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srcu: Make Tree SRCU able to operate without snp_node array

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This commit makes Tree SRCU able to operate without an snp_node
array, that is, when the srcu_data structures' ->mynode pointers
are NULL.  This can result in high contention on the srcu_struct
structure's ->lock, but only when there are lots of call_srcu(),
synchronize_srcu(), and synchronize_srcu_expedited() calls.

Note that when there is no snp_node array, all SRCU callbacks use
CPU 0's callback queue.  This is optimal in the common case of low
update-side load because it removes the need to search each CPU
for the single callback that made the grace period happen.

Co-developed-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Signed-off-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Paul E. McKenney
2022-01-24 09:46:57 -08:00
parent 7b9e9b5856
commit 994f706872
2 changed files with 124 additions and 93 deletions
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14
include/linux/srcutree.h
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@@ -63,8 +63,9 @@ struct srcu_struct {
struct srcu_node node[NUM_RCU_NODES]; /* Combining tree. */
struct srcu_node *level[RCU_NUM_LVLS + 1];
/* First node at each level. */
int srcu_size_state; /* Small-to-big transition state. */
struct mutex srcu_cb_mutex; /* Serialize CB preparation. */
spinlock_t __private lock; /* Protect counters */
spinlock_t __private lock; /* Protect counters and size state. */
struct mutex srcu_gp_mutex; /* Serialize GP work. */
unsigned int srcu_idx; /* Current rdr array element. */
unsigned long srcu_gp_seq; /* Grace-period seq #. */
@@ -83,6 +84,17 @@ struct srcu_struct {
struct lockdep_map dep_map;
};
/* Values for size state variable (->srcu_size_state). */
#define SRCU_SIZE_SMALL 0
#define SRCU_SIZE_ALLOC 1
#define SRCU_SIZE_WAIT_BARRIER 2
#define SRCU_SIZE_WAIT_CALL 3
#define SRCU_SIZE_WAIT_CBS1 4
#define SRCU_SIZE_WAIT_CBS2 5
#define SRCU_SIZE_WAIT_CBS3 6
#define SRCU_SIZE_WAIT_CBS4 7
#define SRCU_SIZE_BIG 8
/* Values for state variable (bottom bits of ->srcu_gp_seq). */
#define SRCU_STATE_IDLE 0
#define SRCU_STATE_SCAN1 1