From: Jiang Biao <benbjiang@gmail.com>
To: Bjorn Helgaas <helgaas@kernel.org>
Cc: Bjorn Helgaas <bhelgaas@google.com>,
linux-pci@vger.kernel.org,
linux-kernel <linux-kernel@vger.kernel.org>,
Jiang Biao <benbjiang@tencent.com>,
Bin Lai <robinlai@tencent.com>
Subject: Re: [PATCH] driver/pci: reduce the single block time in pci_read_config
Date: Sun, 13 Sep 2020 12:27:09 +0800 [thread overview]
Message-ID: <CAPJCdBngxwYdc-CEfSabTAdAXCdnG424Qa2BS47+xcV2wDvJCA@mail.gmail.com> (raw)
In-Reply-To: <20200910015950.GA748330@bjorn-Precision-5520>
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
next prev parent reply other threads:[~2020-09-13 4:27 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-24 5:20 [PATCH] driver/pci: reduce the single block time in pci_read_config Jiang Biao
2020-08-27 9:49 ` Jiang Biao
2020-09-10 1:25 ` Bjorn Helgaas
2020-09-10 1:54 ` Jiang Biao
2020-09-10 1:59 ` Bjorn Helgaas
2020-09-10 2:18 ` Jiang Biao
2020-09-13 4:27 ` Jiang Biao [this message]
2020-09-16 16:56 ` Bjorn Helgaas
2020-09-17 0:22 ` Jiang Biao
2020-09-17 17:32 ` Bjorn Helgaas
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=CAPJCdBngxwYdc-CEfSabTAdAXCdnG424Qa2BS47+xcV2wDvJCA@mail.gmail.com \
--to=benbjiang@gmail.com \
--cc=benbjiang@tencent.com \
--cc=bhelgaas@google.com \
--cc=helgaas@kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-pci@vger.kernel.org \
--cc=robinlai@tencent.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).