1. 09 10月, 2008 40 次提交
    • J
      block: inherit CPU completion on bio->rq and rq->rq merges · ab780f1e
      Jens Axboe 提交于
      Somewhat incomplete, as we do allow merges of requests and bios
      that have different completion CPUs given. This is done on the
      assumption that a larger IO is still more beneficial than CPU
      locality.
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      ab780f1e
    • J
      block: add support for IO CPU affinity · c7c22e4d
      Jens Axboe 提交于
      This patch adds support for controlling the IO completion CPU of
      either all requests on a queue, or on a per-request basis. We export
      a sysfs variable (rq_affinity) which, if set, migrates completions
      of requests to the CPU that originally submitted it. A bio helper
      (bio_set_completion_cpu()) is also added, so that queuers can ask
      for completion on that specific CPU.
      
      In testing, this has been show to cut the system time by as much
      as 20-40% on synthetic workloads where CPU affinity is desired.
      
      This requires a little help from the architecture, so it'll only
      work as designed for archs that are using the new generic smp
      helper infrastructure.
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      c7c22e4d
    • J
      block: make kblockd_schedule_work() take the queue as parameter · 18887ad9
      Jens Axboe 提交于
      Preparatory patch for checking queuing affinity.
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      18887ad9
    • J
      block: split softirq handling into blk-softirq.c · b646fc59
      Jens Axboe 提交于
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      b646fc59
    • J
      0835da67
    • T
      block: allow disk to have extended device number · 3e1a7ff8
      Tejun Heo 提交于
      Now that disk and partition handlings are mostly unified, it's easy to
      allow disk to have extended device number.  This patch makes
      add_disk() use extended device number if disk->minors is zero.  Both
      sd and ide-disk are updated to use this.
      
      * sd_format_disk_name() is implemented which can generically determine
        the drive name.  This removes disk number restriction stemming from
        limited device names.
      
      * If sd index goes over SD_MAX_DISKS (which can be increased now BTW),
        sd simply doesn't initialize minors letting block layer choose
        extended device number.
      
      * If CONFIG_DEBUG_EXT_DEVT is set, both sd and ide-disk always set
        minors to 0 and use extended device numbers.
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      3e1a7ff8
    • T
      block: replace @ext_minors with GENHD_FL_EXT_DEVT · 689d6fac
      Tejun Heo 提交于
      With previous changes, it's meaningless to limit the number of
      partitions.  Replace @ext_minors with GENHD_FL_EXT_DEVT such that
      setting the flag allows the disk to have maximum number of allowed
      partitions (only limited by the number of entries in parsed_partitions
      as determined by MAX_PART constant).
      
      This kills not-too-pretty alloc_disk_ext[_node]() functions and makes
      @minors parameter to alloc_disk[_node]() unnecessary.  The parameter
      is left alone to avoid disturbing the users.
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      689d6fac
    • T
      block: make partition array dynamic · 540eed56
      Tejun Heo 提交于
      disk->__part used to be statically allocated to the maximum possible
      number of partitions.  This patch makes partition array allocation
      dynamic.  The added overhead is minimal as only real change is one
      memory dereference changed to RCU one.  This saves both a bit of
      memory and cpu cycles iterating through unoccupied slots and makes
      increasing partition limit easier.
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      540eed56
    • T
      block: move stats from disk to part0 · 074a7aca
      Tejun Heo 提交于
      Move stats related fields - stamp, in_flight, dkstats - from disk to
      part0 and unify stat handling such that...
      
      * part_stat_*() now updates part0 together if the specified partition
        is not part0.  ie. part_stat_*() are now essentially all_stat_*().
      
      * {disk|all}_stat_*() are gone.
      
      * part_round_stats() is updated similary.  It handles part0 stats
        automatically and disk_round_stats() is killed.
      
      * part_{inc|dec}_in_fligh() is implemented which automatically updates
        part0 stats for parts other than part0.
      
      * disk_map_sector_rcu() is updated to return part0 if no part matches.
        Combined with the above changes, this makes NULL special case
        handling in callers unnecessary.
      
      * Separate stats show code paths for disk are collapsed into part
        stats show code paths.
      
      * Rename disk_stat_lock/unlock() to part_stat_lock/unlock()
      
      While at it, reposition stat handling macros a bit and add missing
      parentheses around macro parameters.
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      074a7aca
    • T
      block: kill GENHD_FL_FAIL and use part0->make_it_fail · eddb2e26
      Tejun Heo 提交于
      GENHD_FL_FAIL for disk is what make_it_fail is for parts.  Kill it and
      use part0->make_it_fail.  Sysfs node handling is unified too.
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      eddb2e26
    • T
      block: always set bdev->bd_part · 0762b8bd
      Tejun Heo 提交于
      Till now, bdev->bd_part is set only if the bdev was for parts other
      than part0.  This patch makes bdev->bd_part always set so that code
      paths don't have to differenciate common handling.
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      0762b8bd
    • T
      block: move holder_dir from disk to part0 · 4c46501d
      Tejun Heo 提交于
      Move disk->holder_dir to part0->holder_dir.  Kill now mostly
      superflous bdev_get_holder().
      
      While at it, kill superflous kobject_get/put() around holder_dir,
      slave_dir and cmd_filter creation and collapse
      disk_sysfs_add_subdirs() into register_disk().  These serve no purpose
      but obfuscating the code.
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      4c46501d
    • T
      block: move policy from disk to part0 · b7db9956
      Tejun Heo 提交于
      Move disk->policy to part0->policy.  Implement and use get_disk_ro().
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      b7db9956
    • T
      block: unify sysfs size node handling · e5610521
      Tejun Heo 提交于
      Now that capacity and __dev are moved to part0, part0 and others can
      share the same method.
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      e5610521
    • T
      block: move __dev from disk to part0 · 548b10eb
      Tejun Heo 提交于
      Move disk->__dev to part0->__dev.  This simplifies bdget_disk() and
      lookup_devt() and allows common sysfs attributes to be unified.
      part_to_disk() is updated to handle part0 -> disk.
      
      Updated to include a fix from Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>,
      he writes:
      
      "part0 is a "special" partition and doesn't need to have capacity set - this
      fixes regression caused by "block: move __dev from disk to part0" commit."
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      548b10eb
    • T
      block: move capacity from disk to part0 · 80795aef
      Tejun Heo 提交于
      Move disk->capacity to part0->nr_sects and convert all users who
      directly accessed the field to use {get|set}_capacity().  This is done
      early to allow the __dev field to be moved.
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      80795aef
    • T
      block: introduce partition 0 · b5d0b9df
      Tejun Heo 提交于
      genhd and partition code handled disk and partitions separately.  All
      information about the whole disk was in struct genhd and partitions in
      struct hd_struct.  However, the whole disk (part0) and other
      partitions have a lot in common and the data structures end up having
      good number of common fields and thus separate code paths doing the
      same thing.  Also, the partition array was indexed by partno - 1 which
      gets pretty confusing at times.
      
      This patch introduces partition 0 and makes the partition array
      indexed by partno.  Following patches will unify the handling of disk
      and parts piece-by-piece.
      
      This patch also implements disk_partitionable() which tests whether a
      disk is partitionable.  With coming dynamic partition array change,
      the most common usage of disk_max_parts() will be testing whether a
      disk is partitionable and the number of max partitions will become
      much less important.
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      b5d0b9df
    • T
      block: implement and use {disk|part}_to_dev() · ed9e1982
      Tejun Heo 提交于
      Implement {disk|part}_to_dev() and use them to access generic device
      instead of directly dereferencing {disk|part}->dev.  To make sure no
      user is left behind, rename generic devices fields to __dev.
      
      This is in preparation of unifying partition 0 handling with other
      partitions.
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      ed9e1982
    • T
      block: implement CONFIG_DEBUG_BLOCK_EXT_DEVT · 870d6656
      Tejun Heo 提交于
      Extended devt introduces non-contiguos device numbers.  This patch
      implements a debug option which forces most devt allocations to be
      from the extended area and spreads them out.  This is enabled by
      default if DEBUG_KERNEL is set and achieves...
      
      1. Detects code paths in kernel or userland which expect predetermined
         consecutive device numbers.
      
      2. When something goes wrong, avoid corruption as adding to the minor
         of earlier partition won't lead to the wrong but valid device.
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      870d6656
    • T
      sd/ide-disk: apply extended minors to sd and ide · f615b48c
      Tejun Heo 提交于
      Update sd and ide-disk such that they can take advantage of extended
      minors.
      
      ide-disk already has 64 minors per device and currently doesn't use
      extended minors although after this patch it can be turned on by
      simply tweaking constants.
      
      sd only had 16 minors per device causing problems on certain peculiar
      configurations.  This patch lifts the restriction and enables it to
      use upto 64 minors.
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      f615b48c
    • T
      block: adjust formatting for large minors and add ext_range sysfs attr · 1f014290
      Tejun Heo 提交于
      With extended minors and the soon-to-follow debug feature, large minor
      numbers for block devices will be common.  This patch does the
      followings to make printouts pretty.
      
      * Adapt print formats such that large minors don't break the
        formatting.
      
      * For extended MAJ:MIN, %02x%02x for MAJ:MIN used in
        printk_all_partitions() doesn't cut it anymore.  Update it such that
        %03x:%05x is used if either MAJ or MIN doesn't fit in %02x.
      
      * Implement ext_range sysfs attribute which shows total minors the
        device can use including both conventional minor space and the
        extended one.
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      1f014290
    • T
      block: implement extended dev numbers · bcce3de1
      Tejun Heo 提交于
      Implement extended device numbers.  A block driver can tell block
      layer that it wants to use extended device numbers.  After the usual
      minor space is used up, block layer automatically allocates devt's
      from EXT_BLOCK_MAJOR.
      
      Currently only one major number is allocated for this but as the
      allocation is strictly on-demand, ~1mil minor space under it should
      suffice unless the system actually has more than ~1mil partitions and
      if that ever happens adding more majors to the extended devt area is
      easy.
      
      Due to internal implementation issues, the first partition can't be
      allocated on the extended area.  In other words, genhd->minors should
      at least be 1.  This limitation will be lifted by later changes.
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      bcce3de1
    • T
      block: fix diskstats access · c9959059
      Tejun Heo 提交于
      There are two variants of stat functions - ones prefixed with double
      underbars which don't care about preemption and ones without which
      disable preemption before manipulating per-cpu counters.  It's unclear
      whether the underbarred ones assume that preemtion is disabled on
      entry as some callers don't do that.
      
      This patch unifies diskstats access by implementing disk_stat_lock()
      and disk_stat_unlock() which take care of both RCU (for partition
      access) and preemption (for per-cpu counter access).  diskstats access
      should always be enclosed between the two functions.  As such, there's
      no need for the versions which disables preemption.  They're removed
      and double underbars ones are renamed to drop the underbars.  As an
      extra argument is added, there's no danger of using the old version
      unconverted.
      
      disk_stat_lock() uses get_cpu() and returns the cpu index and all
      diskstat functions which access per-cpu counters now has @cpu
      argument to help RT.
      
      This change adds RCU or preemption operations at some places but also
      collapses several preemption ops into one at others.  Overall, the
      performance difference should be negligible as all involved ops are
      very lightweight per-cpu ones.
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Cc: Peter Zijlstra <peterz@infradead.org>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      c9959059
    • T
      block: fix disk->part[] dereferencing race · e71bf0d0
      Tejun Heo 提交于
      disk->part[] is protected by its matching bdev's lock.  However,
      non-critical accesses like collecting stats and printing out sysfs and
      proc information used to be performed without any locking.  As
      partitions can come and go dynamically, partitions can go away
      underneath those non-critical accesses.  As some of those accesses are
      writes, this theoretically can lead to silent corruption.
      
      This patch fixes the race by using RCU for the partition array and dev
      reference counter to hold partitions.
      
      * Rename disk->part[] to disk->__part[] to make sure no one outside
        genhd layer proper accesses it directly.
      
      * Use RCU for disk->__part[] dereferencing.
      
      * Implement disk_{get|put}_part() which can be used to get and put
        partitions from gendisk respectively.
      
      * Iterators are implemented to help iterate through all partitions
        safely.
      
      * Functions which require RCU readlock are marked with _rcu suffix.
      
      * Use disk_put_part() in __blkdev_put() instead of directly putting
        the contained kobject.
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      e71bf0d0
    • T
      block: don't depend on consecutive minor space · f331c029
      Tejun Heo 提交于
      * Implement disk_devt() and part_devt() and use them to directly
        access devt instead of computing it from ->major and ->first_minor.
      
        Note that all references to ->major and ->first_minor outside of
        block layer is used to determine devt of the disk (the part0) and as
        ->major and ->first_minor will continue to represent devt for the
        disk, converting these users aren't strictly necessary.  However,
        convert them for consistency.
      
      * Implement disk_max_parts() to avoid directly deferencing
        genhd->minors.
      
      * Update bdget_disk() such that it doesn't assume consecutive minor
        space.
      
      * Move devt computation from register_disk() to add_disk() and make it
        the only one (all other usages use the initially determined value).
      
      These changes clean up the code and will help disk->part dereference
      fix and extended block device numbers.
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      f331c029
    • T
      block: make variable and argument names more consistent · cf771cb5
      Tejun Heo 提交于
      In hd_struct, @partno is used to denote partition number and a number
      of other places use @part to denote hd_struct.  Functions use @part
      and @index instead.  This causes confusion and makes it difficult to
      use consistent variable names for hd_struct.  Always use @partno if a
      variable represents partition number.
      
      Also, print out functions use @f or @part for seq_file argument.  Use
      @seqf uniformly instead.
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      cf771cb5
    • T
      block: misc updates · 310a2c10
      Tejun Heo 提交于
      This patch makes the following misc updates in preparation for
      disk->part dereference fix and extended block devt support.
      
      * implment part_to_disk()
      
      * fix comment about gendisk->part indexing
      
      * rename get_part() to disk_map_sector()
      
      * don't use n which is always zero while printing disk information in
        diskstats_show()
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      310a2c10
    • T
      block: update add_partition() error handling · 88e34126
      Tejun Heo 提交于
      d805dda4 tried to fix error case handling in add_partition() but had a
      few problems.
      
      * disk->part[] entry is set early and left dangling if operation
        fails.
      
      * Once device initialized, the last put_device() is responsible for
        freeing all the resources.  The failure path freed part_stats and p
        regardless of put_device() causing double free.
      
      * holders subdir holds reference to the disk device, so failure path
        should remove it to release resources properly which was missing.
      
      This patch fixes the above problems and while at it move partition
      slot busy check into add_partition() for completeness and inlines
      holders subdirectory creation.  Using separate function for it just
      obfuscates the code.
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Cc: Abdel Benamrouche <draconux@gmail.com>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      88e34126
    • T
      block: allow deleting zero length partition · ec2cdedf
      Tejun Heo 提交于
      delete_partition() was noop for zero length partition.  As the
      addition code allows creating zero lenght partition and deletion is
      assumed to always succeed, this causes memory leak for zero length
      partitions.  Allow zero length partitions to end their meaningless
      lives.
      
      While at it, allow deleting zero lenght partition via
      BLKPG_DEL_PARTITION ioctl too.
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      ec2cdedf
    • T
      block: use class_dev_iterator instead of class_for_each_device() · def4e38d
      Tejun Heo 提交于
      Recent block_class iteration updates 5c6f35c5..27f30251 converted all
      class device iteration to class_for_each_device() and
      class_find_device(), which are correct but pain in the ass to use.
      This pach converts them to newly introduced class_dev_iterator so that
      they can use more natural control structures instead of separate
      callbacks and struct to pass parameters to them.
      
      This results in smaller and easier code.
      
      This patch also restores the original behavior of not printing header
      in /proc/partitions if there's no partition to print.  This is trivial
      but still user-visible behavior.
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Cc: Greg Kroah-Hartman <gregkh@suse.de>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      def4e38d
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      block: don't grab block_class_lock unnecessarily · 2ac3cee5
      Tejun Heo 提交于
      block_class_lock protects major_names array and bdev_map and doesn't
      have anything to do with block class devices.  Don't grab them while
      iterating over block class devices.
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      2ac3cee5
    • T
      block: fix partition info printouts · ac65ece4
      Tejun Heo 提交于
      Recent block_class iteration updates 5c6f35c5..27f30251 broke partition
      info printouts.
      
      * printk_all_partitions(): Partition print out stops when it meets a
        partition hole.  Partition printing inner loop should continue
        instead of exiting on empty partition slot.
      
      * /proc/partitions and /proc/diskstats: If all information can't be
        read in single read(), the information is truncated.  This is
        because find_start() doesn't actually update the counter containing
        the initial seek.  It runs to the end and ends up always reporting
        EOF on the second read.
      
      This patch fixes both problems.
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Cc: Greg Kroah-Hartman <gregkh@suse.de>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      ac65ece4
    • T
      driver-core: use klist for class device list and implement iterator · 5a3ceb86
      Tejun Heo 提交于
      Iterating over entries using callback usually isn't too fun especially
      when the entry being iterated over can't be manipulated freely.  This
      patch converts class->p->class_devices to klist and implements class
      device iterator so that the users can freely build their own control
      structure.  The users are also free to call back into class code
      without worrying about locking.
      
      class_for_each_device() and class_find_device() are converted to use
      the new iterators, so their users don't have to worry about locking
      anymore either.
      
      Note: This depends on klist-dont-iterate-over-deleted-entries patch
      because class_intf->add/remove_dev() depends on proper synchronization
      with device removal.
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Cc: Greg Kroah-Hartman <gregkh@suse.de>
      Cc: Jens Axboe <jens.axboe@oracle.com>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      5a3ceb86
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      klist: don't iterate over deleted entries · a1ed5b0c
      Tejun Heo 提交于
      A klist entry is kept on the list till all its current iterations are
      finished; however, a new iteration after deletion also iterates over
      deleted entries as long as their reference count stays above zero.
      This causes problems for cases where there are users which iterate
      over the list while synchronized against list manipulations and
      natuarally expect already deleted entries to not show up during
      iteration.
      
      This patch implements dead flag which gets set on deletion so that
      iteration can skip already deleted entries.  The dead flag piggy backs
      on the lowest bit of knode->n_klist and only visible to klist
      implementation proper.
      
      While at it, drop klist_iter->i_head as it's redundant and doesn't
      offer anything in semantics or performance wise as klist_iter->i_klist
      is dereferenced on every iteration anyway.
      Signed-off-by: NTejun Heo <tj@kernel.org>
      Cc: Greg Kroah-Hartman <gregkh@suse.de>
      Cc: Alan Stern <stern@rowland.harvard.edu>
      Cc: Jens Axboe <jens.axboe@oracle.com>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      a1ed5b0c
    • R
      Add some block/ source files to the kernel-api docbook. Fix kernel-doc... · 710027a4
      Randy Dunlap 提交于
      Add some block/ source files to the kernel-api docbook. Fix kernel-doc notation in them as needed. Fix changed function parameter names. Fix typos/spellos. In comments, change REQ_SPECIAL to REQ_TYPE_SPECIAL and REQ_BLOCK_PC to REQ_TYPE_BLOCK_PC.
      Signed-off-by: NRandy Dunlap <randy.dunlap@oracle.com>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      710027a4
    • J
      block: make bi_phys_segments an unsigned int instead of short · 5b99c2ff
      Jens Axboe 提交于
      raid5 can overflow with more than 255 stripes, and we can increase it
      to an int for free on both 32 and 64-bit archs due to the padding.
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      5b99c2ff
    • J
      block: raid fixups for removal of bi_hw_segments · 960e739d
      Jens Axboe 提交于
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      960e739d
    • M
      drop vmerge accounting · 5df97b91
      Mikulas Patocka 提交于
      Remove hw_segments field from struct bio and struct request. Without virtual
      merge accounting they have no purpose.
      Signed-off-by: NMikulas Patocka <mpatocka@redhat.com>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      5df97b91
    • M
      block: drop virtual merging accounting · b8b3e16c
      Mikulas Patocka 提交于
      Remove virtual merge accounting.
      Signed-off-by: NMikulas Patocka <mpatocka@redhat.com>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      b8b3e16c
    • A
      block: update documentation for deadline fifo_batch tunable · 6a421c1d
      Aaron Carroll 提交于
      Update the description of fifo_batch to match the current implementation,
      and include a description of how to tune it.
      Signed-off-by: NAaron Carroll <aaronc@gelato.unsw.edu.au>
      Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
      6a421c1d