Re: [PATCH] driver/pci: reduce the single block time in pci_read_config

[Jiang Biao <benbjiang@gmail.com>]

Hi, Bjorn

On Thu, 10 Sep 2020 at 09:59, Bjorn Helgaas <helgaas@kernel.org> wrote:
>
> 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@kernel.org> wrote:
> > >
> > > On Mon, Aug 24, 2020 at 01:20:25PM +0800, Jiang Biao wrote:
> > > > From: Jiang Biao <benbjiang@tencent.com>
> > > >
> > > > 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
We failed to build a test case to produce the latency by setpci command,
AFAIU, setpci could be much less frequently realistically used than lspci.
So, the latency from pci_write_config() path could not be verified for now,
could we apply this patch alone to erase the verified latency introduced
by pci_read_config() path? :)

Thanks a lot.
Regards,
Jiang