linux-kernel - [PATCH] irqchip/gic-v4.1: Optimize the delay time of the poll on the GICR

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Message-ID: <20200915072213.62-1-lushenming@huawei.com>
Date:   Tue, 15 Sep 2020 15:22:13 +0800
From:   Shenming Lu <lushenming@...wei.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Marc Zyngier <maz@...nel.org>, <linux-kernel@...r.kernel.org>
CC:     <wanghaibin.wang@...wei.com>
Subject: [PATCH] irqchip/gic-v4.1: Optimize the delay time of the poll on the GICR_VENPENDBASER.Dirty bit

Every time the vPE is scheduled, the code starts polling the
GICR_VPENDBASER.Dirty bit until it becomes 0. It's OK. But
the delay_us of the poll function is directly set to 10, which
is a long time for most interrupts. In our measurement, it takes
only 1~2 microseconds, or even hundreds of nanoseconds, to finish
parsing the VPT in most cases. However, in the current implementation,
if the GICR_VPENDBASER.Dirty bit is not 0 immediately after the
vPE is scheduled, it will directly wait for 10 microseconds,
resulting in meaningless waiting.

In order to avoid meaningless waiting, we can set the delay_us
to 0, which can exit the poll function immediately when the Dirty
bit becomes 0.

Signed-off-by: Shenming Lu <lushenming@...wei.com>
---
 drivers/irqchip/irq-gic-v3-its.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
index 548de7538632..5cfcf0c2ce1a 100644
--- a/drivers/irqchip/irq-gic-v3-its.c
+++ b/drivers/irqchip/irq-gic-v3-its.c
@@ -3803,7 +3803,7 @@ static void its_wait_vpt_parse_complete(void)
 	WARN_ON_ONCE(readq_relaxed_poll_timeout_atomic(vlpi_base + GICR_VPENDBASER,
 						       val,
 						       !(val & GICR_VPENDBASER_Dirty),
-						       10, 500));
+						       0, 500));
 }

 static void its_vpe_schedule(struct its_vpe *vpe)
-- 
2.19.1