ioc_pd_free() grabs irq-safe ioc->lock without ensuring that irq is disabled
when it can be called with irq disabled or enabled. This has a small chance
of causing A-A deadlocks and triggers lockdep splats. Use irqsave operations
instead.
Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: 7caa47151a ("blkcg: implement blk-iocost")
Cc: stable@vger.kernel.org # v5.4+
Signed-off-by: Jens Axboe <axboe@kernel.dk>
- Untangle the header spaghetti which causes build failures in various
situations caused by the lockdep additions to seqcount to validate that
the write side critical sections are non-preemptible.
- The seqcount associated lock debug addons which were blocked by the
above fallout.
seqcount writers contrary to seqlock writers must be externally
serialized, which usually happens via locking - except for strict per
CPU seqcounts. As the lock is not part of the seqcount, lockdep cannot
validate that the lock is held.
This new debug mechanism adds the concept of associated locks.
sequence count has now lock type variants and corresponding
initializers which take a pointer to the associated lock used for
writer serialization. If lockdep is enabled the pointer is stored and
write_seqcount_begin() has a lockdep assertion to validate that the
lock is held.
Aside of the type and the initializer no other code changes are
required at the seqcount usage sites. The rest of the seqcount API is
unchanged and determines the type at compile time with the help of
_Generic which is possible now that the minimal GCC version has been
moved up.
Adding this lockdep coverage unearthed a handful of seqcount bugs which
have been addressed already independent of this.
While generaly useful this comes with a Trojan Horse twist: On RT
kernels the write side critical section can become preemtible if the
writers are serialized by an associated lock, which leads to the well
known reader preempts writer livelock. RT prevents this by storing the
associated lock pointer independent of lockdep in the seqcount and
changing the reader side to block on the lock when a reader detects
that a writer is in the write side critical section.
- Conversion of seqcount usage sites to associated types and initializers.
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Merge tag 'locking-urgent-2020-08-10' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
Pull locking updates from Thomas Gleixner:
"A set of locking fixes and updates:
- Untangle the header spaghetti which causes build failures in
various situations caused by the lockdep additions to seqcount to
validate that the write side critical sections are non-preemptible.
- The seqcount associated lock debug addons which were blocked by the
above fallout.
seqcount writers contrary to seqlock writers must be externally
serialized, which usually happens via locking - except for strict
per CPU seqcounts. As the lock is not part of the seqcount, lockdep
cannot validate that the lock is held.
This new debug mechanism adds the concept of associated locks.
sequence count has now lock type variants and corresponding
initializers which take a pointer to the associated lock used for
writer serialization. If lockdep is enabled the pointer is stored
and write_seqcount_begin() has a lockdep assertion to validate that
the lock is held.
Aside of the type and the initializer no other code changes are
required at the seqcount usage sites. The rest of the seqcount API
is unchanged and determines the type at compile time with the help
of _Generic which is possible now that the minimal GCC version has
been moved up.
Adding this lockdep coverage unearthed a handful of seqcount bugs
which have been addressed already independent of this.
While generally useful this comes with a Trojan Horse twist: On RT
kernels the write side critical section can become preemtible if
the writers are serialized by an associated lock, which leads to
the well known reader preempts writer livelock. RT prevents this by
storing the associated lock pointer independent of lockdep in the
seqcount and changing the reader side to block on the lock when a
reader detects that a writer is in the write side critical section.
- Conversion of seqcount usage sites to associated types and
initializers"
* tag 'locking-urgent-2020-08-10' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (25 commits)
locking/seqlock, headers: Untangle the spaghetti monster
locking, arch/ia64: Reduce <asm/smp.h> header dependencies by moving XTP bits into the new <asm/xtp.h> header
x86/headers: Remove APIC headers from <asm/smp.h>
seqcount: More consistent seqprop names
seqcount: Compress SEQCNT_LOCKNAME_ZERO()
seqlock: Fold seqcount_LOCKNAME_init() definition
seqlock: Fold seqcount_LOCKNAME_t definition
seqlock: s/__SEQ_LOCKDEP/__SEQ_LOCK/g
hrtimer: Use sequence counter with associated raw spinlock
kvm/eventfd: Use sequence counter with associated spinlock
userfaultfd: Use sequence counter with associated spinlock
NFSv4: Use sequence counter with associated spinlock
iocost: Use sequence counter with associated spinlock
raid5: Use sequence counter with associated spinlock
vfs: Use sequence counter with associated spinlock
timekeeping: Use sequence counter with associated raw spinlock
xfrm: policy: Use sequence counters with associated lock
netfilter: nft_set_rbtree: Use sequence counter with associated rwlock
netfilter: conntrack: Use sequence counter with associated spinlock
sched: tasks: Use sequence counter with associated spinlock
...
We shouldn't skip iocg when its abs_vdebt is not zero.
Fixes: 0b80f9866e ("iocost: protect iocg->abs_vdebt with iocg->waitq.lock")
Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Daniel Wagner <dwagner@suse.de>
Link: https://lkml.kernel.org/r/20200720155530.1173732-21-a.darwish@linutronix.de
Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes.
This code was detected with the help of Coccinelle and, audited and
fixed manually.
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Addresses-KSPP-ID: https://github.com/KSPP/linux/issues/83
Signed-off-by: Jens Axboe <axboe@kernel.dk>
When the QoS targets are met and nothing is being throttled, there's
no way to tell how saturated the underlying device is - it could be
almost entirely idle, at the cusp of saturation or anywhere inbetween.
Given that there's no information, it's best to keep vrate as-is in
this state. Before 7cd806a9a9 ("iocost: improve nr_lagging
handling"), this was the case - if the device isn't missing QoS
targets and nothing is being throttled, busy_level was reset to zero.
While fixing nr_lagging handling, 7cd806a9a9 ("iocost: improve
nr_lagging handling") broke this. Now, while the device is hitting
QoS targets and nothing is being throttled, vrate keeps getting
adjusted according to the existing busy_level.
This led to vrate keeping climing till it hits max when there's an IO
issuer with limited request concurrency if the vrate started low.
vrate starts getting adjusted upwards until the issuer can issue IOs
w/o being throttled. From then on, QoS targets keeps getting met and
nothing on the system needs throttling and vrate keeps getting
increased due to the existing busy_level.
This patch makes the following changes to the busy_level logic.
* Reset busy_level if nr_shortages is zero to avoid the above
scenario.
* Make non-zero nr_lagging block lowering nr_level but still clear
positive busy_level if there's clear non-saturation signal - QoS
targets are met and nr_shortages is non-zero. nr_lagging's role is
preventing adjusting vrate upwards while there are long-running
commands and it shouldn't keep busy_level positive while there's
clear non-saturation signal.
* Restructure code for clarity and add comments.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Andy Newell <newella@fb.com>
Fixes: 7cd806a9a9 ("iocost: improve nr_lagging handling")
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Pull in block-5.7 fixes for 5.8. Mostly to resolve a conflict with
the blk-iocost changes, but we also need the base of the bdi
use-after-free as well as we build on top of it.
* block-5.7:
nvme: fix possible hang when ns scanning fails during error recovery
nvme-pci: fix "slimmer CQ head update"
bdi: add a ->dev_name field to struct backing_dev_info
bdi: use bdi_dev_name() to get device name
bdi: move bdi_dev_name out of line
vboxsf: don't use the source name in the bdi name
iocost: protect iocg->abs_vdebt with iocg->waitq.lock
block: remove the bd_openers checks in blk_drop_partitions
nvme: prevent double free in nvme_alloc_ns() error handling
null_blk: Cleanup zoned device initialization
null_blk: Fix zoned command handling
block: remove unused header
blk-iocost: Fix error on iocost_ioc_vrate_adj
bdev: Reduce time holding bd_mutex in sync in blkdev_close()
buffer: remove useless comment and WB_REASON_FREE_MORE_MEM, reason.
Signed-off-by: Jens Axboe <axboe@kernel.dk>
abs_vdebt is an atomic_64 which tracks how much over budget a given cgroup
is and controls the activation of use_delay mechanism. Once a cgroup goes
over budget from forced IOs, it has to pay it back with its future budget.
The progress guarantee on debt paying comes from the iocg being active -
active iocgs are processed by the periodic timer, which ensures that as time
passes the debts dissipate and the iocg returns to normal operation.
However, both iocg activation and vdebt handling are asynchronous and a
sequence like the following may happen.
1. The iocg is in the process of being deactivated by the periodic timer.
2. A bio enters ioc_rqos_throttle(), calls iocg_activate() which returns
without anything because it still sees that the iocg is already active.
3. The iocg is deactivated.
4. The bio from #2 is over budget but needs to be forced. It increases
abs_vdebt and goes over the threshold and enables use_delay.
5. IO control is enabled for the iocg's subtree and now IOs are attributed
to the descendant cgroups and the iocg itself no longer issues IOs.
This leaves the iocg with stuck abs_vdebt - it has debt but inactive and no
further IOs which can activate it. This can end up unduly punishing all the
descendants cgroups.
The usual throttling path has the same issue - the iocg must be active while
throttled to ensure that future event will wake it up - and solves the
problem by synchronizing the throttling path with a spinlock. abs_vdebt
handling is another form of overage handling and shares a lot of
characteristics including the fact that it isn't in the hottest path.
This patch fixes the above and other possible races by strictly
synchronizing abs_vdebt and use_delay handling with iocg->waitq.lock.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Vlad Dmitriev <vvd@fb.com>
Cc: stable@vger.kernel.org # v5.4+
Fixes: e1518f63f2 ("blk-iocost: Don't let merges push vtime into the future")
Signed-off-by: Jens Axboe <axboe@kernel.dk>
On each IO completion, iocost decides whether the IO met or missed its latency
target. Currently, the targets are fixed numbers per IO type. While this can be
good enough for loose latency targets way higher than typical completion
latencies, the effect of IO size makes it difficult to tighten the latency
target - a target adequate for 4k IOs might be too tight for 512k IOs and
vice-versa.
iocost already has all the necessary information to account for different IO
sizes when testing whether the latency target is met as iocost can calculate the
size vtime cost of a given IO. This patch updates the completion path to
calculate the size vtime cost of the IO, deduct the nsec equivalent from the
observed latency and use the adjusted value to decide whether the target is met.
This makes latency targets independent from IO size and enables determining
adequate latency targets with fixed size fio runs.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Andy Newell <newella@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
The use_delay mechanism was introduced by blk-iolatency to hold memory
allocators accountable for the reclaim and other shared IOs they cause. The
duration of the delay is dynamically balanced between iolatency increasing the
value on each target miss and it auto-decaying as time passes and threads get
delayed on it.
While this works well for iolatency, iocost's control model isn't compatible
with it. There is no repeated "violation" events which can be balanced against
auto-decaying. iocost instead knows how much a given cgroup is over budget and
wants to prevent that cgroup from issuing IOs while over budget. Until now,
iocost has been adding the cost of force-issued IOs. However, this doesn't
reflect the amount which is already over budget and is simply not enough to
counter the auto-decaying allowing anon-memory leaking low priority cgroup to
go over its alloted share of IOs.
As auto-decaying doesn't make much sense for iocost, this patch introduces a
different mode of operation for use_delay - when blkcg_set_delay() are used
insted of blkcg_add/use_delay(), the delay duration is not auto-decayed until it
is explicitly cleared with blkcg_clear_delay(). iocost is updated to keep the
delay duration synchronized to the budget overage amount.
With this change, iocost can effectively police cgroups which generate
significant amount of force-issued IOs.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Systemtap 4.2 is unable to correctly interpret the "u32 (*missed_ppm)[2]"
argument of the iocost_ioc_vrate_adj trace entry defined in
include/trace/events/iocost.h leading to the following error:
/tmp/stapAcz0G0/stap_c89c58b83cea1724e26395efa9ed4939_6321_aux_6.c:78:8:
error: expected ‘;’, ‘,’ or ‘)’ before ‘*’ token
, u32[]* __tracepoint_arg_missed_ppm
That argument type is indeed rather complex and hard to read. Looking
at block/blk-iocost.c. It is just a 2-entry u32 array. By simplifying
the argument to a simple "u32 *missed_ppm" and adjusting the trace
entry accordingly, the compilation error was gone.
Fixes: 7caa47151a ("blkcg: implement blk-iocost")
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
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Merge tag 'for-5.7/block-2020-03-29' of git://git.kernel.dk/linux-block
Pull block updates from Jens Axboe:
- Online capacity resizing (Balbir)
- Number of hardware queue change fixes (Bart)
- null_blk fault injection addition (Bart)
- Cleanup of queue allocation, unifying the node/no-node API
(Christoph)
- Cleanup of genhd, moving code to where it makes sense (Christoph)
- Cleanup of the partition handling code (Christoph)
- disk stat fixes/improvements (Konstantin)
- BFQ improvements (Paolo)
- Various fixes and improvements
* tag 'for-5.7/block-2020-03-29' of git://git.kernel.dk/linux-block: (72 commits)
block: return NULL in blk_alloc_queue() on error
block: move bio_map_* to blk-map.c
Revert "blkdev: check for valid request queue before issuing flush"
block: simplify queue allocation
bcache: pass the make_request methods to blk_queue_make_request
null_blk: use blk_mq_init_queue_data
block: add a blk_mq_init_queue_data helper
block: move the ->devnode callback to struct block_device_operations
block: move the part_stat* helpers from genhd.h to a new header
block: move block layer internals out of include/linux/genhd.h
block: move guard_bio_eod to bio.c
block: unexport get_gendisk
block: unexport disk_map_sector_rcu
block: unexport disk_get_part
block: mark part_in_flight and part_in_flight_rw static
block: mark block_depr static
block: factor out requeue handling from dispatch code
block/diskstats: replace time_in_queue with sum of request times
block/diskstats: accumulate all per-cpu counters in one pass
block/diskstats: more accurate approximation of io_ticks for slow disks
...
vtimes may wrap and time_before/after64() should be used to determine
whether a given vtime is before or after another. iocg_is_idle() was
incorrectly using plain "<" comparison do determine whether done_vtime
is before vtime. Here, the only thing we're interested in is whether
done_vtime matches vtime which indicates that there's nothing in
flight. Let's test for inequality instead.
Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: 7caa47151a ("blkcg: implement blk-iocost")
Cc: stable@vger.kernel.org # v5.4+
Signed-off-by: Jens Axboe <axboe@kernel.dk>
When over-budget IOs are force-issued through root cgroup,
iocg_kick_delay() adjusts the async delay accordingly but doesn't
actually schedule async throttle for the issuing task. This bug is
pretty well masked because sooner or later the offending threads are
gonna get directly throttled on regular IOs or have async delay
scheduled by mem_cgroup_throttle_swaprate().
However, it can affect control quality on filesystem metadata heavy
operations. Let's fix it by invoking blkcg_schedule_throttle() when
iocg_kick_delay() says async delay is needed.
Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: 7caa47151a ("blkcg: implement blk-iocost")
Cc: stable@vger.kernel.org
Reported-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
There is a bug that checking the same active_list over and over again
in iocg_activate(). The intention of the code was checking whether all
the ancestors and self have already been activated. So fix it.
Fixes: 7caa47151a ("blkcg: implement blk-iocost")
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jiufei Xue <jiufei.xue@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This code causes a static analysis warning:
block/blk-iocost.c:2113 ioc_weight_write() error: double lock 'irq'
We disable IRQs in blkg_conf_prep() and re-enable them in
blkg_conf_finish(). IRQ disable/enable should not be nested because
that means the IRQs will be enabled at the first unlock instead of the
second one.
Fixes: 7caa47151a ("blkcg: implement blk-iocost")
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
The default hard disk param sets latency targets at 50ms. As the
default target percentiles are zero, these don't directly regulate
vrate; however, they're still used to calculate the period length -
100ms in this case.
This is excessively low. A SATA drive with QD32 saturated with random
IOs can easily reach avg completion latency of several hundred msecs.
A period duration which is substantially lower than avg completion
latency can lead to wildly fluctuating vrate.
Let's bump up the default latency targets to 250ms so that the period
duration is sufficiently long.
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Some IOs may span multiple periods. As latencies are collected on
completion, the inbetween periods won't register them and may
incorrectly decide to increase vrate. nr_lagging tracks these IOs to
avoid those situations. Currently, whenever there are IOs which are
spanning from the previous period, busy_level is reset to 0 if
negative thus suppressing vrate increase.
This has the following two problems.
* When latency target percentiles aren't set, vrate adjustment should
only be governed by queue depth depletion; however, the current code
keeps nr_lagging active which pulls in latency results and can keep
down vrate unexpectedly.
* When lagging condition is detected, it resets the entire negative
busy_level. This turned out to be way too aggressive on some
devices which sometimes experience extended latencies on a small
subset of commands. In addition, a lagging IO will be accounted as
latency target miss on completion anyway and resetting busy_level
amplifies its impact unnecessarily.
This patch fixes the above two problems by disabling nr_lagging
counting when latency target percentiles aren't set and blocking vrate
increases when there are lagging IOs while leaving busy_level as-is.
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
vrate_adj tracepoint traces vrate changes; however, it does so only
when busy_level is non-zero. busy_level turning to zero can sometimes
be as interesting an event. This patch also enables vrate_adj
tracepoint on other vrate related events - busy_level changes and
non-zero nr_lagging.
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Merges have the same problem that forced-bios had which is fixed by
the previous patch. The cost of a merge is calculated at the time of
issue and force-advances vtime into the future. Until global vtime
catches up, how the cgroup's hweight changes in the meantime doesn't
matter and it often leads to situations where the cost is calculated
at one hweight and paid at a very different one. See the previous
patch for more details.
Fix it by never advancing vtime into the future for merges. If budget
is available, vtime is advanced. Otherwise, the cost is charged as
debt.
This brings merge cost handling in line with issue cost handling in
ioc_rqos_throttle().
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Currently, when a bio needs to be force-charged and there isn't enough
budget, vtime is simply pushed into the future. This means that the
cost of the whole bio is scaled using the current hweight and then
charged immediately. Until the global vtime advances beyond this
future vtime, the cgroup won't be allowed to issue normal IOs.
This is incorrect and can lead to, for example, exploding vrate or
extended stalls if vrate range is constrained. Consider the following
scenario.
1. A cgroup with a very low hweight runs out of budget.
2. A storm of swap-out happens on it. All of them are scaled
according to the current low hweight and charged to vtime pushing
it to a far future.
3. All other cgroups go idle and now the above cgroup has access to
the whole device. However, because vtime is already wound using
the past low hweight, what its current hweight is doesn't matter
until global vtime catches up to the local vtime.
4. As a result, either vrate gets ramped up extremely or the IOs stall
while the underlying device is idle.
This is because the hweight the overage is calculated at is different
from the hweight that it's being paid at.
Fix it by remembering the overage in absoulte vtime and continuously
paying with the actual budget according to the current hweight at each
period.
Note that non-forced bios which wait already remembers the cost in
absolute vtime. This brings forced-bio accounting in line.
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
ioc_pd_free() first cancels the hrtimers and then deactivates the
iocg. However, the iocg timer can run inbetween and reschedule the
hrtimers which will end up running after the iocg is freed leading to
crashes like the following.
general protection fault: 0000 [#1] SMP
...
RIP: 0010:iocg_kick_delay+0xbe/0x1b0
RSP: 0018:ffffc90003598ea0 EFLAGS: 00010046
RAX: 1cee00fd69512b54 RBX: ffff8881bba48400 RCX: 00000000000003e8
RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffff8881bba48400
RBP: 0000000000004e20 R08: 0000000000000002 R09: 00000000000003e8
R10: 0000000000000000 R11: 0000000000000000 R12: ffffc90003598ef0
R13: 00979f3810ad461f R14: ffff8881bba4b400 R15: 25439f950d26e1d1
FS: 0000000000000000(0000) GS:ffff88885f800000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f64328c7e40 CR3: 0000000002409005 CR4: 00000000003606e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<IRQ>
iocg_delay_timer_fn+0x3d/0x60
__hrtimer_run_queues+0xfe/0x270
hrtimer_interrupt+0xf4/0x210
smp_apic_timer_interrupt+0x5e/0x120
apic_timer_interrupt+0xf/0x20
</IRQ>
Fix it by canceling hrtimers after deactivating the iocg.
Fixes: 7caa47151a ("blkcg: implement blk-iocost")
Reported-by: Dave Jones <davej@codemonkey.org.uk>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
blk_iocost_init() forgot to free its percpu stat on the error path.
Fix it.
Fixes: 7caa47151a ("blkcg: implement blk-iocost")
Reported-by: Hillf Danton <hdanton@sina.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Add a script which can be used to generate device-specific iocost
linear model coefficients.
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This patchset implements IO cost model based work-conserving
proportional controller.
While io.latency provides the capability to comprehensively prioritize
and protect IOs depending on the cgroups, its protection is binary -
the lowest latency target cgroup which is suffering is protected at
the cost of all others. In many use cases including stacking multiple
workload containers in a single system, it's necessary to distribute
IO capacity with better granularity.
One challenge of controlling IO resources is the lack of trivially
observable cost metric. The most common metrics - bandwidth and iops
- can be off by orders of magnitude depending on the device type and
IO pattern. However, the cost isn't a complete mystery. Given
several key attributes, we can make fairly reliable predictions on how
expensive a given stream of IOs would be, at least compared to other
IO patterns.
The function which determines the cost of a given IO is the IO cost
model for the device. This controller distributes IO capacity based
on the costs estimated by such model. The more accurate the cost
model the better but the controller adapts based on IO completion
latency and as long as the relative costs across differents IO
patterns are consistent and sensible, it'll adapt to the actual
performance of the device.
Currently, the only implemented cost model is a simple linear one with
a few sets of default parameters for different classes of device.
This covers most common devices reasonably well. All the
infrastructure to tune and add different cost models is already in
place and a later patch will also allow using bpf progs for cost
models.
Please see the top comment in blk-iocost.c and documentation for
more details.
v2: Rebased on top of RQ_ALLOC_TIME changes and folded in Rik's fix
for a divide-by-zero bug in current_hweight() triggered by zero
inuse_sum.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Andy Newell <newella@fb.com>
Cc: Josef Bacik <jbacik@fb.com>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Tejun Heo