The rcu_cpu_kthread_loops variable used to provide debugfs information,
but is no longer used. This commit therefore removes it.
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
Given that RCU has a perfectly good per-CPU rcu_data structure, most
per-CPU quantities should be stored there.
This commit therefore moves the rcu_cpu_kthread_status per-CPU variable
to the rcu_data structure. This also makes this variable unconditionally
present, which should be acceptable given the memory reduction due to the
RCU flavor consolidation and also due to simplifications this will enable.
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
Given that RCU has a perfectly good per-CPU rcu_data structure, most
per-CPU quantities should be stored there.
This commit therefore moves the rcu_cpu_kthread_task per-CPU variable to
the rcu_data structure. This also makes this variable unconditionally
present, which should be acceptable given the memory reduction due to the
RCU flavor consolidation and also due to simplifications this will enable.
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
If rcutorture's forward-progress tests fail while a grace period is not
in progress, it is useful to print the time since the last grace period
ended as a way to detect failure to launch a new grace period. This
commit therefore makes this change.
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
The RCU CPU stall warnings print an estimate of the total number of
RCU callbacks queued in the system, but this estimate leaves out
the callbacks queued for nocbs CPUs. This commit therefore introduces
rcu_get_n_cbs_cpu(), which gives an accurate callback estimate for
both nocbs and normal CPUs, and uses this new function as needed.
This commit also introduces a rcu_get_n_cbs_nocb_cpu() helper function
that returns the number of callbacks for nocbs CPUs or zero otherwise,
and also uses this function in place of direct access to ->nocb_q_count
while in the area (fewer characters, you see).
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
Recent changes have removed the old ->gp_seq_needed field from the
rcu_state structure, which in turn obsoleted a couple of comments in
the rcu_node and rcu_data structures. This commit therefore updates
these comments accordingly.
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: <kernel-team@android.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
The rcu_bh_state and rcu_sched_state variables were removed during the
RCU flavor consolidations, but external declarations remain in tree.h.
This commit therefore removes these obsolete declarations.
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: <kernel-team@android.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
1e64b15a4b ("rcu: Fix grace-period hangs due to race with CPU offline")
added spinlock_t ofl_lock to the rcu_state structure, then takes it with
preemption disabled during CPU offline, which gives the -rt patchset's
sleeping spinlock heartburn.
This commit therefore converts ->ofl_lock to raw_spinlock_t.
Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
The rcu_data structure's ->dynticks_fqs is incremented but never
accesses. Its ->cond_resched_completed field isn't used at all.
This commit therefore removes both fields.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit move ->dynticks from the rcu_dynticks structure to the
rcu_data structure, replacing the field of the same name. It also updates
the code to access ->dynticks from the rcu_data structure and to use the
rcu_data structure rather than following to now-gone ->dynticks field
to the now-gone rcu_dynticks structure. While in the area, this commit
also fixes up comments.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit removes ->dynticks_nesting and ->dynticks_nmi_nesting from
the rcu_dynticks structure and updates the code to access them from the
rcu_data structure.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit removes ->rcu_need_heavy_qs and ->rcu_urgent_qs from the
rcu_dynticks structure and updates the code to access them from the
rcu_data structure.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit removes ->all_lazy, ->nonlazy_posted and ->nonlazy_posted_snap
from the rcu_dynticks structure and updates the code to access them from
the rcu_data structure.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit removes ->last_accelerate and ->last_advance_all from the
rcu_dynticks structure and updates the code to access them from the
rcu_data structure.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit removes ->tick_nohz_enabled_snap from the rcu_dynticks
structure and updates the code to access it from the rcu_data
structure.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Now that there is only ever one rcu_data structure per CPU, there is no
need for a separate rcu_dynticks structure. This commit therefore adds
the rcu_dynticks fields into the rcu_data structure in preparation for
removing the rcu_dynticks structure entirely. Note that the ->dynticks
field will be handled specially because there is a field by that name
in both structures.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Because nohz_full CPUs can leave the scheduler-clock interrupt disabled
even when in kernel mode, RCU cannot rely on rcu_check_callbacks() to
enlist the scheduler's aid in extracting a quiescent state from such CPUs.
This commit therefore more aggressively uses resched_cpu() on nohz_full
CPUs that fail to pass through a quiescent state in a timely manner.
By default, the resched_cpu() beating starts 300 milliseconds into the
quiescent state.
While in the neighborhood, add a ->last_fqs_resched field to the rcu_data
structure in order to rate-limit resched_cpu() calls from the RCU
grace-period kthread.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The ->rcu_qs_ctr counter was intended to allow providing a lightweight
report of a quiescent state to all RCU flavors. But now that there is
only one flavor of RCU in any one running kernel, there is no point in
having this feature. This commit therefore removes the ->rcu_qs_ctr
field from the rcu_dynticks structure and the ->rcu_qs_ctr_snap field
from the rcu_data structure. This results in the "rqc" option to the
rcu_fqs trace event no longer being used, so this commit also removes the
"rqc" description from the header comment.
While in the neighborhood, this commit also causes the forward-progress
request .rcu_need_heavy_qs be set one jiffies_till_sched_qs interval
later in the grace period than the first setting of .rcu_urgent_qs.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Because rcu_barrier() is a one-line wrapper function for _rcu_barrier()
and because nothing else calls _rcu_barrier(), this commit inlines
_rcu_barrier() into rcu_barrier().
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Consolidation of the RCU flavors into one makes increment_cpu_stall_ticks()
a trivial one-line function with only one caller. This commit therefore
inlines it.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Now that there is only ever a single flavor of RCU in a given kernel
build, there isn't a whole lot of point in having a flavor-traversal
macro. This commit therefore removes it and converts calls to it to
straightline code, inlining trivial functions as appropriate.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Now that there is only one rcu_state structure, there is no need for the
rcu_data structure to indicate which it corresponds to. This commit
therefore removes the rcu_data structure's ->rsp field, replacing all
remaining uses of it with &rcu_state.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
There now is only one rcu_state structure in a given build of the
Linux kernel, so there is no need to pass it as a parameter to
RCU's functions. This commit therefore removes the rsp parameter
from the code in kernel/rcu/tree_exp.h, and removes all of the
rsp local variables while in the area.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
There now is only one rcu_state structure in a given build of the
Linux kernel, so there is no need to pass it as a parameter to
RCU's functions. This commit therefore removes the rsp parameter
from rcu_nocb_cpu_needs_barrier(), rcu_spawn_one_nocb_kthread(),
rcu_organize_nocb_kthreads(), rcu_nocb_cpu_needs_barrier(), and
rcu_nohz_full_cpu().
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
There now is only one rcu_state structure in a given build of the
Linux kernel, so there is no need to pass it as a parameter to RCU's
functions. This commit therefore removes the rsp parameter from
print_cpu_stall_info().
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
There now is only one rcu_state structure in a given build of the
Linux kernel, so there is no need to pass it as a parameter to RCU's
functions. This commit therefore removes the rsp parameter from
rcu_spawn_one_boost_kthread().
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
There now is only one rcu_state structure in a given build of the
Linux kernel, so there is no need to pass it as a parameter to RCU's
functions. This commit therefore removes the rsp parameter from
dump_blkd_tasks() and rcu_preempt_blocked_readers_cgp().
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
There now is only one rcu_state structure in a given build of the
Linux kernel, so there is no need to pass it as a parameter to RCU's
functions. This commit therefore removes the rsp parameter from
rcu_print_detail_task_stall().
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The rcu_state structure's ->rda field was used to find the per-CPU
rcu_data structures corresponding to that rcu_state structure. But now
there is only one rcu_state structure (creatively named "rcu_state")
and one set of per-CPU rcu_data structures (creatively named "rcu_data").
Therefore, uses of the ->rda field can always be replaced by "rcu_data,
and this commit makes that change and removes the ->rda field.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The rcu_state structure's ->call field references the corresponding RCU
flavor's call_rcu() function. However, now that there is only ever one
rcu_state structure in a given build of the Linux kernel, and that flavor
uses plain old call_rcu(), there is not a lot of point in continuing to
have the ->call field. This commit therefore removes it.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Now that a given build of the Linux kernel has only one set of rcu_state,
rcu_node, and rcu_data structures, there is no point in creating a macro
to declare and compile-time initialize them. This commit therefore
just does normal declaration and compile-time initialization of these
structures. While in the area, this commit also removes #ifndefs of
the no-longer-ever-defined preprocessor macro RCU_TREE_NONCORE.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Now that RCU-preempt knows about preemption disabling, its implementation
of synchronize_rcu() works for synchronize_sched(), and likewise for the
other RCU-sched update-side API members. This commit therefore confines
the RCU-sched update-side code to CONFIG_PREEMPT=n builds, and defines
RCU-sched's update-side API members in terms of those of RCU-preempt.
This means that any given build of the Linux kernel has only one
update-side flavor of RCU, namely RCU-preempt for CONFIG_PREEMPT=y builds
and RCU-sched for CONFIG_PREEMPT=n builds. This in turn means that kernels
built with CONFIG_RCU_NOCB_CPU=y have only one rcuo kthread per CPU.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Andi Kleen <ak@linux.intel.com>
One necessary step towards consolidating the three flavors of RCU is to
make sure that the resulting consolidated "one flavor to rule them all"
correctly handles networking denial-of-service attacks. One thing that
allows RCU-bh to do so is that __do_softirq() invokes rcu_bh_qs() every
so often, and so something similar has to happen for consolidated RCU.
This must be done carefully. For example, if a preemption-disabled
region of code takes an interrupt which does softirq processing before
returning, consolidated RCU must ignore the resulting rcu_bh_qs()
invocations -- preemption is still disabled, and that means an RCU
reader for the consolidated flavor.
This commit therefore creates a new rcu_softirq_qs() that is called only
from the ksoftirqd task, thus avoiding the interrupted-a-preempted-region
problem. This new rcu_softirq_qs() function invokes rcu_sched_qs(),
rcu_preempt_qs(), and rcu_preempt_deferred_qs(). The latter call handles
any deferred quiescent states.
Note that __do_softirq() still invokes rcu_bh_qs(). It will continue to
do so until a later stage of cleanup when the RCU-bh flavor is removed.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Fix !SMP issue located by kbuild test robot. ]
This commit defers reporting of RCU-preempt quiescent states at
rcu_read_unlock_special() time when any of interrupts, softirq, or
preemption are disabled. These deferred quiescent states are reported
at a later RCU_SOFTIRQ, context switch, idle entry, or CPU-hotplug
offline operation. Of course, if another RCU read-side critical
section has started in the meantime, the reporting of the quiescent
state will be further deferred.
This also means that disabling preemption, interrupts, and/or
softirqs will act as an RCU-preempt read-side critical section.
This is enforced by checking preempt_count() as needed.
Some special cases must be handled on an ad-hoc basis, for example,
context switch is a quiescent state even though both the scheduler and
do_exit() disable preemption. In these cases, additional calls to
rcu_preempt_deferred_qs() override the preemption disabling. Similar
logic overrides disabled interrupts in rcu_preempt_check_callbacks()
because in this case the quiescent state happened just before the
corresponding scheduling-clock interrupt.
In theory, this change lifts a long-standing restriction that required
that if interrupts were disabled across a call to rcu_read_unlock()
that the matching rcu_read_lock() also be contained within that
interrupts-disabled region of code. Because the reporting of the
corresponding RCU-preempt quiescent state is now deferred until
after interrupts have been enabled, it is no longer possible for this
situation to result in deadlocks involving the scheduler's runqueue and
priority-inheritance locks. This may allow some code simplification that
might reduce interrupt latency a bit. Unfortunately, in practice this
would also defer deboosting a low-priority task that had been subjected
to RCU priority boosting, so real-time-response considerations might
well force this restriction to remain in place.
Because RCU-preempt grace periods are now blocked not only by RCU
read-side critical sections, but also by disabling of interrupts,
preemption, and softirqs, it will be possible to eliminate RCU-bh and
RCU-sched in favor of RCU-preempt in CONFIG_PREEMPT=y kernels. This may
require some additional plumbing to provide the network denial-of-service
guarantees that have been traditionally provided by RCU-bh. Once these
are in place, CONFIG_PREEMPT=n kernels will be able to fold RCU-bh
into RCU-sched. This would mean that all kernels would have but
one flavor of RCU, which would open the door to significant code
cleanup.
Moving to a single flavor of RCU would also have the beneficial effect
of reducing the NOCB kthreads by at least a factor of two.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Apply rcu_read_unlock_special() preempt_count() feedback
from Joel Fernandes. ]
[ paulmck: Adjust rcu_eqs_enter() call to rcu_preempt_deferred_qs() in
response to bug reports from kbuild test robot. ]
[ paulmck: Fix bug located by kbuild test robot involving recursion
via rcu_preempt_deferred_qs(). ]
The rcu_kick_nohz_cpu() function is no longer used, and the functionality
it used to provide is now provided by a call to resched_cpu() in the
force-quiescent-state function rcu_implicit_dynticks_qs(). This commit
therefore removes rcu_kick_nohz_cpu().
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
CPUs are expected to report quiescent states when coming online and
when going offline, and grace-period initialization is supposed to
handle any race conditions where a CPU's ->qsmask bit is set just after
it goes offline. This commit adds diagnostics for the case where an
offline CPU nevertheless has a grace period waiting on it.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Grace-period initialization first processes any recent CPU-hotplug
operations, and then initializes state for the new grace period. These
two phases of initialization are currently not distinguished in debug
prints, but the distinction is valuable in a number of debug situations.
This commit therefore introduces two new values for ->gp_state,
RCU_GP_ONOFF and RCU_GP_INIT, in order to make this distinction.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Interactions between CPU-hotplug operations and grace-period
initialization can result in dump_blkd_tasks(). One of the first
debugging actions in this case is to search back in dmesg to work
out which of the affected rcu_node structure's CPUs are online and to
determine the last CPU-hotplug operation affecting any of those CPUs.
This can be laborious and error-prone, especially when console output
is lost.
This commit therefore causes dump_blkd_tasks() to dump the state of
the affected rcu_node structure's CPUs and the last grace period during
which the last offline and online operation affected each of these CPUs.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Now that quiescent states for newly offlined CPUs are reported either
when that CPU goes offline or at the end of grace-period initialization,
the CPU-hotplug failsafe in the force-quiescent-state code path is no
longer needed.
This commit therefore removes this failsafe.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Without special fail-safe quiescent-state-propagation checks, grace-period
hangs can result from the following scenario:
1. CPU 1 goes offline.
2. Because CPU 1 is the only CPU in the system blocking the current
grace period, the grace period ends as soon as
rcu_cleanup_dying_idle_cpu()'s call to rcu_report_qs_rnp()
returns.
3. At this point, the leaf rcu_node structure's ->lock is no longer
held: rcu_report_qs_rnp() has released it, as it must in order
to awaken the RCU grace-period kthread.
4. At this point, that same leaf rcu_node structure's ->qsmaskinitnext
field still records CPU 1 as being online. This is absolutely
necessary because the scheduler uses RCU (in this case on the
wake-up path while awakening RCU's grace-period kthread), and
->qsmaskinitnext contains RCU's idea as to which CPUs are online.
Therefore, invoking rcu_report_qs_rnp() after clearing CPU 1's
bit from ->qsmaskinitnext would result in a lockdep-RCU splat
due to RCU being used from an offline CPU.
5. RCU's grace-period kthread awakens, sees that the old grace period
has completed and that a new one is needed. It therefore starts
a new grace period, but because CPU 1's leaf rcu_node structure's
->qsmaskinitnext field still shows CPU 1 as being online, this new
grace period is initialized to wait for a quiescent state from the
now-offline CPU 1.
6. Without the fail-safe force-quiescent-state checks, there would
be no quiescent state from the now-offline CPU 1, which would
eventually result in RCU CPU stall warnings and memory exhaustion.
It would be good to get rid of the special fail-safe quiescent-state
propagation checks, and thus it would be good to fix things so that
the above scenario cannot happen. This commit therefore adds a new
->ofl_lock to the rcu_state structure. This lock is held by rcu_gp_init()
across the applying of buffered online and offline operations to the
rcu_node tree, and it is also held by rcu_cleanup_dying_idle_cpu()
when buffering a new offline operation. This prevents rcu_gp_init()
from acquiring the leaf rcu_node structure's lock during the interval
between when rcu_cleanup_dying_idle_cpu() invokes rcu_report_qs_rnp(),
which releases ->lock and the re-acquisition of that same lock.
This in turn prevents the failure scenario outlined above, and will
hopefully eventually allow removal of the offline-CPU checks from the
force-quiescent-state code path.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Now that everything has been converted to use ->gp_seq instead of
->gpnum and ->completed, this commit removes ->gpnum and ->completed.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
One problem with the ->need_future_gp[] array is that the grace-period
assignment of each element changes as the grace periods complete.
This means that it is necessary to hold a lock when checking this
array to learn if a given grace period has already been requested.
This increase lock contention, which is the opposite of helpful.
This commit therefore replaces the ->need_future_gp[] with a single
->gp_seq_needed value and keeps it updated in the rcu_data structure.
This will enable reliable lockless checking of whether or not a given
grace period has already been requested.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit converts the grace-period request code paths from ->completed
and ->gpnum to ->gp_seq. The need_future_gp_element() macro encapsulates
the shift operation required to use ->gp_seq as an index to the
->need_future_gp[] array. The rcu_cbs_completed() function is removed
in favor of the rcu_seq_snap() function. The rcu_start_this_gp()
gets some temporary consistency checks and uses rcu_seq_done(),
rcu_seq_current(), rcu_seq_state(), and rcu_gp_in_progress() in place
of the earlier open-coded comparisons of ->gpnum and ->completed.
The rcu_future_gp_cleanup() function replaces use of ->completed
with ->gp_seq. The rcu_accelerate_cbs() function replaces a call to
rcu_cbs_completed() with one to rcu_seq_snap(). The rcu_advance_cbs()
function replaces an access to >completed with one to ->gp_seq and adds
some temporary warnings. The rcu_nocb_wait_gp() function replaces a
call to rcu_cbs_completed() with one to rcu_seq_snap() and an open-coded
comparison with rcu_seq_done().
The temporary warnings will be removed when the various ->gpnum and
->completed fields are removed. Their purpose is to locate code who
might still be using ->gpnum and ->completed. (Much easier that way
than trying to trace down the causes of too-short grace periods and
grace-period hangs!)
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit switches the interrupt-disabled detection mechanism to
->gp_seq. This mechanism is used as part of RCU CPU stall warnings,
and detects cases where the stall is due to a CPU having interrupts
disabled.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit adds grace-period sequence numbers (->gp_seq) to the
rcu_state, rcu_node, and rcu_data structures, and updates them.
It also checks for consistency between rsp->gpnum and rsp->gp_seq.
These ->gp_seq counters will eventually replace the existing ->gpnum
and ->completed counters, allowing a single memory access to determine
whether or not a grace period is in progress and if so, which one.
This in turn will enable changes that will reduce ->lock contention on
the leaf rcu_node structures.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit causes a splat if RCU is idle and a request for a new grace
period is ignored for more than one second. This splat normally indicates
that some code path asked for a new grace period, but failed to wake up
the RCU grace-period kthread.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Fix bug located by Dan Carpenter and his static checker. ]
[ paulmck: Fix self-deadlock bug located 0day test robot. ]
[ paulmck: Disable unless CONFIG_PROVE_RCU=y. ]
The WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp()) in
rcu_gp_cleanup() triggers (inexplicably, of course) every so often.
This commit therefore extracts more information.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
There is no longer any need for ->need_future_gp[] to count the number of
requests for future grace periods, so this commit converts the additions
to assignments to "true" and reduces the size of each element to one byte.
While we are in the area, fix an obsolete comment.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
Paul E. McKenney