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This cycle, a lot of workqueue changes including some that are significant
and invasive.
- During v6.6 cycle, unbound workqueues were updated so that they are more
topology aware and flexible, which among other things improved workqueue
behavior on modern multi-L3 CPUs. In the process, 636b927eba
("workqueue: Make unbound workqueues to use per-cpu pool_workqueues")
switched unbound workqueues to use per-CPU frontend pool_workqueues as a
part of increasing front-back mapping flexibility.
An unwelcome side effect of this change was that this made max concurrency
enforcement per-CPU blowing up the maximum number of allowed concurrent
executions. I incorrectly assumed that this wouldn't cause practical
problems as most unbound workqueue users are self-regulate max
concurrency; however, there definitely are which don't (e.g. on IO paths)
and the drastic increase in the allowed max concurrency led to noticeable
perf regressions in some use cases.
This is now addressed by separating out max concurrency enforcement to a
separate struct - wq_node_nr_active - which makes @max_active consistently
mean system-wide max concurrency regardless of the number of CPUs or
(finally) NUMA nodes. This is a rather invasive and, in places, a bit
clunky; however, the clunkiness rises from the the inherent requirement to
handle the disagreement between the execution locality domain and max
concurrency enforcement domain on some modern machines. See 5797b1c189
("workqueue: Implement system-wide nr_active enforcement for unbound
workqueues") for more details.
- BH workqueue support is added. They are similar to per-CPU workqueues but
execute work items in the softirq context. This is expected to replace
tasklet. However, currently, it's missing the ability to disable and
enable work items which is needed to convert many tasklet users. To avoid
crowding this merge window too much, this will be included in the next
merge window. A separate pull request will be sent for the couple
conversion patches that are currently pending.
- Waiman plugged a long-standing hole in workqueue CPU isolation where
ordered workqueues didn't follow wq_unbound_cpumask updates. Ordered
workqueues now follow the same rules as other unbound workqueues.
- More CPU isolation improvements: Juri fixed another deficit in workqueue
isolation where unbound rescuers don't respect wq_unbound_cpumask.
Leonardo fixed delayed_work timers firing on isolated CPUs.
- Other misc changes.
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Merge tag 'wq-for-6.9' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq
Pull workqueue updates from Tejun Heo:
"This cycle, a lot of workqueue changes including some that are
significant and invasive.
- During v6.6 cycle, unbound workqueues were updated so that they are
more topology aware and flexible, which among other things improved
workqueue behavior on modern multi-L3 CPUs. In the process, commit
636b927eba ("workqueue: Make unbound workqueues to use per-cpu
pool_workqueues") switched unbound workqueues to use per-CPU
frontend pool_workqueues as a part of increasing front-back mapping
flexibility.
An unwelcome side effect of this change was that this made max
concurrency enforcement per-CPU blowing up the maximum number of
allowed concurrent executions. I incorrectly assumed that this
wouldn't cause practical problems as most unbound workqueue users
are self-regulate max concurrency; however, there definitely are
which don't (e.g. on IO paths) and the drastic increase in the
allowed max concurrency led to noticeable perf regressions in some
use cases.
This is now addressed by separating out max concurrency enforcement
to a separate struct - wq_node_nr_active - which makes @max_active
consistently mean system-wide max concurrency regardless of the
number of CPUs or (finally) NUMA nodes. This is a rather invasive
and, in places, a bit clunky; however, the clunkiness rises from
the the inherent requirement to handle the disagreement between the
execution locality domain and max concurrency enforcement domain on
some modern machines.
See commit 5797b1c189 ("workqueue: Implement system-wide
nr_active enforcement for unbound workqueues") for more details.
- BH workqueue support is added.
They are similar to per-CPU workqueues but execute work items in
the softirq context. This is expected to replace tasklet. However,
currently, it's missing the ability to disable and enable work
items which is needed to convert many tasklet users. To avoid
crowding this merge window too much, this will be included in the
next merge window. A separate pull request will be sent for the
couple conversion patches that are currently pending.
- Waiman plugged a long-standing hole in workqueue CPU isolation
where ordered workqueues didn't follow wq_unbound_cpumask updates.
Ordered workqueues now follow the same rules as other unbound
workqueues.
- More CPU isolation improvements: Juri fixed another deficit in
workqueue isolation where unbound rescuers don't respect
wq_unbound_cpumask. Leonardo fixed delayed_work timers firing on
isolated CPUs.
- Other misc changes"
* tag 'wq-for-6.9' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq: (54 commits)
workqueue: Drain BH work items on hot-unplugged CPUs
workqueue: Introduce from_work() helper for cleaner callback declarations
workqueue: Control intensive warning threshold through cmdline
workqueue: Make @flags handling consistent across set_work_data() and friends
workqueue: Remove clear_work_data()
workqueue: Factor out work_grab_pending() from __cancel_work_sync()
workqueue: Clean up enum work_bits and related constants
workqueue: Introduce work_cancel_flags
workqueue: Use variable name irq_flags for saving local irq flags
workqueue: Reorganize flush and cancel[_sync] functions
workqueue: Rename __cancel_work_timer() to __cancel_timer_sync()
workqueue: Use rcu_read_lock_any_held() instead of rcu_read_lock_held()
workqueue: Cosmetic changes
workqueue, irq_work: Build fix for !CONFIG_IRQ_WORK
workqueue: Fix queue_work_on() with BH workqueues
async: Use a dedicated unbound workqueue with raised min_active
workqueue: Implement workqueue_set_min_active()
workqueue: Fix kernel-doc comment of unplug_oldest_pwq()
workqueue: Bind unbound workqueue rescuer to wq_unbound_cpumask
kernel/workqueue: Let rescuers follow unbound wq cpumask changes
...
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