linux-kernel - Re: /sys/module/pcie_aspm/parameters/policy not writable?

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Date:	Wed, 10 Jul 2013 15:29:50 +0200
From:	Pavel Machek <pavel@....cz>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	Greg KH <greg@...ah.com>,
	kernel list <linux-kernel@...r.kernel.org>,
	Joe Lawrence <joe.lawrence@...atus.com>,
	Myron Stowe <myron.stowe@...hat.com>
Subject: Re: /sys/module/pcie_aspm/parameters/policy not writable?

Hi!

> >> But:
> >> 1) it should not list unavailable options
> >>
> >> 2) operation not permitted seems like wrong error code for
> >> operation not supported.
> >
> > So I forcibly enabled ASPM, and now ping latencies are in normal
> > range... no matter how I set
> > /sys/module/pcie_aspm/parameters/policy. Strange.
> >
> > Any ideas what correct solution is?
> >                                                                         Pavel
> > Signed-off-by: Pavel Machek <pavel@....cz>
> > (but don't apply)
...
> > diff --git a/drivers/pci/pci-acpi.c b/drivers/pci/pci-acpi.c
> > index e4b1fb2..9a1b63e 100644
> > --- a/drivers/pci/pci-acpi.c
> > +++ b/drivers/pci/pci-acpi.c
> > @@ -382,7 +382,7 @@ static int __init acpi_pci_init(void)
> >
> >         if (acpi_gbl_FADT.boot_flags & ACPI_FADT_NO_ASPM) {
> >                 printk(KERN_INFO"ACPI FADT declares the system doesn't support PCIe ASPM, so disable it\n");
> > -               pcie_no_aspm();
> > +//             pcie_no_aspm();
> >         }
> >
> >         ret = register_acpi_bus_type(&acpi_pci_bus);
> 
> Hi Pavel,
> 
> Interesting.  Can you collect dmesg and "lspci -vvv" output for both
> cases (high ping latency and normal ping latency)?  

Will do. Results are in attachment (200KB...)

> Also, how much
> difference does this make in ping latency?  

The ping latency goes from 100msec range to <2msec.

> If ASPM is enabled for a
> device, e.g., your NIC, the link may be put in a low power state when
> the device is idle.  It takes time to exit that low power state, of
> course, but I would expect that time to be in the microsecond time and
> probably not observable via ping.

I'd hope so. 100msec ping makes ssh unpleasant to use.

Thanks,
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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