From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
To: linux-kernel@vger.kernel.org
Cc: "Joel Fernandes (Google)" <joel@joelfernandes.org>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Josh Triplett <josh@joshtriplett.org>,
Steven Rostedt <rostedt@goodmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Lai Jiangshan <jiangshanlai@gmail.com>,
byungchul.park@lge.com, kernel-team@android.com
Subject: [PATCH RFC 1/8] rcu: Add comment documenting how rcu_seq_snap works
Date: Sun, 13 May 2018 20:15:34 -0700 [thread overview]
Message-ID: <20180514031541.67247-2-joel@joelfernandes.org> (raw)
In-Reply-To: <20180514031541.67247-1-joel@joelfernandes.org>
rcu_seq_snap may be tricky for someone looking at it for the first time.
Lets document how it works with an example to make it easier.
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
kernel/rcu/rcu.h | 24 +++++++++++++++++++++++-
1 file changed, 23 insertions(+), 1 deletion(-)
diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index 003671825d62..fc3170914ac7 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -91,7 +91,29 @@ static inline void rcu_seq_end(unsigned long *sp)
WRITE_ONCE(*sp, rcu_seq_endval(sp));
}
-/* Take a snapshot of the update side's sequence number. */
+/*
+ * Take a snapshot of the update side's sequence number.
+ *
+ * This function predicts what the grace period number will be the next
+ * time an RCU callback will be executed, given the current grace period's
+ * number. This can be gp+1 if RCU is idle, or gp+2 if a grace period is
+ * already in progress.
+ *
+ * We do this with a single addition and masking.
+ * For example, if RCU_SEQ_STATE_MASK=1 and the least significant bit (LSB) of
+ * the seq is used to track if a GP is in progress or not, its sufficient if we
+ * add (2+1) and mask with ~1. Let's see why with an example:
+ *
+ * Say the current seq is 6 which is 0b110 (gp is 3 and state bit is 0).
+ * To get the next GP number, we have to at least add 0b10 to this (0x1 << 1)
+ * to account for the state bit. However, if the current seq is 7 (gp is 3 and
+ * state bit is 1), then it means the current grace period is already in
+ * progress so the next time the callback will run is at the end of grace
+ * period number gp+2. To account for the extra +1, we just overflow the LSB by
+ * adding another 0x1 and masking with ~0x1. In case no GP was in progress (RCU
+ * is idle), then the addition of the extra 0x1 and masking will have no
+ * effect. This is calculated as below.
+ */
static inline unsigned long rcu_seq_snap(unsigned long *sp)
{
unsigned long s;
--
2.17.0.441.gb46fe60e1d-goog
next prev parent reply other threads:[~2018-05-14 3:16 UTC|newest]
Thread overview: 42+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-14 3:15 [PATCH RFC 0/8] rcu fixes, clean ups for rcu/dev Joel Fernandes (Google)
2018-05-14 3:15 ` Joel Fernandes (Google) [this message]
2018-05-14 3:47 ` [PATCH RFC 1/8] rcu: Add comment documenting how rcu_seq_snap works Randy Dunlap
2018-05-14 5:05 ` Joel Fernandes
2018-05-14 17:38 ` Paul E. McKenney
2018-05-15 1:51 ` Joel Fernandes
2018-05-15 3:59 ` Paul E. McKenney
2018-05-15 7:02 ` Joel Fernandes
2018-05-15 12:55 ` Paul E. McKenney
2018-05-15 18:41 ` Joel Fernandes
2018-05-15 19:08 ` Paul E. McKenney
2018-05-15 22:55 ` Joel Fernandes
2018-05-16 15:45 ` Paul E. McKenney
2018-05-16 23:21 ` Joel Fernandes
2018-05-14 3:15 ` [PATCH RFC 2/8] rcu: Clarify usage of cond_resched for tasks-RCU Joel Fernandes (Google)
2018-05-14 14:54 ` Steven Rostedt
2018-05-14 17:22 ` Paul E. McKenney
2018-05-15 0:35 ` Joel Fernandes
2018-05-15 3:42 ` Paul E. McKenney
2018-05-14 3:15 ` [PATCH RFC 3/8] rcu: Add back the cpuend tracepoint Joel Fernandes (Google)
2018-05-14 18:12 ` Paul E. McKenney
2018-05-15 0:43 ` Joel Fernandes
2018-05-14 3:15 ` [PATCH RFC 4/8] rcu: Get rid of old c variable from places in tree RCU Joel Fernandes (Google)
2018-05-14 17:57 ` Paul E. McKenney
2018-05-15 0:41 ` Joel Fernandes
2018-05-14 3:15 ` [PATCH RFC 5/8] rcu: Use rcu_node as temporary variable in funnel locking loop Joel Fernandes (Google)
2018-05-14 18:00 ` Paul E. McKenney
2018-05-15 0:43 ` Joel Fernandes
2018-05-14 3:15 ` [PATCH RFC 6/8] rcu: Add back the Startedleaf tracepoint Joel Fernandes (Google)
2018-05-14 18:38 ` Paul E. McKenney
2018-05-15 0:57 ` Joel Fernandes
2018-05-15 3:46 ` Paul E. McKenney
2018-05-15 23:04 ` Joel Fernandes
2018-05-16 15:48 ` Paul E. McKenney
2018-05-16 23:13 ` Joel Fernandes
2018-05-14 3:15 ` [PATCH RFC 7/8] rcu: trace CleanupMore condition only if needed Joel Fernandes (Google)
2018-05-14 19:20 ` Paul E. McKenney
2018-05-15 1:01 ` Joel Fernandes
2018-05-15 3:47 ` Paul E. McKenney
2018-05-14 3:15 ` [PATCH RFC 8/8] rcu: Fix cpustart tracepoint gp_seq number Joel Fernandes (Google)
2018-05-14 20:33 ` Paul E. McKenney
2018-05-15 1:02 ` Joel Fernandes
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