a copy of it then updates the child's pointer to it. But if
   the allocation fails, the duplication function leaves the child with
   a pointer to its parent's descriptor. When the child cleans up its
   data, it will free the parent's descriptor while the parent is still
   using it.

   In the duplication function, set the child's user_event_mm to NULL
   before testing if the allocation succeeded, and when it exits it will
   not free the parent's descriptor.

 - Fix retry exhaustion in simple ring buffer reader swap

   simple_ring_buffer_swap_reader_page() starts with retry set to 8 and
   post-decrements it only after a failed link replacement. On the final
   attempt, a successful replacement leaves retry at zero, while a
   failed replacement leaves it at -1.

   But the check for success expects the retry value to be non-zero and
   exits with an error on zero. This is the opposite result. Fix it.

 - Fail nicely when the remote swap_reader_page() returns an error

   Currently, if the swap_reader_page() of a remote buffer fails, it
   triggers a WARN_ON_ONCE() and continues normally. Instead, have it
   exit with an error and a pr_warn() print instead of a full WARNING.

* tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace:
  ring-buffer: Stop remote reader update when page swap fails
  tracing: Fix retry exhaustion in simple ring buffer reader swap
  tracing/user_events: Clear copied tracing state before fork duplication
  samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-multi-modify
  samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-modify
  eventfs: Initialize ei->children and ei->list in init_ei()
  tracing: Fix use-after-free in trace_pipe read on sub-buffer order change
  tracing: Fix crash passing ERR_PTR to kthread_stop()
  tracing: Fix use-after-free with same-name named triggers
  tracing: Fix logged instance name on creation failure

1 files changed, 187 insertions, 0 deletions

diff --git a/Documentation/admin-guide/iostats.rst b/Documentation/admin-guide/iostats.rst
new file mode 100644
index 000000000..9453196ad
--- /dev/null
+++ b/Documentation/admin-guide/iostats.rst
@@ -0,0 +1,187 @@
+=====================
+I/O statistics fields
+=====================
+
+The kernel exposes disk statistics via ``/proc/diskstats`` and
+``/sys/block/<device>/stat``. These stats are usually accessed via tools
+such as ``sar`` and ``iostat``.
+
+Here are examples using a disk with two partitions::
+
+   /proc/diskstats:
+     259       0 nvme0n1 255999 814 12369153 47919 996852 81 36123024 425995 0 301795 580470 0 0 0 0 60602 106555
+     259       1 nvme0n1p1 492 813 17572 96 848 81 108288 210 0 76 307 0 0 0 0 0 0
+     259       2 nvme0n1p2 255401 1 12343477 47799 996004 0 36014736 425784 0 344336 473584 0 0 0 0 0 0
+
+   /sys/block/nvme0n1/stat:
+     255999 814 12369153 47919 996858 81 36123056 426009 0 301809 580491 0 0 0 0 60605 106562
+
+   /sys/block/nvme0n1/nvme0n1p1/stat:
+     492 813 17572 96 848 81 108288 210 0 76 307 0 0 0 0 0 0
+
+Both files contain the same 17 statistics. ``/sys/block/<device>/stat``
+contains the fields for ``<device>``. In ``/proc/diskstats`` the fields
+are prefixed with the major and minor device numbers and the device
+name. In the example above, the first stat value for ``nvme0n1`` is
+255999 in both files.
+
+The sysfs ``stat`` file is efficient for monitoring a small, known set
+of disks. If you're tracking a large number of devices,
+``/proc/diskstats`` is often the better choice since it avoids the
+overhead of opening and closing multiple files for each snapshot.
+
+All fields are cumulative, monotonic counters, except for field 9, which
+resets to zero as I/Os complete. The remaining fields reset at boot, on
+device reattachment or reinitialization, or when the underlying counter
+overflows. Applications reading these counters should detect and handle
+resets when comparing stat snapshots.
+
+Each set of stats only applies to the indicated device; if you want
+system-wide stats you'll have to find all the devices and sum them all up.
+
+Field  1 -- # of reads completed (unsigned long)
+    This is the total number of reads completed successfully.
+
+Field  2 -- # of reads merged, field 6 -- # of writes merged (unsigned long)
+    Reads and writes which are adjacent to each other may be merged for
+    efficiency.  Thus two 4K reads may become one 8K read before it is
+    ultimately handed to the disk, and so it will be counted (and queued)
+    as only one I/O.  This field lets you know how often this was done.
+
+Field  3 -- # of sectors read (unsigned long)
+    This is the total number of sectors read successfully.
+
+Field  4 -- # of milliseconds spent reading (unsigned int)
+    This is the total number of milliseconds spent by all reads (as
+    measured from blk_mq_alloc_request() to __blk_mq_end_request()).
+
+Field  5 -- # of writes completed (unsigned long)
+    This is the total number of writes completed successfully.
+
+Field  6 -- # of writes merged  (unsigned long)
+    See the description of field 2.
+
+Field  7 -- # of sectors written (unsigned long)
+    This is the total number of sectors written successfully.
+
+Field  8 -- # of milliseconds spent writing (unsigned int)
+    This is the total number of milliseconds spent by all writes (as
+    measured from blk_mq_alloc_request() to __blk_mq_end_request()).
+
+Field  9 -- # of I/Os currently in progress (unsigned int)
+    The only field that should go to zero. Incremented as requests are
+    given to appropriate struct request_queue and decremented as they finish.
+
+Field 10 -- # of milliseconds spent doing I/Os (unsigned int)
+    This field increases so long as field 9 is nonzero.
+
+    Since 5.0 this field counts jiffies when at least one request was
+    started or completed. If request runs more than 2 jiffies then some
+    I/O time might be not accounted in case of concurrent requests.
+
+Field 11 -- weighted # of milliseconds spent doing I/Os (unsigned int)
+    This field is incremented at each I/O start, I/O completion, I/O
+    merge, or read of these stats by the number of I/Os in progress
+    (field 9) times the number of milliseconds spent doing I/O since the
+    last update of this field.  This can provide an easy measure of both
+    I/O completion time and the backlog that may be accumulating.
+
+Field 12 -- # of discards completed (unsigned long)
+    This is the total number of discards completed successfully.
+
+Field 13 -- # of discards merged (unsigned long)
+    See the description of field 2
+
+Field 14 -- # of sectors discarded (unsigned long)
+    This is the total number of sectors discarded successfully.
+
+Field 15 -- # of milliseconds spent discarding (unsigned int)
+    This is the total number of milliseconds spent by all discards (as
+    measured from blk_mq_alloc_request() to __blk_mq_end_request()).
+
+Field 16 -- # of flush requests completed
+    This is the total number of flush requests completed successfully.
+
+    Block layer combines flush requests and executes at most one at a time.
+    This counts flush requests executed by disk. Not tracked for partitions.
+
+Field 17 -- # of milliseconds spent flushing
+    This is the total number of milliseconds spent by all flush requests.
+
+To avoid introducing performance bottlenecks, no locks are held while
+modifying these counters.  This implies that minor inaccuracies may be
+introduced when changes collide, so (for instance) adding up all the
+read I/Os issued per partition should equal those made to the disks ...
+but due to the lack of locking it may only be very close.
+
+In 2.6+, there are counters for each CPU, which make the lack of locking
+almost a non-issue.  When the statistics are read, the per-CPU counters
+are summed (possibly overflowing the unsigned long variable they are
+summed to) and the result given to the user.  There is no convenient
+user interface for accessing the per-CPU counters themselves.
+
+Since 4.19 request times are measured with nanoseconds precision and
+truncated to milliseconds before showing in this interface.
+
+Disks vs Partitions
+-------------------
+
+There were significant changes between 2.4 and 2.6+ in the I/O subsystem.
+As a result, some statistic information disappeared. The translation from
+a disk address relative to a partition to the disk address relative to
+the host disk happens much earlier.  All merges and timings now happen
+at the disk level rather than at both the disk and partition level as
+in 2.4.  Consequently, you'll see a different statistics output on 2.6+ for
+partitions from that for disks.  There are only *four* fields available
+for partitions on 2.6+ machines.  This is reflected in the examples above.
+
+Field  1 -- # of reads issued
+    This is the total number of reads issued to this partition.
+
+Field  2 -- # of sectors read
+    This is the total number of sectors requested to be read from this
+    partition.
+
+Field  3 -- # of writes issued
+    This is the total number of writes issued to this partition.
+
+Field  4 -- # of sectors written
+    This is the total number of sectors requested to be written to
+    this partition.
+
+Note that since the address is translated to a disk-relative one, and no
+record of the partition-relative address is kept, the subsequent success
+or failure of the read cannot be attributed to the partition.  In other
+words, the number of reads for partitions is counted slightly before time
+of queuing for partitions, and at completion for whole disks.  This is
+a subtle distinction that is probably uninteresting for most cases.
+
+More significant is the error induced by counting the numbers of
+reads/writes before merges for partitions and after for disks. Since a
+typical workload usually contains a lot of successive and adjacent requests,
+the number of reads/writes issued can be several times higher than the
+number of reads/writes completed.
+
+In 2.6.25, the full statistic set is again available for partitions and
+disk and partition statistics are consistent again. Since we still don't
+keep record of the partition-relative address, an operation is attributed to
+the partition which contains the first sector of the request after the
+eventual merges. As requests can be merged across partition, this could lead
+to some (probably insignificant) inaccuracy.
+
+Additional notes
+----------------
+
+In 2.6+, sysfs is not mounted by default.  If your distribution of
+Linux hasn't added it already, here's the line you'll want to add to
+your ``/etc/fstab``::
+
+	none /sys sysfs defaults 0 0
+
+
+In 2.6+, all disk statistics were removed from ``/proc/stat``.  In 2.4, they
+appear in both ``/proc/partitions`` and ``/proc/stat``, although the ones in
+``/proc/stat`` take a very different format from those in ``/proc/partitions``
+(see proc(5), if your system has it.)
+
+-- ricklind@us.ibm.com