This commit makes the rcu_future_grace_period tracepoint use gp_seq
instead of ->gpnum and ->completed.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit makes the rcu_grace_period tracepoint use gp_seq instead
of ->gpnum or ->completed. It also introduces a "cpuofl-bgp" string to
less obscurely indicate when a CPU has gone offline while a grace period
is waiting on it.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit makes rcu_nocb_wait_gp() check rdp->gp_seq_needed to see
if the current CPU already knows about the needed grace period having
already been requested. If so, it avoids acquiring the corresponding
leaf rcu_node structure's ->lock, thus decreasing contention. This
optimization is intended for cases where either multiple leader rcuo
kthreads are running on the same CPU or these kthreads are running on
a non-offloaded (e.g., housekeeping) CPU.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Move lock release past "if" as suggested by Joel Fernandes. ]
[ paulmck: Fix caching of furthest-future requested grace period. ]
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>
SRCU has long used ->srcu_gp_seq, and now RCU uses ->gp_seq. This
commit therefore moves the rcutorture_get_gp_data() function from
a ->gpnum / ->completed pair to ->gp_seq.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit makes the RCU CPU stall-warning code in print_other_cpu_stall(),
print_cpu_stall(), and check_cpu_stall() use ->gp_seq instead of ->gpnum
and ->completed.
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 quiescent-state no-backtracking checks from
->gpnum and ->completed to ->gp_seq.
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 makes rcu_gp_in_progress() use ->gp_seq instead of
->completed and ->gpnum. The READ_ONCE() invocations are buried
in rcu_seq_current().
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit makes rcu_try_advance_all_cbs() use ->gp_seq. It uses
rcu_seq_ctr() in order to shift away the state bits, so that the
low-order bits of the result may safely be used to index ->nocb_gp_wq[].
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit makes rcu_try_advance_all_cbs() use ->gp_seq, with the
exception of tracing, which will be converted later.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit makes rcu_implicit_dynticks_qs() use ->gp_seq, with the
exception of tracing, which will be converted later.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit converts rcu_gpnum_ovf() to use ->gp_seq instead of ->gpnum.
Same size unsigned long, so same approach.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit moves __note_gp_changes(), note_gp_changes(), and
__rcu_pending() to ->gp_seq, creating new rcu_seq_completed_gp() and
rcu_seq_new_gp() functions for this purpose.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Reinstate "cpuend: trace as suggested by Joel Fernandes. ]
This commit converts get_state_synchronize_rcu(), cond_synchronize_rcu(),
get_state_synchronize_sched(), and cond_synchronize_sched() from ->gpnum
and ->completed to ->gp_seq. Note that this also introduces a full
memory barrier in the already-done paths off cond_synchronize_rcu() and
cond_synchronize_sched(), as work with LKMM indicates that the earlier
smp_load_acquire() were insufficiently strong in some situations where
these two functions were called just as the grace period ended. In such
cases, these two functions would not gain the benefit of memory ordering
at the end of the grace period.
Please note that the performance impact is negligible, as you shouldn't
be using either function anywhere near a fastpath in any case.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit switches the functions reporting quiescent states from
use of ->gpnum to ->gp_seq. In either case, the point is to handle
races where a given grace period ends before a quiescent state can
be reported. Failing to catch these races would result in too-short
grace periods, hence the checking.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit switches rcu_check_gp_kthread_starvation() from printing
->gpnum and ->completed to printing ->gp_seq upon detecting a starving
RCU grace-period kthread during an RCU CPU stall warning.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The rcutorture test invokes rcu_batches_started(),
rcu_batches_completed(), rcu_batches_started_bh(),
rcu_batches_completed_bh(), rcu_batches_started_sched(), and
rcu_batches_completed_sched() to do grace-period consistency checks,
and rcuperf uses the _completed variants for statistics.
These functions use ->gpnum and ->completed. This commit therefore
replaces them with rcu_get_gp_seq(), rcu_bh_get_gp_seq(), and
rcu_sched_get_gp_seq(), adjusting rcutorture and rcuperf to make
use of them.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit moves rcu_gp_slow() to ->gp_seq. This function only uses
the grace-period number to modulate delay, so rcu_seq_ctr(rsp->gp_seq)
gets the same effect, at least in cases where the delay is to happen
more than four times per wrap of an unsigned long.
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>
At the end of rcu_gp_cleanup(), if another grace period is needed, but
not via rcu_accelerate_cbs(), the ->gp_flags field is written twice,
once when making the new grace-period request, and once when clearing
all other types of requests. This commit therefore adds an else-clause
to avoid this double write.
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. ]
Currently, the parallelized initialization of expedited grace periods uses
the workqueue associated with each rcu_node structure's ->grplo field.
This works fine unless that CPU is offline. This commit therefore uses
the CPU corresponding to the lowest-numbered online CPU, or just queues
the work on WORK_CPU_UNBOUND if there are no online CPUs corresponding
to this rcu_node structure.
Note that this patch uses cpu_is_offline() instead of the usual approach
of checking bits in the rcu_node structure's ->qsmaskinitnext field. This
is safe because preemption is disabled across both the cpu_is_offline()
check and the call to queue_work_on().
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
[ paulmck: Disable preemption to close offline race window. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Apply Peter Zijlstra feedback on CPU selection. ]
Tested-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
There is a two-jiffy delay between the time that a CPU will self-report
an RCU CPU stall warning and the time that some other CPU will report a
warning on behalf of the first CPU. This has worked well in the past,
but on busy systems, it is possible for the two warnings to overlap,
which makes interpreting them extremely difficult.
This commit therefore uses a cmpxchg-based timing decision that
allows only one report in a given one-minute period (assuming default
stall-warning Kconfig parameters). This approach will of course fail
if you are seeing minute-long vCPU preemption, but in that case the
overlapping RCU CPU stall warnings are the least of your worries.
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Sparse reported this:
| kernel/rcu/tree_plugin.h:814:9: warning: incorrect type in argument 1 (different modifiers)
| kernel/rcu/tree_plugin.h:814:9: expected struct lockdep_map const *lock
| kernel/rcu/tree_plugin.h:814:9: got struct lockdep_map [noderef] *<noident>
This is caused by using vanilla lockdep annotations on rcu_node::lock,
and that requires accessing ->lock of rcu_node directly. However we need
to keep rcu_node::lock __private to avoid breaking its extra ordering
guarantee. And we have a dedicated lockdep annotation for
rcu_node::lock, so use it.
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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>
This commit adds "#define pr_fmt(fmt) fmt" to the torture-test files
in order to keep the current dmesg format. Once Joe's commits have
hit mainline, these definitions will be changed in order to automatically
generate the dmesg line prefix that the scripts expect. This will have
the beneficial side-effect of allowing printk() formats to be used more
widely and of shortening some pr_*() lines.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Joe Perches <joe@perches.com>
Some bugs reproduce quickly only at high CPU-hotplug rates, so the
rcutorture TREE03 scenario now has only 200 milliseconds spacing between
CPU-hotplug operations. At this rate, the torture-test pair of console
messages per operation becomes a bit voluminous. This commit therefore
converts the torture-test set of "verbose" kernel-boot arguments from
bool to int, and prints the extra console messages only when verbose=2.
The default is still verbose=1.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit adds the address of the first callback to the per-CPU rcutorture
output in order to allow lost wakeups to be more efficiently tracked down.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit updates the header comment of srcu_funnel_gp_start() to
document the fact that srcu_funnel_gp_start() does the work of
srcu_funnel_exp_start(), in some cases by invoking it directly.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit simply changes some copy-pasta call_rcu() instances to
the correct call_srcu().
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
During expedited grace-period initialization, a work item is scheduled
for each leaf rcu_node structure. However, that initialization code
is itself (normally) executing from a workqueue, so one of the leaf
rcu_node structures could just as well be handled by that pre-existing
workqueue, and with less overhead. This commit therefore uses a
shiny new rcu_is_leaf_node() macro to execute the last leaf rcu_node
structure's initialization directly from the pre-existing workqueue.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The x86/mtrr code does horrific things because hardware. It uses
stop_machine_from_inactive_cpu(), which does a wakeup (of the stopper
thread on another CPU), which uses RCU, all before the CPU is onlined.
RCU complains about this, because wakeups use RCU and RCU does
(rightfully) not consider offline CPUs for grace-periods.
Fix this by initializing RCU way early in the MTRR case.
Tested-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Add !SMP support, per 0day Test Robot report. ]
This commit adds end-of-test state printout to help check whether RCU
shut down nicely. Note that this printout only helps for flavors of
RCU that are not used much by the kernel. In particular, for normal
RCU having a grace period in progress is expected behavior.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
Now that grace-period requests use funnel locking and now that they
set ->gp_flags to RCU_GP_FLAG_INIT even when the RCU grace-period
kthread has not yet started, rcu_gp_kthread() no longer needs to check
need_any_future_gp() at startup time. This commit therefore removes
this check.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
Now that RCU no longer relies on failsafe checks, cpu_needs_another_gp()
can be greatly simplified. This simplification eliminates the last
call to rcu_future_needs_gp() and to rcu_segcblist_future_gp_needed(),
both of which which can then be eliminated. And then, because
cpu_needs_another_gp() is called only from __rcu_pending(), it can be
inlined and eliminated.
This commit carries out the simplification, inlining, and elimination
called out above.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
All of the cpu_needs_another_gp() function's checks (except for
newly arrived callbacks) have been subsumed into the rcu_gp_cleanup()
function's scan of the rcu_node tree. This commit therefore drops the
call to cpu_needs_another_gp(). The check for newly arrived callbacks
is supplied by rcu_accelerate_cbs(). Any needed advancing (as in the
earlier rcu_advance_cbs() call) will be supplied when the corresponding
CPU becomes aware of the end of the now-completed grace period.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
If rcu_start_this_gp() is invoked with a requested grace period more
than three in the future, then either the ->need_future_gp[] array
needs to be bigger or the caller needs to be repaired. This commit
therefore adds a WARN_ON_ONCE() checking for this condition.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
The rcu_start_this_gp() function had a simple form of funnel locking that
used only the leaves and root of the rcu_node tree, which is fine for
systems with only a few hundred CPUs, but sub-optimal for systems having
thousands of CPUs. This commit therefore adds full-tree funnel locking.
This variant of funnel locking is unusual in the following ways:
1. The leaf-level rcu_node structure's ->lock is held throughout.
Other funnel-locking implementations drop the leaf-level lock
before progressing to the next level of the tree.
2. Funnel locking can be started at the root, which is convenient
for code that already holds the root rcu_node structure's ->lock.
Other funnel-locking implementations start at the leaves.
3. If an rcu_node structure other than the initial one believes
that a grace period is in progress, it is not necessary to
go further up the tree. This is because grace-period cleanup
scans the full tree, so that marking the need for a subsequent
grace period anywhere in the tree suffices -- but only if
a grace period is currently in progress.
4. It is possible that the RCU grace-period kthread has not yet
started, and this case must be handled appropriately.
However, the general approach of using a tree to control lock contention
is still in place.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
The rcu_accelerate_cbs() function selects a grace-period target, which
it uses to have rcu_segcblist_accelerate() assign numbers to recently
queued callbacks. Then it invokes rcu_start_future_gp(), which selects
a grace-period target again, which is a bit pointless. This commit
therefore changes rcu_start_future_gp() to take the grace-period target as
a parameter, thus avoiding double selection. This commit also changes
the name of rcu_start_future_gp() to rcu_start_this_gp() to reflect
this change in functionality, and also makes a similar change to the
name of trace_rcu_future_gp().
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
The rcu_start_gp_advanced() is invoked only from rcu_start_future_gp() and
much of its code is redundant when invoked from that context. This commit
therefore inlines rcu_start_gp_advanced() into rcu_start_future_gp(),
then removes rcu_start_gp_advanced().
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
Once the grace period has ended, any RCU_GP_FLAG_FQS requests are
irrelevant: The grace period has ended, so there is no longer any
point in forcing quiescent states in order to try to make it end sooner.
This commit therefore causes rcu_gp_cleanup() to clear any bits other
than RCU_GP_FLAG_INIT from ->gp_flags at the end of the grace period.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
It is true that currently only the low-order two bits are used, so
there should be no problem given modern machines and compilers, but
good hygiene and maintainability dictates use of an unsigned long
instead of an int. This commit therefore makes this change.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
The __rcu_process_callbacks() function currently checks to see if
the current CPU needs a grace period and also if there is any other
reason to kick off a new grace period. This is one of the fail-safe
checks that has been rendered unnecessary by the changes that increase
the accuracy of rcu_gp_cleanup()'s estimate as to whether another grace
period is required. Because this particular fail-safe involved acquiring
the root rcu_node structure's ->lock, which has seen excessive contention
in real life, this fail-safe needs to go.
However, one check must remain, namely the check for newly arrived
RCU callbacks that have not yet been associated with a grace period.
One might hope that the checks in __note_gp_changes(), which is invoked
indirectly from rcu_check_quiescent_state(), would suffice, but this
function won't be invoked at all if RCU is idle. It is therefore necessary
to replace the fail-safe checks with a simpler check for newly arrived
callbacks during an RCU idle period, which is exactly what this commit
does. This change removes the final call to rcu_start_gp(), so this
function is removed as well.
Note that lockless use of cpu_needs_another_gp() is racy, but that
these races are harmless in this case. If RCU really is idle, the
values will not change, so the return value from cpu_needs_another_gp()
will be correct. If RCU is not idle, the resulting redundant call to
rcu_accelerate_cbs() will be harmless, and might even have the benefit
of reducing grace-period latency a bit.
This commit also moves interrupt disabling into the "if" statement to
improve real-time response a bit.
Reported-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
When __call_rcu_core() notices excessive numbers of callbacks pending
on the current CPU, we know that at least one of them is not yet
classified, namely the one that was just now queued. Therefore, it
is not necessary to invoke rcu_start_gp() and thus not necessary to
acquire the root rcu_node structure's ->lock. This commit therefore
replaces the rcu_start_gp() with rcu_accelerate_cbs(), thus replacing
an acquisition of the root rcu_node structure's ->lock with that of
this CPU's leaf rcu_node structure.
This decreases contention on the root rcu_node structure's ->lock.
Reported-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
