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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2017-01-27 14:17:50 -0800
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2017-04-18 11:38:18 -0700
commit9226b10d78ffe7895549045fe388dc5e73b87eac (patch)
tree29d959a37f8d502c97948e49f895ae3269793b12 /Documentation/RCU/Design
parent0f9be8cabbc343218dd2807af7308656be113045 (diff)
rcu: Place guard on rcu_all_qs() and rcu_note_context_switch() actions
The rcu_all_qs() and rcu_note_context_switch() do a series of checks, taking various actions to supply RCU with quiescent states, depending on the outcomes of the various checks. This is a bit much for scheduling fastpaths, so this commit creates a separate ->rcu_urgent_qs field in the rcu_dynticks structure that acts as a global guard for these checks. Thus, in the common case, rcu_all_qs() and rcu_note_context_switch() check the ->rcu_urgent_qs field, find it false, and simply return. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org>
Diffstat (limited to 'Documentation/RCU/Design')
-rw-r--r--Documentation/RCU/Design/Data-Structures/Data-Structures.html11
1 files changed, 10 insertions, 1 deletions
diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.html b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
index e4bf20a68fa3..4dec89097559 100644
--- a/Documentation/RCU/Design/Data-Structures/Data-Structures.html
+++ b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
@@ -1106,6 +1106,7 @@ Its fields are as follows:
3 atomic_t dynticks;
4 bool rcu_need_heavy_qs;
5 unsigned long rcu_qs_ctr;
+ 6 bool rcu_urgent_qs;
</pre>
<p>The <tt>-&gt;dynticks_nesting</tt> field counts the
@@ -1131,12 +1132,20 @@ it is willing to call for heavy-weight dyntick-counter operations.
This flag is checked by RCU's context-switch and <tt>cond_resched()</tt>
code, which provide a momentary idle sojourn in response.
-</p><p>Finally the <tt>-&gt;rcu_qs_ctr</tt> field is used to record
+</p><p>The <tt>-&gt;rcu_qs_ctr</tt> field is used to record
quiescent states from <tt>cond_resched()</tt>.
Because <tt>cond_resched()</tt> can execute quite frequently, this
must be quite lightweight, as in a non-atomic increment of this
per-CPU field.
+</p><p>Finally, the <tt>-&gt;rcu_urgent_qs</tt> field is used to record
+the fact that the RCU core code would really like to see a quiescent
+state from the corresponding CPU, with the various other fields indicating
+just how badly RCU wants this quiescent state.
+This flag is checked by RCU's context-switch and <tt>cond_resched()</tt>
+code, which, if nothing else, non-atomically increment <tt>-&gt;rcu_qs_ctr</tt>
+in response.
+
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>