linux-kernel - Re: [PATCH] driver/pci: reduce the single block time in pci_read

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Date:   Wed, 9 Sep 2020 20:59:50 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Jiang Biao <benbjiang@...il.com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Jiang Biao <benbjiang@...cent.com>,
        Bin Lai <robinlai@...cent.com>
Subject: Re: [PATCH] driver/pci: reduce the single block time in
 pci_read_config

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@...nel.org> wrote:
> >
> > On Mon, Aug 24, 2020 at 01:20:25PM +0800, Jiang Biao wrote:
> > > From: Jiang Biao <benbjiang@...cent.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