Revision 474095e46cd14421821da3201a9fd6a4c070996b authored by Linus Torvalds on 24 April 2015, 16:28:01 UTC, committed by Linus Torvalds on 24 April 2015, 16:28:01 UTC
Pull md updates from Neil Brown: "More updates that usual this time. A few have performance impacts which hould mostly be positive, but RAID5 (in particular) can be very work-load ensitive... We'll have to wait and see. Highlights: - "experimental" code for managing md/raid1 across a cluster using DLM. Code is not ready for general use and triggers a WARNING if used. However it is looking good and mostly done and having in mainline will help co-ordinate development. - RAID5/6 can now batch multiple (4K wide) stripe_heads so as to handle a full (chunk wide) stripe as a single unit. - RAID6 can now perform read-modify-write cycles which should help performance on larger arrays: 6 or more devices. - RAID5/6 stripe cache now grows and shrinks dynamically. The value set is used as a minimum. - Resync is now allowed to go a little faster than the 'mininum' when there is competing IO. How much faster depends on the speed of the devices, so the effective minimum should scale with device speed to some extent" * tag 'md/4.1' of git://neil.brown.name/md: (58 commits) md/raid5: don't do chunk aligned read on degraded array. md/raid5: allow the stripe_cache to grow and shrink. md/raid5: change ->inactive_blocked to a bit-flag. md/raid5: move max_nr_stripes management into grow_one_stripe and drop_one_stripe md/raid5: pass gfp_t arg to grow_one_stripe() md/raid5: introduce configuration option rmw_level md/raid5: activate raid6 rmw feature md/raid6 algorithms: xor_syndrome() for SSE2 md/raid6 algorithms: xor_syndrome() for generic int md/raid6 algorithms: improve test program md/raid6 algorithms: delta syndrome functions raid5: handle expansion/resync case with stripe batching raid5: handle io error of batch list RAID5: batch adjacent full stripe write raid5: track overwrite disk count raid5: add a new flag to track if a stripe can be batched raid5: use flex_array for scribble data md raid0: access mddev->queue (request queue member) conditionally because it is not set when accessed from dm-raid md: allow resync to go faster when there is competing IO. md: remove 'go_faster' option from ->sync_request() ...
rcu.txt
RCU Concepts
The basic idea behind RCU (read-copy update) is to split destructive
operations into two parts, one that prevents anyone from seeing the data
item being destroyed, and one that actually carries out the destruction.
A "grace period" must elapse between the two parts, and this grace period
must be long enough that any readers accessing the item being deleted have
since dropped their references. For example, an RCU-protected deletion
from a linked list would first remove the item from the list, wait for
a grace period to elapse, then free the element. See the listRCU.txt
file for more information on using RCU with linked lists.
Frequently Asked Questions
o Why would anyone want to use RCU?
The advantage of RCU's two-part approach is that RCU readers need
not acquire any locks, perform any atomic instructions, write to
shared memory, or (on CPUs other than Alpha) execute any memory
barriers. The fact that these operations are quite expensive
on modern CPUs is what gives RCU its performance advantages
in read-mostly situations. The fact that RCU readers need not
acquire locks can also greatly simplify deadlock-avoidance code.
o How can the updater tell when a grace period has completed
if the RCU readers give no indication when they are done?
Just as with spinlocks, RCU readers are not permitted to
block, switch to user-mode execution, or enter the idle loop.
Therefore, as soon as a CPU is seen passing through any of these
three states, we know that that CPU has exited any previous RCU
read-side critical sections. So, if we remove an item from a
linked list, and then wait until all CPUs have switched context,
executed in user mode, or executed in the idle loop, we can
safely free up that item.
Preemptible variants of RCU (CONFIG_PREEMPT_RCU) get the
same effect, but require that the readers manipulate CPU-local
counters. These counters allow limited types of blocking within
RCU read-side critical sections. SRCU also uses CPU-local
counters, and permits general blocking within RCU read-side
critical sections. These variants of RCU detect grace periods
by sampling these counters.
o If I am running on a uniprocessor kernel, which can only do one
thing at a time, why should I wait for a grace period?
See the UP.txt file in this directory.
o How can I see where RCU is currently used in the Linux kernel?
Search for "rcu_read_lock", "rcu_read_unlock", "call_rcu",
"rcu_read_lock_bh", "rcu_read_unlock_bh", "call_rcu_bh",
"srcu_read_lock", "srcu_read_unlock", "synchronize_rcu",
"synchronize_net", "synchronize_srcu", and the other RCU
primitives. Or grab one of the cscope databases from:
http://www.rdrop.com/users/paulmck/RCU/linuxusage/rculocktab.html
o What guidelines should I follow when writing code that uses RCU?
See the checklist.txt file in this directory.
o Why the name "RCU"?
"RCU" stands for "read-copy update". The file listRCU.txt has
more information on where this name came from, search for
"read-copy update" to find it.
o I hear that RCU is patented? What is with that?
Yes, it is. There are several known patents related to RCU,
search for the string "Patent" in RTFP.txt to find them.
Of these, one was allowed to lapse by the assignee, and the
others have been contributed to the Linux kernel under GPL.
There are now also LGPL implementations of user-level RCU
available (http://lttng.org/?q=node/18).
o I hear that RCU needs work in order to support realtime kernels?
This work is largely completed. Realtime-friendly RCU can be
enabled via the CONFIG_PREEMPT_RCU kernel configuration
parameter. However, work is in progress for enabling priority
boosting of preempted RCU read-side critical sections. This is
needed if you have CPU-bound realtime threads.
o Where can I find more information on RCU?
See the RTFP.txt file in this directory.
Or point your browser at http://www.rdrop.com/users/paulmck/RCU/.
o What are all these files in this directory?
See 00-INDEX for the list.
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