linux-stable.git/kernel/rcu/tree_plugin.h, branch v4.18.2

Merge branches 'exp.2018.05.15a', 'fixes.2018.05.15a', 'lock.2018.05.15a' and 'torture.2018.05.15a' into HEAD

2018-05-15T17:33:05+00:00

exp.2018.05.15a: Parallelize expedited grace-period initialization.
fixes.2018.05.15a: Miscellaneous fixes.
lock.2018.05.15a: Decrease lock contention on root rcu_node structure,
	which is a step towards merging RCU flavors.
torture.2018.05.15a: Torture-test updates.

rcu: Make rcu_start_future_gp() caller select grace period

2018-05-15T17:30:32+00:00

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 
Tested-by: Nicholas Piggin

rcu: Make rcu_gp_cleanup() more accurately predict need for new GP

2018-05-15T17:29:28+00:00

Currently, rcu_gp_cleanup() scans the rcu_node tree in order to reset
state to reflect the end of the grace period.  It also checks to see
whether a new grace period is needed, but in a number of cases, rather
than directly cause the new grace period to be immediately started, it
instead leaves the grace-period-needed state where various fail-safes
can find it.  This works fine, but results in higher contention on the
root rcu_node structure's ->lock, which is undesirable, and contention
on that lock has recently become noticeable.

This commit therefore makes rcu_gp_cleanup() immediately start a new
grace period if there is any need for one.

It is quite possible that it will later be necessary to throttle the
grace-period rate, but that can be dealt with when and if.

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

rcu: Add accessor macros for the ->need_future_gp[] array

2018-05-15T17:29:18+00:00

Accessors for the ->need_future_gp[] array are currently open-coded,
which makes them difficult to change.  To improve maintainability, this
commit adds need_future_gp_mask() to compute the indexing mask from the
array size, need_future_gp_element() to access the element corresponding
to the specified grace-period number, and need_any_future_gp() to
determine if any future grace period is needed.  This commit also applies
need_future_gp_element() to existing open-coded single-element accesses.

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

rcu: Add leaf-node macros

2018-05-15T17:28:07+00:00

This commit adds rcu_first_leaf_node() that returns a pointer to
the first leaf rcu_node structure in the specified RCU flavor and an
rcu_is_leaf_node() that returns true iff the specified rcu_node structure
is a leaf.  This commit also uses these macros where appropriate.

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

rcu: Update rcu_bind_gp_kthread() header comment

2018-05-15T17:27:46+00:00

The header comment for rcu_bind_gp_kthread() refers to sysidle, which
is no longer with us.  However, it is still important to bind RCU's
grace-period kthreads to the housekeeping CPU(s), so rather than remove
rcu_bind_gp_kthread(), this commit updates the comment.

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

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: Call wake_nocb_leader_defer() with 'FORCE' when nocb_q_count is high

2018-05-15T17:27:18+00:00

If an excessive number of callbacks have been queued, but the NOCB
leader kthread's wakeup must be deferred, then we should wake up the
leader unconditionally once it is safe to do so.

This was handled correctly in commit fbce7497ee ("rcu: Parallelize and
economize NOCB kthread wakeups"), but then commit 8be6e1b15c ("rcu:
Use timer as backstop for NOCB deferred wakeups") passed RCU_NOCB_WAKE
instead of the correct RCU_NOCB_WAKE_FORCE to wake_nocb_leader_defer().
As an interesting aside, RCU_NOCB_WAKE_FORCE is never passed to anything,
which should have been taken as a hint.  ;-)

This commit therefore passes RCU_NOCB_WAKE_FORCE instead of RCU_NOCB_WAKE
to wake_nocb_leader_defer() when a callback is queued onto a NOCB CPU
that already has an excessive number of callbacks pending.

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

rcu: Don't allocate rcu_nocb_mask if no one needs it

2018-05-15T17:27:11+00:00

Commit 44c65ff2e3b0 ("rcu: Eliminate NOCBs CPU-state Kconfig options")
made allocation of rcu_nocb_mask depend only on the rcu_nocbs=,
nohz_full=, or isolcpus= kernel boot parameters.  However, it failed
to change the initial value of rcu_init_nohz()'s local variable
need_rcu_nocb_mask to false, which can result in useless allocation
of an all-zero rcu_nocb_mask.  This commit therefore fixes this bug by
changing the initial value of need_rcu_nocb_mask to false.

While we are in the area, also correct the error message that is printed
when someone specifies that can-never-exist CPUs should be NOCBs CPUs.

Reported-by: Byungchul Park 
Signed-off-by: Paul E. McKenney 
Acked-by: Byungchul Park 
Tested-by: Nicholas Piggin