linux.git/kernel/rcu/update.c, branch v4.19

rcu: Add comment to the last sleep in the rcu tasks loop

2018-07-12T22:39:21+00:00

At the end of rcu_tasks_kthread() there's a lonely
schedule_timeout_uninterruptible() call with no apparent rationale for
its existence. But there is. It is to keep the thread from going into
a tight loop if there's some anomaly. That really needs a comment.

Link: http://lkml.kernel.org/r/20180524223839.GU3803@linux.vnet.ibm.com
Signed-off-by: Steven Rostedt (VMware) 
Signed-off-by: Paul E. McKenney

rcu: Speed up calling of RCU tasks callbacks

2018-07-12T22:39:21+00:00

Joel Fernandes found that the synchronize_rcu_tasks() was taking a
significant amount of time. He demonstrated it with the following test:

 # cd /sys/kernel/tracing
 # while [ 1 ]; do x=1; done &
 # echo '__schedule_bug:traceon' > set_ftrace_filter
 # time echo '!__schedule_bug:traceon' > set_ftrace_filter;

real	0m1.064s
user	0m0.000s
sys	0m0.004s

Where it takes a little over a second to perform the synchronize,
because there's a loop that waits 1 second at a time for tasks to get
through their quiescent points when there's a task that must be waited
for.

After discussion we came up with a simple way to wait for holdouts but
increase the time for each iteration of the loop but no more than a
full second.

With the new patch we have:

 # time echo '!__schedule_bug:traceon' > set_ftrace_filter;

real	0m0.131s
user	0m0.000s
sys	0m0.004s

Which drops it down to 13% of what the original wait time was.

Link: http://lkml.kernel.org/r/20180523063815.198302-2-joel@joelfernandes.org
Reported-by: Joel Fernandes (Google) 
Suggested-by: Joel Fernandes (Google) 
Signed-off-by: Steven Rostedt (VMware) 
Signed-off-by: Paul E. McKenney

rcu: Improve RCU-tasks naming and comments

2018-07-12T22:39:15+00:00

The naming and comments associated with some RCU-tasks code make
the faulty assumption that context switches due to cond_resched()
are voluntary.  As several people pointed out, this is not the case.
This commit therefore updates function names and comments to better
reflect current reality.

Reported-by: Byungchul Park 
Reported-by: Joel Fernandes 
Reported-by: Steven Rostedt 
Signed-off-by: Paul E. McKenney

rcu: Move __rcu_read_lock() and __rcu_read_unlock() to tree_plugin.h

2018-05-15T17:27:41+00:00

The __rcu_read_lock() and __rcu_read_unlock() functions were moved
to kernel/rcu/update.c in order to implement tiny preemptible RCU.
However, tiny preemptible RCU was removed from the kernel a long time
ago, so this commit belatedly moves them back into the only remaining
preemptible-RCU code.

Signed-off-by: Paul E. McKenney 
Tested-by: Nicholas Piggin

rcu: Rename cond_resched_rcu_qs() to cond_resched_tasks_rcu_qs()

2018-05-15T17:27:29+00:00

Commit e31d28b6ab8f ("trace: Eliminate cond_resched_rcu_qs() in favor
of cond_resched()") substituted cond_resched() for the earlier call
to cond_resched_rcu_qs().  However, the new-age cond_resched() does
not do anything to help RCU-tasks grace periods because (1) RCU-tasks
is only enabled when CONFIG_PREEMPT=y and (2) cond_resched() is a
complete no-op when preemption is enabled.  This situation results
in hangs when running the trace benchmarks.

A number of potential fixes were discussed on LKML
(https://lkml.kernel.org/r/20180224151240.0d63a059@vmware.local.home),
including making cond_resched() not be a no-op; making cond_resched()
not be a no-op, but only when running tracing benchmarks; reverting
the aforementioned commit (which works because cond_resched_rcu_qs()
does provide an RCU-tasks quiescent state; and adding a call to the
scheduler/RCU rcu_note_voluntary_context_switch() function.  All were
deemed unsatisfactory, either due to added cond_resched() overhead or
due to magic functions inviting cargo culting.

This commit renames cond_resched_rcu_qs() to cond_resched_tasks_rcu_qs(),
which provides a clear hint as to what this function is doing and
why and where it should be used, and then replaces the call to
cond_resched() with cond_resched_tasks_rcu_qs() in the trace benchmark's
benchmark_event_kthread() function.

Reported-by: Steven Rostedt 
Signed-off-by: Paul E. McKenney 
Tested-by: Nicholas Piggin

rcu: Export init_rcu_head() and destroy_rcu_head() to GPL modules

2017-12-08T00:51:49+00:00

Use of init_rcu_head() and destroy_rcu_head() from modules results in
the following build-time error with CONFIG_DEBUG_OBJECTS_RCU_HEAD=y:

	ERROR: "init_rcu_head" [drivers/scsi/scsi_mod.ko] undefined!
	ERROR: "destroy_rcu_head" [drivers/scsi/scsi_mod.ko] undefined!

This commit therefore adds EXPORT_SYMBOL_GPL() for each to allow them to
be used by GPL-licensed kernel modules.

Reported-by: Bart Van Assche 
Reported-by: Stephen Rothwell 
Signed-off-by: Paul E. McKenney 
Signed-off-by: Martin K. Petersen

Merge branch 'sched-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

2017-11-13T21:37:52+00:00

Pull scheduler updates from Ingo Molnar:
 "The main updates in this cycle were:

   - Group balancing enhancements and cleanups (Brendan Jackman)

   - Move CPU isolation related functionality into its separate
     kernel/sched/isolation.c file, with related 'housekeeping_*()'
     namespace and nomenclature et al. (Frederic Weisbecker)

   - Improve the interactive/cpu-intense fairness calculation (Josef
     Bacik)

   - Improve the PELT code and related cleanups (Peter Zijlstra)

   - Improve the logic of pick_next_task_fair() (Uladzislau Rezki)

   - Improve the RT IPI based balancing logic (Steven Rostedt)

   - Various micro-optimizations:

   - better !CONFIG_SCHED_DEBUG optimizations (Patrick Bellasi)

   - better idle loop (Cheng Jian)

   - ... plus misc fixes, cleanups and updates"

* 'sched-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (54 commits)
  sched/core: Optimize sched_feat() for !CONFIG_SCHED_DEBUG builds
  sched/sysctl: Fix attributes of some extern declarations
  sched/isolation: Document isolcpus= boot parameter flags, mark it deprecated
  sched/isolation: Add basic isolcpus flags
  sched/isolation: Move isolcpus= handling to the housekeeping code
  sched/isolation: Handle the nohz_full= parameter
  sched/isolation: Introduce housekeeping flags
  sched/isolation: Split out new CONFIG_CPU_ISOLATION=y config from CONFIG_NO_HZ_FULL
  sched/isolation: Rename is_housekeeping_cpu() to housekeeping_cpu()
  sched/isolation: Use its own static key
  sched/isolation: Make the housekeeping cpumask private
  sched/isolation: Provide a dynamic off-case to housekeeping_any_cpu()
  sched/isolation, watchdog: Use housekeeping_cpumask() instead of ad-hoc version
  sched/isolation: Move housekeeping related code to its own file
  sched/idle: Micro-optimize the idle loop
  sched/isolcpus: Fix "isolcpus=" boot parameter handling when !CONFIG_CPUMASK_OFFSTACK
  x86/tsc: Append the 'tsc=' description for the 'tsc=unstable' boot parameter
  sched/rt: Simplify the IPI based RT balancing logic
  block/ioprio: Use a helper to check for RT prio
  sched/rt: Add a helper to test for a RT task
  ...

sched/isolation: Introduce housekeeping flags

2017-10-27T07:55:29+00:00

Before we implement isolcpus under housekeeping, we need the isolation
features to be more finegrained. For example some people want NOHZ_FULL
without the full scheduler isolation, others want full scheduler
isolation without NOHZ_FULL.

So let's cut all these isolation features piecewise, at the risk of
overcutting it right now. We can still merge some flags later if they
always make sense together.

Signed-off-by: Frederic Weisbecker 
Acked-by: Thomas Gleixner 
Cc: Chris Metcalf 
Cc: Christoph Lameter 
Cc: Linus Torvalds 
Cc: Luiz Capitulino 
Cc: Mike Galbraith 
Cc: Paul E. McKenney 
Cc: Peter Zijlstra 
Cc: Rik van Riel 
Cc: Wanpeng Li 
Link: http://lkml.kernel.org/r/1509072159-31808-9-git-send-email-frederic@kernel.org
Signed-off-by: Ingo Molnar

sched/isolation: Move housekeeping related code to its own file

2017-10-27T07:55:24+00:00

The housekeeping code is currently tied to the NOHZ code. As we are
planning to make housekeeping independent from it, start with moving
the relevant code to its own file.

Signed-off-by: Frederic Weisbecker 
Acked-by: Thomas Gleixner 
Acked-by: Paul E. McKenney 
Cc: Chris Metcalf 
Cc: Christoph Lameter 
Cc: Linus Torvalds 
Cc: Luiz Capitulino 
Cc: Mike Galbraith 
Cc: Peter Zijlstra 
Cc: Rik van Riel 
Cc: Wanpeng Li 
Link: http://lkml.kernel.org/r/1509072159-31808-2-git-send-email-frederic@kernel.org
Signed-off-by: Ingo Molnar

Merge branches 'doc.2017.10.20a', 'fixes.2017.10.19a', 'stall.2017.10.09a' and 'torture.2017.10.09a' into HEAD

2017-10-20T18:11:15+00:00

doc.2017.10.20a: Documentation updates.
fixes.2017.10.19a: Miscellaneous fixes.
stall.2017.10.09a: RCU CPU stall-warning updates.
torture.2017.10.09a: Torture-test updates.