linux-kernel - [PATCH] pci: call _cond_resched() after pci_bus_write

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Message-Id: <20211013125542.759696-1-imagedong@tencent.com>
Date:   Wed, 13 Oct 2021 20:55:42 +0800
From:   menglong8.dong@...il.com
To:     bhelgaas@...gle.com
Cc:     linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        Menglong Dong <imagedong@...cent.com>
Subject: [PATCH] pci: call _cond_resched() after pci_bus_write_config

From: Menglong Dong <imagedong@...cent.com>

While the system is running in KVM, pci config writing for virtio devices
may cost long time(about 1-2ms), as it causes VM-exit. During
__pci_bus_assign_resources(), pci_setup_bridge, which can do pci config
writing up to 10 times, can be called many times without any
_cond_resched(). So __pci_bus_assign_resources can cause 25+ms scheduling
latency with !CONFIG_PREEMPT.

To solve this problem, call _cond_resched() after pci config writing.

Signed-off-by: Menglong Dong <imagedong@...cent.com>
---
 drivers/pci/access.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/pci/access.c b/drivers/pci/access.c
index 46935695cfb9..babed43702df 100644
--- a/drivers/pci/access.c
+++ b/drivers/pci/access.c
@@ -57,6 +57,7 @@ int noinline pci_bus_write_config_##size \
 	pci_lock_config(flags);						\
 	res = bus->ops->write(bus, devfn, pos, len, value);		\
 	pci_unlock_config(flags);					\
+	_cond_resched();						\
 	return res;							\
 }

-- 
2.27.0