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rcu: Eliminate flavor scan in rcu_momentary_dyntick_idle()

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The rcu_momentary_dyntick_idle() function scans the RCU flavors, checking
that one of them still needs a quiescent state before doing an expensive
atomic operation on the ->dynticks counter.  However, this check reduces
overhead only after a rare race condition, and increases complexity.  This
commit therefore removes the scan and the mechanism enabling the scan.

Signed-off-by: Paul E. McKenney <[email protected]>
This commit is contained in:
Paul E. McKenney 2017-01-27 13:17:02 -08:00
commit 0f9be8cabb
3 changed files with 15 additions and 54 deletions
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4
Documentation/RCU/Design/Data-Structures/Data-Structures.html
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@ -1104,7 +1104,7 @@ Its fields are as follows:
1 int dynticks_nesting;
2 int dynticks_nmi_nesting;
3 atomic_t dynticks;
4 int rcu_sched_qs_mask;
4 bool rcu_need_heavy_qs;
5 unsigned long rcu_qs_ctr;
</pre>
@ -1124,7 +1124,7 @@ CPU's transitions to and from dyntick-idle mode, so that this counter
has an even value when the CPU is in dyntick-idle mode and an odd
value otherwise.
</p><p>The <tt>-&gt;rcu_sched_qs_mask</tt> field is used
</p><p>The <tt>-&gt;rcu_need_heavy_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, so much so that
it is willing to call for heavy-weight dyntick-counter operations.