Merge branch 'for-linus' of git://git.kernel.dk/linux-block

Pull block fixes from Jens Axboe:
 "A smaller collection of fixes that should go into -rc1. This contains:

   - A fix from Christoph, fixing a regression with the WRITE_SAME and
     partial completions. Caused a BUG() on ppc.

   - Fixup for __blk_mq_stop_hw_queues(), it should be static. From
     Colin.

   - Removal of dmesg error messages on elevator switching, when invoked
     from sysfs. From me.

   - Fix for blk-stat, using this_cpu_ptr() in a section only protected
     by rcu_read_lock(). This breaks when PREEMPT_RCU is enabled. From
     me.

   - Two fixes for BFQ from Paolo, one fixing a crash and one updating
     the documentation.

   - An error handling lightnvm memory leak, from Rakesh.

   - The previous blk-mq hot unplug lock reversal depends on the CPU
     hotplug rework that isn't in mainline yet. This caused a lockdep
     splat when people unplugged CPUs with blk-mq devices. From Wanpeng.

   - A regression fix for DIF/DIX on blk-mq. From Wen"

* 'for-linus' of git://git.kernel.dk/linux-block:
  block: handle partial completions for special payload requests
  blk-mq: NVMe 512B/4K+T10 DIF/DIX format returns I/O error on dd with split op
  blk-stat: don't use this_cpu_ptr() in a preemptable section
  elevator: remove redundant warnings on IO scheduler switch
  block, bfq: stress that low_latency must be off to get max throughput
  block, bfq: use pointer entity->sched_data only if set
  nvme: lightnvm: fix memory leak
  blk-mq: make __blk_mq_stop_hw_queues static
  lightnvm: remove unused rq parameter of nvme_nvm_rqtocmd() to kill warning
  block/mq: fix potential deadlock during cpu hotplug

1 files changed, 16 insertions, 1 deletions

diff --git a/Documentation/block/bfq-iosched.txt b/Documentation/block/bfq-iosched.txt
index 1b87df6cd4761..05e2822a80b34 100644
--- a/Documentation/block/bfq-iosched.txt
+++ b/Documentation/block/bfq-iosched.txt
@@ -11,6 +11,13 @@ controllers), BFQ's main features are:
   groups (switching back to time distribution when needed to keep
   throughput high).
 
+In its default configuration, BFQ privileges latency over
+throughput. So, when needed for achieving a lower latency, BFQ builds
+schedules that may lead to a lower throughput. If your main or only
+goal, for a given device, is to achieve the maximum-possible
+throughput at all times, then do switch off all low-latency heuristics
+for that device, by setting low_latency to 0. Full details in Section 3.
+
 On average CPUs, the current version of BFQ can handle devices
 performing at most ~30K IOPS; at most ~50 KIOPS on faster CPUs. As a
 reference, 30-50 KIOPS correspond to very high bandwidths with
@@ -375,11 +382,19 @@ default, low latency mode is enabled. If enabled, interactive and soft
 real-time applications are privileged and experience a lower latency,
 as explained in more detail in the description of how BFQ works.
 
-DO NOT enable this mode if you need full control on bandwidth
+DISABLE this mode if you need full control on bandwidth
 distribution. In fact, if it is enabled, then BFQ automatically
 increases the bandwidth share of privileged applications, as the main
 means to guarantee a lower latency to them.
 
+In addition, as already highlighted at the beginning of this document,
+DISABLE this mode if your only goal is to achieve a high throughput.
+In fact, privileging the I/O of some application over the rest may
+entail a lower throughput. To achieve the highest-possible throughput
+on a non-rotational device, setting slice_idle to 0 may be needed too
+(at the cost of giving up any strong guarantee on fairness and low
+latency).
+
 timeout_sync
 ------------