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rcu: Place guard on rcu_all_qs() and rcu_note_context_switch() actions

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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 <[email protected]>
Cc: Peter Zijlstra <[email protected]>
This commit is contained in:
Paul E. McKenney 2017-01-27 14:17:50 -08:00
commit 9226b10d78
5 changed files with 44 additions and 18 deletions
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11
Documentation/RCU/Design/Data-Structures/Data-Structures.html
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@ -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>