[PATCH v2] pci: call cond_resched() after pci bus config writing

[PATCH v2] pci: call cond_resched() after pci bus config writing

[menglong8.dong]

From: Menglong Dong <imagedong@tencent.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 write pci
config 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@tencent.com>
---
v2:
- use cond_resched() instead of _cond_resched()
---
 drivers/pci/access.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/pci/access.c b/drivers/pci/access.c
index 46935695cfb9..4c52a50f2c46 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

Re: [PATCH v2] pci: call cond_resched() after pci bus config writing

[Bjorn Helgaas]

On Fri, Oct 15, 2021 at 10:40:25AM +0800, menglong8.dong@gmail.com wrote:
> From: Menglong Dong <imagedong@tencent.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 write pci
> config 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.

See https://lore.kernel.org/r/20211013190014.GA1909934@bhelgaas
for comments about subject line, s/pci/PCI/, line length, etc.

> Signed-off-by: Menglong Dong <imagedong@tencent.com>
> ---
> v2:
> - use cond_resched() instead of _cond_resched()
> ---
>  drivers/pci/access.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/pci/access.c b/drivers/pci/access.c
> index 46935695cfb9..4c52a50f2c46 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
>