Re: tasks-trace RCU: question about grace period forward progress
From: Mathieu Desnoyers
Date: Thu Feb 25 2021 - 15:22:01 EST
----- On Feb 25, 2021, at 1:33 PM, paulmck paulmck@xxxxxxxxxx wrote:
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commit 581f79546b6be406a9c7280b2d3511b60821efe0
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Author: Paul E. McKenney <paulmck@xxxxxxxxxx>
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Date: Thu Feb 25 10:26:00 2021 -0800
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rcu-tasks: Add block comment laying out RCU Tasks Trace design
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This commit adds a block comment that gives a high-level overview of
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how RCU tasks trace grace periods progress. It also adds a note about
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how exiting tasks are handles, plus it gives an overview of the memory
handles -> handled
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ordering.
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Reported-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
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Reported-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
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Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
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diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
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index 17c8ebe..f818357 100644
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--- a/kernel/rcu/tasks.h
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+++ b/kernel/rcu/tasks.h
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@@ -726,6 +726,42 @@ EXPORT_SYMBOL_GPL(show_rcu_tasks_rude_gp_kthread);
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// flavors, rcu_preempt and rcu_sched. The fact that RCU Tasks Trace
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// readers can operate from idle, offline, and exception entry/exit in no
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// way allows rcu_preempt and rcu_sched readers to also do so.
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+//
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+// The implementation uses rcu_tasks_wait_gp(), which relies on function
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+// pointers in the rcu_tasks structure. The rcu_spawn_tasks_trace_kthread()
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+// function sets these function pointers up so that rcu_tasks_wait_gp()
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+// invokes these functions in this order:
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+//
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+// rcu_tasks_trace_pregp_step():
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+// Initialize the count of readers and block CPU-hotplug operations.
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+// rcu_tasks_trace_pertask(), invoked on every non-idle task:
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+// Initialize per-task state and attempt to identify an immediate
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+// quiescent state for that task, or, failing that, attempt to set
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+// that task's .need_qs flag so that that task's next outermost
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+// rcu_read_unlock_trace() will report the quiescent state (in which
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+// case the count of readers is incremented). If both attempts fail,
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+// the task is added to a "holdout" list.
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+// rcu_tasks_trace_postscan():
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+// Initialize state and attempt to identify an immediate quiescent
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+// state as above (but only for idle tasks), unblock CPU-hotplug
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+// operations, and wait for an RCU grace period to avoid races with
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+// tasks that are in the process of exiting.
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+// check_all_holdout_tasks_trace(), repeatedly until holdout list is empty:
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+// Scans the holdout list, attempting to identify a quiescent state
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+// for each task on the list. If there is a quiescent state, the
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+// corresponding task is removed from the holdout list.
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+// rcu_tasks_trace_postgp():
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+// Wait for the count of readers do drop to zero, reporting any stalls.
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+// Also execute full memory barriers to maintain ordering with code
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+// executing after the grace period.
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+//
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+// The exit_tasks_rcu_finish_trace() synchronizes with exiting tasks.
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+//
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+// Pre-grace-period update-side code is ordered before the grace
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+// period via the ->cbs_lock and barriers in rcu_tasks_kthread().
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+// Pre-grace-period read-side code is ordered before the grace period by
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+// atomic_dec_and_test() of the count of readers (for IPIed readers) and by
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+// scheduler context-switch ordering (for locked-down non-running readers).
The rest looks good, thanks!
Mathieu
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// The lockdep state must be outside of #ifdef to be useful.
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com