lkml.org 
[lkml]   [2012]   [Feb]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH tip/core/rcu 16/47] rcu: Don't make callbacks go through second full grace period
Date
From: "Paul E. McKenney" <paul.mckenney@linaro.org>

RCU's current CPU-offline code path dumps all of the outgoing CPU's
callbacks onto the RCU_NEXT_TAIL portion of the surviving CPU's
callback list. This means that all the ready-to-invoke callbacks from
the outgoing CPU must wait for another full RCU grace period. This was
just fine when CPU-hotplug events were rare, but there is increasing
evidence that users are planning to make increasing use of CPU hotplug.

Therefore, this commit changes the callback-dumping procedure so that
callbacks that are ready to invoke are moved to the RCU_DONE_TAIL
portion of the surviving CPU's callback list. This avoids running
these callbacks through a second unnecessary grace period.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
kernel/rcutree.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++------
1 files changed, 46 insertions(+), 6 deletions(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index ac3a810..7789e66 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1270,24 +1270,64 @@ static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
struct rcu_data *receive_rdp = per_cpu_ptr(rsp->rda, receive_cpu);
struct rcu_node *rnp = rdp->mynode; /* For dying CPU. */

- /* Move callbacks to some other CPU. */
+ /* First, adjust the counts. */
+ if (rdp->nxtlist != NULL) {
+ receive_rdp->qlen_lazy += rdp->qlen_lazy;
+ receive_rdp->qlen += rdp->qlen;
+ rdp->qlen_lazy = 0;
+ rdp->qlen = 0;
+ }
+
+ /*
+ * Next, move ready-to-invoke callbacks to be invoked on some
+ * other CPU. These will not be required to pass through another
+ * grace period: They are done, regardless of CPU.
+ */
+ if (rdp->nxtlist != NULL &&
+ rdp->nxttail[RCU_DONE_TAIL] != &rdp->nxtlist) {
+ struct rcu_head *oldhead;
+ struct rcu_head **oldtail;
+ struct rcu_head **newtail;
+
+ oldhead = rdp->nxtlist;
+ oldtail = receive_rdp->nxttail[RCU_DONE_TAIL];
+ rdp->nxtlist = *rdp->nxttail[RCU_DONE_TAIL];
+ *rdp->nxttail[RCU_DONE_TAIL] = *oldtail;
+ *receive_rdp->nxttail[RCU_DONE_TAIL] = oldhead;
+ newtail = rdp->nxttail[RCU_DONE_TAIL];
+ for (i = RCU_DONE_TAIL; i < RCU_NEXT_SIZE; i++) {
+ if (receive_rdp->nxttail[i] == oldtail)
+ receive_rdp->nxttail[i] = newtail;
+ if (rdp->nxttail[i] == newtail)
+ rdp->nxttail[i] = &rdp->nxtlist;
+ }
+ }
+
+ /*
+ * Finally, put the rest of the callbacks at the end of the list.
+ * The ones that made it partway through get to start over: We
+ * cannot assume that grace periods are synchronized across CPUs.
+ * (We could splice RCU_WAIT_TAIL into RCU_NEXT_READY_TAIL, but
+ * this does not seem compelling. Not yet, anyway.)
+ */
if (rdp->nxtlist != NULL) {
*receive_rdp->nxttail[RCU_NEXT_TAIL] = rdp->nxtlist;
receive_rdp->nxttail[RCU_NEXT_TAIL] =
rdp->nxttail[RCU_NEXT_TAIL];
- receive_rdp->qlen_lazy += rdp->qlen_lazy;
- receive_rdp->qlen += rdp->qlen;
receive_rdp->n_cbs_adopted += rdp->qlen;
rdp->n_cbs_orphaned += rdp->qlen;

rdp->nxtlist = NULL;
for (i = 0; i < RCU_NEXT_SIZE; i++)
rdp->nxttail[i] = &rdp->nxtlist;
- rdp->qlen_lazy = 0;
- rdp->qlen = 0;
}

- /* Record a quiescent state for the dying CPU. */
+ /*
+ * Record a quiescent state for the dying CPU. This is safe
+ * only because we have already cleared out the callbacks.
+ * (Otherwise, the RCU core might try to schedule the invocation
+ * of callbacks on this now-offline CPU, which would be bad.)
+ */
mask = rdp->grpmask; /* rnp->grplo is constant. */
trace_rcu_grace_period(rsp->name,
rnp->gpnum + 1 - !!(rnp->qsmask & mask),
--
1.7.8


\
 
 \ /
  Last update: 2012-02-04 02:55    [W:0.230 / U:0.516 seconds]
©2003-2017 Jasper Spaans. hosted at Digital OceanAdvertise on this site