On Fri, Apr 14, 2017 at 5:17 PM, Sinan Kaya <okaya@xxxxxxxxxxxxxx> wrote: > On 4/14/2017 5:44 PM, Bjorn Helgaas wrote: >> I think there's an argument to be made that if we care about ASPM >> configuration, we should be using a non-POLICY_DEFAULT setting. And I >> think there's value in having POLICY_DEFAULT be the absolute lowest- >> risk setting, which I think means option 1. >> >> What do you think? > > I see your point. The counter argument is that most of the users do not > know what an ASPM kernel command line is unless they understand PCI > language. I don't think the answer is using the "pcie_aspm.policy=" boot argument. I certainly don't want users to have to deal with that. I wish we didn't even have that parameter. I think we need runtime knobs instead (and I guess we already have /sys/module/pcie_aspm/parameters/policy and /sys/.../link_state), and distro userspace should use them. I'm envisioning something in "System Settings / Power" or similar. Basically I think the policy doesn't *have* to be dictated by a kernel boot-time parameter, so it should not be. > I have been using the powersave policy option until now. I recently realized > that nobody except me is using this option. Therefore, we are wasting > power by default following a hotplug insertion. > > This is the case where I'm trying to avoid. With the introduction of NVMe > u.2 drives, hotplug is becoming more and more mainstream. I decided to > take the matters into my hand with this series for this very reason. > > Like you said, BIOS is out of the picture with pciehp. There is nobody > to configure ASPM following a hotplug insertion. > > I can also claim that If user wants performance, they should boot with > the performance policy or pcie_aspm=off parameters. > > I saw this recommendation in multiple DPDK tuning documents. > > Like you said, what do we do by default is the question. Should we opt > for safe like we are doing, or try to save some power. I think safety is paramount. Every user should be able to boot safely without any kernel parameters. We don't want users to have a problem booting and then have to search for a workaround like booting with "pcie_aspm=off". Most users will never do that. Here's a long-term strawman proposal, see what you think: - Deprecate CONFIG_PCIEASPM_DEFAULT, CONFIG_PCIEASPM_POWERSAVE, etc. - Default aspm_policy is POLICY_DEFAULT always. - POLICY_DEFAULT means Linux doesn't touch anything: if BIOS enabled ASPM, we leave it that way; we leave ASPM disabled on hot-added devices. - Deprecate kernel boot parameters (possibly keep pcie_aspm=off for debugging use). - Use /sys/module/pcie_aspm/parameters/policy for run-time system-wide control, including for future hot-added devices. - Remove CONFIG_PCIEASPM_DEBUG and enable that code always, so we have fine-grained run-time control. > Maybe, we are missing a HPP option from the PCI spec. That's an interesting idea. _HPX does have provision for manipulating Link Control bits (see ACPI r5.0, sec 6.2.8.3), but I don't think very many systems implement it. And there's currently no connection between program_hpp_type2() and aspm.c, so I'm a little worried that we might have issues if a system did implement an _HPX that sets any of the ASPM bits. Bjorn
