Match previous subject lines (use "git log --oneline
drivers/pci/access.c" to see them).
On Wed, Oct 13, 2021 at 08:55:42PM +0800, menglong8.dong@xxxxxxxxx wrote:
> From: Menglong Dong <imagedong@xxxxxxxxxxx>
>
> 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.
s/pci/PCI/ above.
Add space before "(".
Add "()" after function names consistently (some have it, some don't).
What exactly is the problem? I expect __pci_bus_assign_resources() to
be used mostly during boot-time enumeration. How much of a problem is
the latency at that point? Why is this particularly a problem in the
KVM environment? Or is it also a problem on bare metal?
Are there other config write paths that should have a similar change?
_cond_resched() only appears here:
$ git grep "\<_cond_resched\>"
include/linux/sched.h:static __always_inline int _cond_resched(void)
include/linux/sched.h:static inline int _cond_resched(void)
include/linux/sched.h:static inline int _cond_resched(void) { return 0; }
include/linux/sched.h: _cond_resched();
so I don't believe PCI is so special that this needs to be the only
other use. Maybe a different resched interface is more appropriate?
> Signed-off-by: Menglong Dong <imagedong@xxxxxxxxxxx>
> ---
> 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
>
