On Thu, Sep 10, 2020 at 09:54:02AM +0800, Jiang Biao wrote:
> Hi,
>
> On Thu, 10 Sep 2020 at 09:25, Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
> >
> > On Mon, Aug 24, 2020 at 01:20:25PM +0800, Jiang Biao wrote:
> > > From: Jiang Biao <benbjiang@xxxxxxxxxxx>
> > >
> > > pci_read_config() could block several ms in kernel space, mainly
> > > caused by the while loop to call pci_user_read_config_dword().
> > > Singel pci_user_read_config_dword() loop could consume 130us+,
> > > | pci_user_read_config_dword() {
> > > | _raw_spin_lock_irq() {
> > > ! 136.698 us | native_queued_spin_lock_slowpath();
> > > ! 137.582 us | }
> > > | pci_read() {
> > > | raw_pci_read() {
> > > | pci_conf1_read() {
> > > 0.230 us | _raw_spin_lock_irqsave();
> > > 0.035 us | _raw_spin_unlock_irqrestore();
> > > 8.476 us | }
> > > 8.790 us | }
> > > 9.091 us | }
> > > ! 147.263 us | }
> > > and dozens of the loop could consume ms+.
> > >
> > > If we execute some lspci commands concurrently, ms+ scheduling
> > > latency could be detected.
> > >
> > > Add scheduling chance in the loop to improve the latency.
> >
> > Thanks for the patch, this makes a lot of sense.
> >
> > Shouldn't we do the same in pci_write_config()?
> Yes, IMHO, that could be helpful too.
If it's feasible, it would be nice to actually verify that it makes a
difference. I know config writes should be faster than reads, but
they're certainly not as fast as a CPU can pump out data, so there
must be *some* mechanism that slows the CPU down.
Bjorn
