[PATCH 4/5] ARCv2: Elide sending new cross core intr if receiver didn't ack prev

Vineet Gupta
2016-02-23T09:12:33Z

ARConnect/MCIP IPI sending has a retry-wait loop in case caller had
not seen a previous such interrupt. Turns out that it is not needed at
all. Linux cross core calling allows coalescing multiple IPIs to same
receiver - it is fine as long as there is one.

This logic is built into upper layer already, at a higher level of
abstraction. ipi_send_msg_one() sets the actual msg payload, but it only
calls MCIP IPI sending if msg holder was empty (using
atomic-set-new-and-get-old construct). Thus it is unlikely that the
retry-wait looping was ever getting exercised at all.

Cc: Chuck Jordan <cjordan@synopsys.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
---
arch/arc/kernel/mcip.c | 27 ++++++++++-----------------
1 file changed, 10 insertions(+), 17 deletions(-)

diff --git a/arch/arc/kernel/mcip.c b/arch/arc/kernel/mcip.c
index e30d5d428330..7afc3c703ed1 100644
--- a/arch/arc/kernel/mcip.c
+++ b/arch/arc/kernel/mcip.c
@@ -40,26 +40,19 @@ static void mcip_ipi_send(int cpu)
return;
}

+ raw_spin_lock_irqsave(&mcip_lock, flags);
+
/*
- * NOTE: We must spin here if the other cpu hasn't yet
- * serviced a previous message. This can burn lots
- * of time, but we MUST follows this protocol or
- * ipi messages can be lost!!!
- * Also, we must release the lock in this loop because
- * the other side may get to this same loop and not
- * be able to ack -- thus causing deadlock.
+ * If receiver already has a pending interrupt, elide sending this one.
+ * Linux cross core calling works well with concurrent IPIs
+ * coalesced into one
+ * see arch/arc/kernel/smp.c: ipi_send_msg_one()
*/
+ __mcip_cmd(CMD_INTRPT_READ_STATUS, cpu);
+ ipi_was_pending = read_aux_reg(ARC_REG_MCIP_READBACK);
+ if (!ipi_was_pending)
+ __mcip_cmd(CMD_INTRPT_GENERATE_IRQ, cpu);

- do {
- raw_spin_lock_irqsave(&mcip_lock, flags);
- __mcip_cmd(CMD_INTRPT_READ_STATUS, cpu);
- ipi_was_pending = read_aux_reg(ARC_REG_MCIP_READBACK);
- if (ipi_was_pending == 0)
- break; /* break out but keep lock */
- raw_spin_unlock_irqrestore(&mcip_lock, flags);
- } while (1);
-
- __mcip_cmd(CMD_INTRPT_GENERATE_IRQ, cpu);
raw_spin_unlock_irqrestore(&mcip_lock, flags);

#ifdef CONFIG_ARC_IPI_DBG
--
2.5.0